Home / Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules

Stereochemistry and Chirality

By James Ashenhurst

  • Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules

Last updated: February 20th, 2024 |

Assigning  R and  S Configurations With the Cahn-Ingold-Prelog (CIP) Rules

  • Enantiomers are stereoisomers that are non-superimposable mirror images of each other (by the way, molecules that are superimposable mirror images of each other are considered to be identical molecules). 
  • Enantiomers rotate plane-polarized light in equal and opposite directions, but there is no straightforward way to trace back the absolute configuration of a molecule to the direction of optical rotation
  • A tetrahedral atom with four different substituents (a chiral center) can have two different configurations. A naming scheme developed by Cahn, Ingold and Prelog (CIP) is used for assigning the terms  R or  S  to each chiral center.  (When all the (R,S) designations of a molecule are specified, this is referred to as its “ absolute configuration”.)
  • In the CIP protocol, each atom attached to the chiral center is ranked in order of atomic number (highest = #1, lowest = #4).  (We go further down the chain in the event of ties. )
  • With the #4 substituent in the back: if #1, #2, and #3 trace a clockwise path, the chiral center is assigned (R) . If they trace a counterclockwise path the chiral center is (S) .
  • When #4 is in the front or on the side, some useful tips and tricks can be used to avoid having to rotate the whole molecule ( See also: How To Draw The Enantiomer of A Chiral Molecule ).
  • For breaking ties, it’s useful to keep track of which carbons you’re working on with the “dot method”.

Table of Contents

  • Chiral Centers And The Problem Of Naming
  • The Cahn-Ingold-Prelog System For Naming Chiral Centers
  • Oh No! What Do We Do When #4 Is Not In The Back?
  • Assigning R/S When #4 Is In The Front: A Short Cut
  • Assigning R/S When #4 Is In The Plane Of The Page
  • Breaking Ties With The “Dot Technique”
  • Conclusion: Assigning R and S With CIP

(Advanced) References and Further Reading

This post was co-authored by Matt Pierce of Organic Chemistry Solutions .  Ask Matt about scheduling an online tutoring session here .

1. Chiral Centers And The Problem Of Naming

Previously on MOC we’ve described enantiomers : molecules that are non-superimposable mirror images of each other. Perhaps the most memorable example is these “enantiocats”.

drawing-of-enantiocats-master-organic-chemistry-graeme-mackay-look-at-the-legs

Each of these cats is said to be “chiral”: they lack a plane of symmetry.

What causes molecules to have chirality?

The most common source of chirality is a “chiral centre”: typically a  tetrahedral carbon attached to four different “groups”, or “substituents”.  For each chiral centre there are  two  (and only two!) different ways of arranging the 4 different substituents, which gives rise to two different configurations.  [If you don’t believe there are only two, see Single Swap Rule ].

The purpose of this post is to introduce and describe the nomenclature we use to describe these configurations: the (R)/(S) notation, or Cahn-Ingold Prelog Rules.

Let’s look at a simple example.

Both of these molecules are 1-bromo-1-chloroethane. But they are not  exactly the same molecule, in the same way that your left shoe is not exactly the same as your right. They are non-superimposable mirror images of each other. How do we communicate this difference?

One way would be to describe their physical properties. For example, although these two molecules have the same boiling point, melting point, and share many other physical properties, they rotate plane-polarized light in equal and opposite directions, a property called  optical rotation ( See Optical Rotation and Optical Activity ) We could use (+)-1-bromo-1-chloroethane to refer to the isomer that rotates polarized light to the right (clockwise, or “dextrorotatory”) and use (-)-1-bromo-1-chloroethane to refer to the isomer that rotates polarized light to the left (counterclockwise, or “levorotatory”).

However this nomenclature suffers from a serious problem. There is no simple correlation between the arrangement of substituents around a chiral centre and the direction in which polarized light is rotated . Another solution is needed.

2. The Cahn Ingold Prelog (CIP) System For Naming Chiral Centers

A solution to this quandary was proposed by Robert Cahn, Chris Ingold, and Vladimir Prelog in 1966. The resulting “CIP” protocol works as follows:

  • Prioritize the four groups around a chiral center according to atomic number . The highest atomic number is assigned priority #1, and the lowest atomic number is assigned priority #4.
  • Orient the chiral centre such that the #4 priority substituent is pointing away from the viewer. For our purposes, it’s enough for it merely to be attached to a “dashed” bond.
  • If the path traced from 1-2-3   is clockwise , the chiral center is assigned ( R ) (from Latin,  rectus )
  • If the path traced is counter clockwise , the chiral center is assigned ( S ) (from the Latin  sinister)
  • Now we have a better way to describe molecules [A] and [B] shown above. Molecule [A] is named ( R )-1-bromo-1-chloroethane, and molecule [B] is named ( S )-1-bromo-1-chloroethane:

We should reiterate that the designations (R) and (S) bear no relationship to whether a molecule rotates plane-polarized light clockwise (+) or counterclockwise (-). For example the most common naturally occurring configuration of the amino acid alanine is (S), but its optical rotation (in aqueous acid solution) is (+).

3. What About When #4 Is Not In The Back?

That seems simple enough! “Is that it?”, you might ask.

Uh, no. As it happens, there’s a few bumps in the road toward determining (R)/(S) once we get beyond the simple example above.

These “trickier cases” fall into three main categories.

  • What if the #4 substituent is not helpfully pointing away from the viewer , as it was in our example above? What if it’s in the “front” (i.e. attached to a “wedged” bond) or, heaven forbid, in the plane of the page?
  • Assigning priorities in complex situations . What do we do in situations where a chiral centre has two or more identical atoms attached? In other words, how do we break ties? 
  • What do we do if the molecule is drawn a peculiar way , such as in  Fischer or Newman projections ?

We’re not going to be able to fully address all of these issues in this post. But we can certainly deal with #1 and make some headway with #2. For #3, see How To Determine R/S On A Fischer Projection and How to Determine R/S on a Newman Projection

4. Determining R/S When The #4 Substituent Is In Front (i.e. on a “Wedge”): A Short Cut

Let’s first consider the molecule below. The name of this molecule is ( R )-1-fluoroethanol. It is listed below with priorities assigned based on atomic number. In this case F>O>C>H. So F is #1 and H is #4. The tricky part here is that the #4 priority is pointing out of the page (on a “wedge”).

How do we determine (R)/(S) in this case?  There are two ways to do it.

Many instructors will tell you to “simply” rotate the molecule in your head so that the #4 priority is on a dash. Then you can assign R or S in the traditional way. This “simple” advice is not always an easy task for beginners.

Thankfully, it is technically unnecessary to perform such a mental rotation.

Here’s  a way around this. When the #4 priority is on a wedge you can just reverse the rules. So now we have two sets of rules:

If the #4 priority is on a dash :

  • Clockwise = R
  • Counterclockwise = S

If the #4 priority is on a wedge , reverse the typical rules:

  • Clockwise = S
  • Counterclockwise = R

R and S can easily be assigned to either picture of the molecule. I still encourage you to use a model kit and learn how to do so, however. Organic chemistry is much easier to understand, and much more beautiful, if you can master how to visualize a tetrahedral carbon atom.

See also, How To Draw The Enantiomer of A Chiral Molecule

5. Determining R/S When The #4 Group Is In The Plane Of The Page?

What if the #4 priority is in the plane of the paper, for example on a line? In this case it’s impossible to assign R and S in the traditional way. You’d have a 50:50 shot of getting it correct: not good odds. Again, if you can redraw the molecule in your head, then it’s better to just do that. If you can’t do this reliably then you need to learn the “single swap” concept.

Here’s how it works.  Swapping any two groups on a chiral centre will flip the configuration of the chiral centre from R to S (and vice versa). [ We previously talked about the “single swap rule” here ]

Knowing this, we can do a nifty trick.

  • Take the #4 substituent (in the plane of the page) and  swap it with the substituent in the back [If the configuration is R, this will switch it to S. If the configuration is S, this will flip it to R. We’ll need to account for this in step #3].
  • Redraw the chiral centre, and determine R/S on the new chiral centre which now has group #4 in the back.
  •  Whatever result you got,  flip it to its opposite to account for the fact that you did a single swap in step #1.

Here’s an example. Note that here  I first switched #4 and #3, but the main point is to switch two groups so that #4 is out of the plane of the paper.

This method always works, assuming you’ve determined the four priorities accurately. (It also works for cases when #4 is on a wedge).

However, sometimes we’re not in the position of dealing with 4 different atoms attached to a chiral carbon. For instance, it’s possible to have chiral carbons which are attached to 4  carbons . So how do we break the ties in these cases?

6. Determining CIP Priorities: Breaking “Ties” With The “Dot Technique”

The quick answer is to use the “dot technique”. Here’s how it works. Let’s do it for 4-ethyl-4-methyloctane, above.

  • Go outward from the chiral centre to each of the surrounding 4 atoms and assign priorities (based on atomic number) to each of these atoms. [Sometimes it’s helpful to draw  dots on each of these atoms.]

3. Compare each list, atom by atom. In our example, since C>H, (C,H,H) takes priority over (H,H,H) so the CH 3 group is assigned priority #4.

4. If there is still a tie, move the dots to the highest ranking atom in the list (i.e. the atom with highest atomic number). The dots are helpful because they help you to keep track of where you are, which can be important in complex examples.

5. In this case, we keep moving along the chain. By the way, if you ever reach the end of the chain without determining a difference, that means that the groups are identical and it isn’t a chiral centre after all.

6. By this point we have enough information to assign (R)/(S). Since priority #4 is in the front, we can also break out our “opposite rule” for good measure:

7. Conclusion: The Cahn-Ingold-Prelog Rules For Assigning R and S Configurations

In the next post we’ll go into some trickier examples with determining R/S, including how to deal with double bonds, rings, and isotopes. In a future post, we’ll get into determining R/S in the Fischer and Newman projections.

Thanks to Matt Pierce for making major contributions to this article.  

Ask Matt about scheduling an online tutoring session  here .

Related Articles

  • Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) – The Method of Dots
  • How To Draw The Enantiomer Of A Chiral Molecule
  • How To Determine R and S Configurations On A Fischer Projection
  • Assigning R/S To Newman Projections (And Converting Newman To Line Diagrams)
  • Types of Isomers: Constitutional Isomers, Stereoisomers, Enantiomers, and Diastereomers
  • On Cats, Part 4: Enantiocats
  • On Cats, Part 6: Stereocenters
  • Stereochemistry Practice Problems and Quizzes (MOC Membership)
  • How To Draw A Bond Rotation
  • Specification of Molecular Chirality R. S. Cahn, Sir Christopher Ingold, V. Prelog Angew. Chem. Int. Ed. 1966, 5 (4), 385-415 DOI: 10.1002/anie.196603851 This is not the first paper on the topic by the authors (see Refs. 4 and 5), but it is a major publication and an attempt to consolidate all the information on chirality in a single place. This paper discusses the various types of chirality possible in chemistry (not just at tetrahedral carbons!) and how to assign chirality unambiguously.
  • Basic Principles of the CIP‐System and Proposals for a Revision Dr. Vladlmir Prelog and Prof. Dr. Günter Helmchen Angew. Chem. Int. Ed. 1982 , 21 (8), 567-583 DOI: 10.1002/anie.198205671 An update to Ref. #1, which addresses a lot of edge cases that may come up in complex stereochemical assignments.
  • CHIRALITY IN CHEMISTRY Vladimir Prelog Nobel Lecture, 1975 Prelog’s Nobel Lecture. Nobel Lectures are fascinating to read as they give insight into the life of scientists and the path to discovery, which is rarely linear.
  • “Absolutely” simple stereochemistry Philip S. Beauchamp Journal of Chemical Education 1984, 61 (8), 666 DOI : 10.1021/ed061p666 This paper describes a simple method for determining stereochemistry of tetrahedral carbons using the hands, suitable for undergraduate students of organic chemistry.
  • A simple hand method for Cahn-Ingold-Prelog assignment of R and S configuration to chiral carbons Martin P. Aalund and James A. Pincock Journal of Chemical Education 1986, 63 (7), 600 DOI : 10.1021/ed063p600 A follow-up to the previous paper (Ref #4), but sadly it is incomplete!
  • A Web-Based Stereochemistry Tool to Improve Students’ Ability to Draw Newman Projections and Chair Conformations and Assign R/S Labels Nimesh Mistry, Ravi Singh, and Jamie Ridley Journal of Chemical Education 2020, 97 (4), 1157-1161 DOI : 10.1021/acs.jchemed.9b00688 This paper discusses a web-based tool that helps students with visualization of chiral compounds and assignment of stereochemistry as per the Cahn-Ingold-Prelog (CIL) rules. See ref. 34 in the paper for the link.

00 General Chemistry Review

  • Lewis Structures
  • Ionic and Covalent Bonding
  • Chemical Kinetics
  • Chemical Equilibria
  • Valence Electrons of the First Row Elements
  • How Concepts Build Up In Org 1 ("The Pyramid")

01 Bonding, Structure, and Resonance

  • How Do We Know Methane (CH4) Is Tetrahedral?
  • Hybrid Orbitals and Hybridization
  • How To Determine Hybridization: A Shortcut
  • Orbital Hybridization And Bond Strengths
  • Sigma bonds come in six varieties: Pi bonds come in one
  • A Key Skill: How to Calculate Formal Charge
  • The Four Intermolecular Forces and How They Affect Boiling Points
  • 3 Trends That Affect Boiling Points
  • How To Use Electronegativity To Determine Electron Density (and why NOT to trust formal charge)
  • Introduction to Resonance
  • How To Use Curved Arrows To Interchange Resonance Forms
  • Evaluating Resonance Forms (1) - The Rule of Least Charges
  • How To Find The Best Resonance Structure By Applying Electronegativity
  • Evaluating Resonance Structures With Negative Charges
  • Evaluating Resonance Structures With Positive Charge
  • Exploring Resonance: Pi-Donation
  • Exploring Resonance: Pi-acceptors
  • In Summary: Evaluating Resonance Structures
  • Drawing Resonance Structures: 3 Common Mistakes To Avoid
  • How to apply electronegativity and resonance to understand reactivity
  • Bond Hybridization Practice
  • Structure and Bonding Practice Quizzes
  • Resonance Structures Practice

02 Acid Base Reactions

  • Introduction to Acid-Base Reactions
  • Acid Base Reactions In Organic Chemistry
  • The Stronger The Acid, The Weaker The Conjugate Base
  • Walkthrough of Acid-Base Reactions (3) - Acidity Trends
  • Five Key Factors That Influence Acidity
  • Acid-Base Reactions: Introducing Ka and pKa
  • How to Use a pKa Table
  • The pKa Table Is Your Friend
  • A Handy Rule of Thumb for Acid-Base Reactions
  • Acid Base Reactions Are Fast
  • pKa Values Span 60 Orders Of Magnitude
  • How Protonation and Deprotonation Affect Reactivity
  • Acid Base Practice Problems

03 Alkanes and Nomenclature

  • Meet the (Most Important) Functional Groups
  • Condensed Formulas: Deciphering What the Brackets Mean
  • Hidden Hydrogens, Hidden Lone Pairs, Hidden Counterions
  • Don't Be Futyl, Learn The Butyls
  • Primary, Secondary, Tertiary, Quaternary In Organic Chemistry
  • Branching, and Its Affect On Melting and Boiling Points
  • The Many, Many Ways of Drawing Butane
  • Wedge And Dash Convention For Tetrahedral Carbon
  • Common Mistakes in Organic Chemistry: Pentavalent Carbon
  • Table of Functional Group Priorities for Nomenclature
  • Summary Sheet - Alkane Nomenclature
  • Organic Chemistry IUPAC Nomenclature Demystified With A Simple Puzzle Piece Approach
  • Boiling Point Quizzes
  • Organic Chemistry Nomenclature Quizzes

04 Conformations and Cycloalkanes

  • Staggered vs Eclipsed Conformations of Ethane
  • Conformational Isomers of Propane
  • Newman Projection of Butane (and Gauche Conformation)
  • Introduction to Cycloalkanes (1)
  • Geometric Isomers In Small Rings: Cis And Trans Cycloalkanes
  • Calculation of Ring Strain In Cycloalkanes
  • Cycloalkanes - Ring Strain In Cyclopropane And Cyclobutane
  • Cyclohexane Conformations
  • Cyclohexane Chair Conformation: An Aerial Tour
  • How To Draw The Cyclohexane Chair Conformation
  • The Cyclohexane Chair Flip
  • The Cyclohexane Chair Flip - Energy Diagram
  • Substituted Cyclohexanes - Axial vs Equatorial
  • Ranking The Bulkiness Of Substituents On Cyclohexanes: "A-Values"
  • Cyclohexane Chair Conformation Stability: Which One Is Lower Energy?
  • Fused Rings - Cis-Decalin and Trans-Decalin
  • Naming Bicyclic Compounds - Fused, Bridged, and Spiro
  • Bredt's Rule (And Summary of Cycloalkanes)
  • Newman Projection Practice
  • Cycloalkanes Practice Problems

05 A Primer On Organic Reactions

  • The Most Important Question To Ask When Learning a New Reaction
  • Learning New Reactions: How Do The Electrons Move?
  • The Third Most Important Question to Ask When Learning A New Reaction
  • 7 Factors that stabilize negative charge in organic chemistry
  • 7 Factors That Stabilize Positive Charge in Organic Chemistry
  • Nucleophiles and Electrophiles
  • Curved Arrows (for reactions)
  • Curved Arrows (2): Initial Tails and Final Heads
  • Nucleophilicity vs. Basicity
  • The Three Classes of Nucleophiles
  • What Makes A Good Nucleophile?
  • What makes a good leaving group?
  • 3 Factors That Stabilize Carbocations
  • Equilibrium and Energy Relationships
  • What's a Transition State?
  • Hammond's Postulate
  • Learning Organic Chemistry Reactions: A Checklist (PDF)
  • Introduction to Free Radical Substitution Reactions
  • Introduction to Oxidative Cleavage Reactions

06 Free Radical Reactions

  • Bond Dissociation Energies = Homolytic Cleavage
  • Free Radical Reactions
  • 3 Factors That Stabilize Free Radicals
  • What Factors Destabilize Free Radicals?
  • Bond Strengths And Radical Stability
  • Free Radical Initiation: Why Is "Light" Or "Heat" Required?
  • Initiation, Propagation, Termination
  • Monochlorination Products Of Propane, Pentane, And Other Alkanes
  • Selectivity In Free Radical Reactions
  • Selectivity in Free Radical Reactions: Bromination vs. Chlorination
  • Halogenation At Tiffany's
  • Allylic Bromination
  • Bonus Topic: Allylic Rearrangements
  • In Summary: Free Radicals
  • Synthesis (2) - Reactions of Alkanes
  • Free Radicals Practice Quizzes

07 Stereochemistry and Chirality

  • Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) - The Method of Dots
  • Enantiomers vs Diastereomers vs The Same? Two Methods For Solving Problems
  • The Meso Trap
  • Optical Rotation, Optical Activity, and Specific Rotation
  • Optical Purity and Enantiomeric Excess
  • What's a Racemic Mixture?
  • Chiral Allenes And Chiral Axes
  • Stereochemistry Practice Problems and Quizzes

08 Substitution Reactions

  • Introduction to Nucleophilic Substitution Reactions
  • Walkthrough of Substitution Reactions (1) - Introduction
  • Two Types of Nucleophilic Substitution Reactions
  • The SN2 Mechanism
  • Why the SN2 Reaction Is Powerful
  • The SN1 Mechanism
  • The Conjugate Acid Is A Better Leaving Group
  • Comparing the SN1 and SN2 Reactions
  • Polar Protic? Polar Aprotic? Nonpolar? All About Solvents
  • Steric Hindrance is Like a Fat Goalie
  • Common Blind Spot: Intramolecular Reactions
  • The Conjugate Base is Always a Stronger Nucleophile
  • Substitution Practice - SN1
  • Substitution Practice - SN2

09 Elimination Reactions

  • Elimination Reactions (1): Introduction And The Key Pattern
  • Elimination Reactions (2): The Zaitsev Rule
  • Elimination Reactions Are Favored By Heat
  • Two Elimination Reaction Patterns
  • The E1 Reaction
  • The E2 Mechanism
  • E1 vs E2: Comparing the E1 and E2 Reactions
  • Antiperiplanar Relationships: The E2 Reaction and Cyclohexane Rings
  • Bulky Bases in Elimination Reactions
  • Comparing the E1 vs SN1 Reactions
  • Elimination (E1) Reactions With Rearrangements
  • E1cB - Elimination (Unimolecular) Conjugate Base
  • Elimination (E1) Practice Problems And Solutions
  • Elimination (E2) Practice Problems and Solutions

10 Rearrangements

  • Introduction to Rearrangement Reactions
  • Rearrangement Reactions (1) - Hydride Shifts
  • Carbocation Rearrangement Reactions (2) - Alkyl Shifts
  • Pinacol Rearrangement
  • The SN1, E1, and Alkene Addition Reactions All Pass Through A Carbocation Intermediate

11 SN1/SN2/E1/E2 Decision

  • Identifying Where Substitution and Elimination Reactions Happen
  • Deciding SN1/SN2/E1/E2 (1) - The Substrate
  • Deciding SN1/SN2/E1/E2 (2) - The Nucleophile/Base
  • SN1 vs E1 and SN2 vs E2 : The Temperature
  • Deciding SN1/SN2/E1/E2 - The Solvent
  • Wrapup: The Quick N' Dirty Guide To SN1/SN2/E1/E2
  • Alkyl Halide Reaction Map And Summary
  • SN1 SN2 E1 E2 Practice Problems

12 Alkene Reactions

  • E and Z Notation For Alkenes (+ Cis/Trans)
  • Alkene Stability
  • Addition Reactions: Elimination's Opposite
  • Stereoselective and Stereospecific Reactions
  • Regioselectivity In Alkene Addition Reactions
  • Stereoselectivity In Alkene Addition Reactions: Syn vs Anti Addition
  • Hydrohalogenation of Alkenes and Markovnikov's Rule
  • Hydration of Alkenes With Aqueous Acid
  • Rearrangements in Alkene Addition Reactions
  • Halogenation of Alkenes and Halohydrin Formation
  • Oxymercuration Demercuration of Alkenes
  • Hydroboration Oxidation of Alkenes
  • m-CPBA (meta-chloroperoxybenzoic acid)
  • OsO4 (Osmium Tetroxide) for Dihydroxylation of Alkenes
  • Palladium on Carbon (Pd/C) for Catalytic Hydrogenation of Alkenes
  • Cyclopropanation of Alkenes
  • A Fourth Alkene Addition Pattern - Free Radical Addition
  • Alkene Reactions: Ozonolysis
  • Summary: Three Key Families Of Alkene Reaction Mechanisms
  • Synthesis (4) - Alkene Reaction Map, Including Alkyl Halide Reactions
  • Alkene Reactions Practice Problems

13 Alkyne Reactions

  • Acetylides from Alkynes, And Substitution Reactions of Acetylides
  • Partial Reduction of Alkynes With Lindlar's Catalyst
  • Partial Reduction of Alkynes With Na/NH3 To Obtain Trans Alkenes
  • Alkyne Hydroboration With "R2BH"
  • Hydration and Oxymercuration of Alkynes
  • Hydrohalogenation of Alkynes
  • Alkyne Halogenation: Bromination, Chlorination, and Iodination of Alkynes
  • Alkyne Reactions - The "Concerted" Pathway
  • Alkenes To Alkynes Via Halogenation And Elimination Reactions
  • Alkynes Are A Blank Canvas
  • Synthesis (5) - Reactions of Alkynes
  • Alkyne Reactions Practice Problems With Answers

14 Alcohols, Epoxides and Ethers

  • Alcohols - Nomenclature and Properties
  • Alcohols Can Act As Acids Or Bases (And Why It Matters)
  • Alcohols - Acidity and Basicity
  • The Williamson Ether Synthesis
  • Ethers From Alkenes, Tertiary Alkyl Halides and Alkoxymercuration
  • Alcohols To Ethers via Acid Catalysis
  • Cleavage Of Ethers With Acid
  • Epoxides - The Outlier Of The Ether Family
  • Opening of Epoxides With Acid
  • Epoxide Ring Opening With Base
  • Making Alkyl Halides From Alcohols
  • Tosylates And Mesylates
  • PBr3 and SOCl2
  • Elimination Reactions of Alcohols
  • Elimination of Alcohols To Alkenes With POCl3
  • Alcohol Oxidation: "Strong" and "Weak" Oxidants
  • Demystifying The Mechanisms of Alcohol Oxidations
  • Protecting Groups For Alcohols
  • Thiols And Thioethers
  • Calculating the oxidation state of a carbon
  • Oxidation and Reduction in Organic Chemistry
  • Oxidation Ladders
  • SOCl2 Mechanism For Alcohols To Alkyl Halides: SN2 versus SNi
  • Alcohol Reactions Roadmap (PDF)
  • Alcohol Reaction Practice Problems
  • Epoxide Reaction Quizzes
  • Oxidation and Reduction Practice Quizzes

15 Organometallics

  • What's An Organometallic?
  • Formation of Grignard and Organolithium Reagents
  • Organometallics Are Strong Bases
  • Reactions of Grignard Reagents
  • Protecting Groups In Grignard Reactions
  • Synthesis Problems Involving Grignard Reagents
  • Grignard Reactions And Synthesis (2)
  • Organocuprates (Gilman Reagents): How They're Made
  • Gilman Reagents (Organocuprates): What They're Used For
  • The Heck, Suzuki, and Olefin Metathesis Reactions (And Why They Don't Belong In Most Introductory Organic Chemistry Courses)
  • Reaction Map: Reactions of Organometallics
  • Grignard Practice Problems

16 Spectroscopy

  • Degrees of Unsaturation (or IHD, Index of Hydrogen Deficiency)
  • Conjugation And Color (+ How Bleach Works)
  • Introduction To UV-Vis Spectroscopy
  • UV-Vis Spectroscopy: Absorbance of Carbonyls
  • UV-Vis Spectroscopy: Practice Questions
  • Bond Vibrations, Infrared Spectroscopy, and the "Ball and Spring" Model
  • Infrared Spectroscopy: A Quick Primer On Interpreting Spectra
  • IR Spectroscopy: 4 Practice Problems
  • 1H NMR: How Many Signals?
  • Homotopic, Enantiotopic, Diastereotopic
  • Diastereotopic Protons in 1H NMR Spectroscopy: Examples
  • C13 NMR - How Many Signals
  • Liquid Gold: Pheromones In Doe Urine
  • Natural Product Isolation (1) - Extraction
  • Natural Product Isolation (2) - Purification Techniques, An Overview
  • Structure Determination Case Study: Deer Tarsal Gland Pheromone

17 Dienes and MO Theory

  • What To Expect In Organic Chemistry 2
  • Are these molecules conjugated?
  • Conjugation And Resonance In Organic Chemistry
  • Bonding And Antibonding Pi Orbitals
  • Molecular Orbitals of The Allyl Cation, Allyl Radical, and Allyl Anion
  • Pi Molecular Orbitals of Butadiene
  • Reactions of Dienes: 1,2 and 1,4 Addition
  • Thermodynamic and Kinetic Products
  • More On 1,2 and 1,4 Additions To Dienes
  • s-cis and s-trans
  • The Diels-Alder Reaction
  • Cyclic Dienes and Dienophiles in the Diels-Alder Reaction
  • Stereochemistry of the Diels-Alder Reaction
  • Exo vs Endo Products In The Diels Alder: How To Tell Them Apart
  • HOMO and LUMO In the Diels Alder Reaction
  • Why Are Endo vs Exo Products Favored in the Diels-Alder Reaction?
  • Diels-Alder Reaction: Kinetic and Thermodynamic Control
  • The Retro Diels-Alder Reaction
  • The Intramolecular Diels Alder Reaction
  • Regiochemistry In The Diels-Alder Reaction
  • The Cope and Claisen Rearrangements
  • Electrocyclic Reactions
  • Electrocyclic Ring Opening And Closure (2) - Six (or Eight) Pi Electrons
  • Diels Alder Practice Problems
  • Molecular Orbital Theory Practice

18 Aromaticity

  • Introduction To Aromaticity
  • Rules For Aromaticity
  • Huckel's Rule: What Does 4n+2 Mean?
  • Aromatic, Non-Aromatic, or Antiaromatic? Some Practice Problems
  • Antiaromatic Compounds and Antiaromaticity
  • The Pi Molecular Orbitals of Benzene
  • The Pi Molecular Orbitals of Cyclobutadiene
  • Frost Circles
  • Aromaticity Practice Quizzes

19 Reactions of Aromatic Molecules

  • Electrophilic Aromatic Substitution: Introduction
  • Activating and Deactivating Groups In Electrophilic Aromatic Substitution
  • Electrophilic Aromatic Substitution - The Mechanism
  • Ortho-, Para- and Meta- Directors in Electrophilic Aromatic Substitution
  • Understanding Ortho, Para, and Meta Directors
  • Why are halogens ortho- para- directors?
  • Disubstituted Benzenes: The Strongest Electron-Donor "Wins"
  • Electrophilic Aromatic Substitutions (1) - Halogenation of Benzene
  • Electrophilic Aromatic Substitutions (2) - Nitration and Sulfonation
  • EAS Reactions (3) - Friedel-Crafts Acylation and Friedel-Crafts Alkylation
  • Intramolecular Friedel-Crafts Reactions
  • Nucleophilic Aromatic Substitution (NAS)
  • Nucleophilic Aromatic Substitution (2) - The Benzyne Mechanism
  • Reactions on the "Benzylic" Carbon: Bromination And Oxidation
  • The Wolff-Kishner, Clemmensen, And Other Carbonyl Reductions
  • More Reactions on the Aromatic Sidechain: Reduction of Nitro Groups and the Baeyer Villiger
  • Aromatic Synthesis (1) - "Order Of Operations"
  • Synthesis of Benzene Derivatives (2) - Polarity Reversal
  • Aromatic Synthesis (3) - Sulfonyl Blocking Groups
  • Birch Reduction
  • Synthesis (7): Reaction Map of Benzene and Related Aromatic Compounds
  • Aromatic Reactions and Synthesis Practice
  • Electrophilic Aromatic Substitution Practice Problems

20 Aldehydes and Ketones

  • What's The Alpha Carbon In Carbonyl Compounds?
  • Nucleophilic Addition To Carbonyls
  • Aldehydes and Ketones: 14 Reactions With The Same Mechanism
  • Sodium Borohydride (NaBH4) Reduction of Aldehydes and Ketones
  • Grignard Reagents For Addition To Aldehydes and Ketones
  • Wittig Reaction
  • Hydrates, Hemiacetals, and Acetals
  • Imines - Properties, Formation, Reactions, and Mechanisms
  • All About Enamines
  • Breaking Down Carbonyl Reaction Mechanisms: Reactions of Anionic Nucleophiles (Part 2)
  • Aldehydes Ketones Reaction Practice

21 Carboxylic Acid Derivatives

  • Nucleophilic Acyl Substitution (With Negatively Charged Nucleophiles)
  • Addition-Elimination Mechanisms With Neutral Nucleophiles (Including Acid Catalysis)
  • Basic Hydrolysis of Esters - Saponification
  • Transesterification
  • Proton Transfer
  • Fischer Esterification - Carboxylic Acid to Ester Under Acidic Conditions
  • Lithium Aluminum Hydride (LiAlH4) For Reduction of Carboxylic Acid Derivatives
  • LiAlH[Ot-Bu]3 For The Reduction of Acid Halides To Aldehydes
  • Di-isobutyl Aluminum Hydride (DIBAL) For The Partial Reduction of Esters and Nitriles
  • Amide Hydrolysis
  • Thionyl Chloride (SOCl2)
  • Diazomethane (CH2N2)
  • Carbonyl Chemistry: Learn Six Mechanisms For the Price Of One
  • Making Music With Mechanisms (PADPED)
  • Carboxylic Acid Derivatives Practice Questions

22 Enols and Enolates

  • Keto-Enol Tautomerism
  • Enolates - Formation, Stability, and Simple Reactions
  • Kinetic Versus Thermodynamic Enolates
  • Aldol Addition and Condensation Reactions
  • Reactions of Enols - Acid-Catalyzed Aldol, Halogenation, and Mannich Reactions
  • Claisen Condensation and Dieckmann Condensation
  • Decarboxylation
  • The Malonic Ester and Acetoacetic Ester Synthesis
  • The Michael Addition Reaction and Conjugate Addition
  • The Robinson Annulation
  • Haloform Reaction
  • The Hell–Volhard–Zelinsky Reaction
  • Enols and Enolates Practice Quizzes
  • The Amide Functional Group: Properties, Synthesis, and Nomenclature
  • Basicity of Amines And pKaH
  • 5 Key Basicity Trends of Amines
  • The Mesomeric Effect And Aromatic Amines
  • Nucleophilicity of Amines
  • Alkylation of Amines (Sucks!)
  • Reductive Amination
  • The Gabriel Synthesis
  • Some Reactions of Azides
  • The Hofmann Elimination
  • The Hofmann and Curtius Rearrangements
  • The Cope Elimination
  • Protecting Groups for Amines - Carbamates
  • The Strecker Synthesis of Amino Acids
  • Introduction to Peptide Synthesis
  • Reactions of Diazonium Salts: Sandmeyer and Related Reactions
  • Amine Practice Questions

24 Carbohydrates

  • D and L Notation For Sugars
  • Pyranoses and Furanoses: Ring-Chain Tautomerism In Sugars
  • What is Mutarotation?
  • Reducing Sugars
  • The Big Damn Post Of Carbohydrate-Related Chemistry Definitions
  • The Haworth Projection
  • Converting a Fischer Projection To A Haworth (And Vice Versa)
  • Reactions of Sugars: Glycosylation and Protection
  • The Ruff Degradation and Kiliani-Fischer Synthesis
  • Isoelectric Points of Amino Acids (and How To Calculate Them)
  • Carbohydrates Practice
  • Amino Acid Quizzes

25 Fun and Miscellaneous

  • A Gallery of Some Interesting Molecules From Nature
  • Screw Organic Chemistry, I'm Just Going To Write About Cats
  • On Cats, Part 1: Conformations and Configurations
  • On Cats, Part 2: Cat Line Diagrams
  • Organic Chemistry Is Shit
  • The Organic Chemistry Behind "The Pill"
  • Maybe they should call them, "Formal Wins" ?
  • Why Do Organic Chemists Use Kilocalories?
  • The Principle of Least Effort
  • Organic Chemistry GIFS - Resonance Forms
  • Reproducibility In Organic Chemistry
  • What Holds The Nucleus Together?
  • How Reactions Are Like Music
  • Organic Chemistry and the New MCAT

26 Organic Chemistry Tips and Tricks

  • Common Mistakes: Formal Charges Can Mislead
  • Partial Charges Give Clues About Electron Flow
  • Draw The Ugly Version First
  • Organic Chemistry Study Tips: Learn the Trends
  • The 8 Types of Arrows In Organic Chemistry, Explained
  • Top 10 Skills To Master Before An Organic Chemistry 2 Final
  • Common Mistakes with Carbonyls: Carboxylic Acids... Are Acids!
  • Planning Organic Synthesis With "Reaction Maps"
  • Alkene Addition Pattern #1: The "Carbocation Pathway"
  • Alkene Addition Pattern #2: The "Three-Membered Ring" Pathway
  • Alkene Addition Pattern #3: The "Concerted" Pathway
  • Number Your Carbons!
  • The 4 Major Classes of Reactions in Org 1
  • How (and why) electrons flow
  • Grossman's Rule
  • Three Exam Tips
  • A 3-Step Method For Thinking Through Synthesis Problems
  • Putting It Together
  • Putting Diels-Alder Products in Perspective
  • The Ups and Downs of Cyclohexanes
  • The Most Annoying Exceptions in Org 1 (Part 1)
  • The Most Annoying Exceptions in Org 1 (Part 2)
  • The Marriage May Be Bad, But the Divorce Still Costs Money
  • 9 Nomenclature Conventions To Know
  • Nucleophile attacks Electrophile

27 Case Studies of Successful O-Chem Students

  • Success Stories: How Corina Got The The "Hard" Professor - And Got An A+ Anyway
  • How Helena Aced Organic Chemistry
  • From a "Drop" To B+ in Org 2 – How A Hard Working Student Turned It Around
  • How Serge Aced Organic Chemistry
  • Success Stories: How Zach Aced Organic Chemistry 1
  • Success Stories: How Kari Went From C– to B+
  • How Esther Bounced Back From a "C" To Get A's In Organic Chemistry 1 And 2
  • How Tyrell Got The Highest Grade In Her Organic Chemistry Course
  • This Is Why Students Use Flashcards
  • Success Stories: How Stu Aced Organic Chemistry
  • How John Pulled Up His Organic Chemistry Exam Grades
  • Success Stories: How Nathan Aced Organic Chemistry (Without It Taking Over His Life)
  • How Chris Aced Org 1 and Org 2
  • Interview: How Jay Got an A+ In Organic Chemistry
  • How to Do Well in Organic Chemistry: One Student's Advice
  • "America's Top TA" Shares His Secrets For Teaching O-Chem
  • "Organic Chemistry Is Like..." - A Few Metaphors
  • How To Do Well In Organic Chemistry: Advice From A Tutor
  • Guest post: "I went from being afraid of tests to actually looking forward to them".

Comment section

22 thoughts on “ introduction to assigning (r) and (s): the cahn-ingold-prelog rules ”.

In a chiral molecule, two groups are attached to it with the normal line bond ,the third is shown through a wedge and hydrogen is not shown..can I conclude that the hydrogen is a dash ?

Yes! The dashed hydrogen is implied!

Thanks. Move the dots. Could not find this before.

Glad you found it useful James!

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During my studies for 11th grade and 12th grade, we had a brilliant Organic Chemistry teacher who taught the concepts beautifully. In addition, I had a passion (more of a “study crush”) on Chemistry in general and Organic Chemistry in particular. To such an extent that this topic of R and S enantiomers is still ingrained in memory. Though I am in a completely different area now of Machine Learning and Analytics in the Healthcare space in Industry, primarily a Software Engg job. Out of sheer curiosity, I googled “Chirality Detection Machine Learning” and voila !! such cool, intereesting papers I came across where they combine Bayesian Learning and Convolutional Neural Networks (Advanced ML Theory) to detect chirality in Nanoparticles. So application of ML in cutting edge Physics. Amazing stuff :!

Most people don’t learn chirality until 2nd year university in north america, so you are ahead of the curve

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I just only want to know the CIP system of Nomenclature

Man this website proved to be a boon for me in quarantine…keep it up🔥🔥 The best content of organic chem I could get in such an incredible way

Thank you so much!! :) This was a great refresher on chirality and you explained it in such a straightforward manner. Appreciate it!

What to do if the compound is not denoted using the dash and wedge but simple bond line notation or expanded notation ?

Can you show an example? There has to be some kind of indicator. If all four bonds from the chiral center are shown as simple line notation there is no way to tell if it is R or S. It’s ambiguous.

Thank you so much, you are a true life saver???

I have a lot of trouble rotating molecules in my head, so these tips feel like magic to me!!! Thank you soooo much :DDDD Btw I also go to McGill!

The molecule used to explain the dot technique is labelled as 3-ethyl-3-methyloctane, however shouldn’t the molecule be named as 4-ethyl-4-methyloctane? The branches are on the fourth carbon…

Shoot. You are right. Thanks for the catch. Fixed!

Thank You so much :)

Thanks!! You saved my org chem exam

I was having trouble with this when 4 was in the plane of the page. This technique is so easy. Thanks

Kindly take my work into consideration in your website.

Abstract:- “The Keval’s Method” is developed for the determination of absolute configuration of a chiral carbon in a Fisher Projection and Wedge-Dash Projection just by simple calculations. This method is easily applicable over both Fisher as well as Wedge-Dash Projection. Various methods for determining absolute configuration have been developed and published till now, some of them used fingers and hands and other used exchanging elements. “Keval’s Method” is the first method in which a chiral carbon is taken to be an origin and the branches to axes, also it is purely calculation based method where absolute configuration is found based on the nature of calculated answer without using fingers and hands and also without exchanging elements.

Your’ Thankfully Keval Chetanbhai Purohit 5th-Computer Engineering, Vishwakarma Government Engineering College, Mo- 7226953531

Thank you very much, I now understand the R/S, its not easy to rotate a compound in your mind……

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Find R and S configuration of the following compounds.

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Chemistry Steps

Determining R and S Configuration on a Fischer Projection

Organic Chemistry

Stereochemistry.

To determine the R and S configuration of the chiral carbon atoms in a Fischer projection, we need first recall the concept of the Fischer projection. And that is; the horizontal groups are pointing towards the viewer (wedge) , and the groups on the vertical axis are pointing away from the viewer (dash) even though all the bonds are shown in plain lines.

assign r and s configuration to the following compound 1 cooh

The rules for determining the absolute configurations are all the same that we learned in an earlier article: How to Determine the R and S configuration

For example, lets determine the configuration of the chiral carbon in the following Fischer projection:

assign r and s configuration to the following compound 1 cooh

Step 1. Draw the horizontal bonds as wedge lines:

assign r and s configuration to the following compound 1 cooh

Step 2. Assign the priorities of the four groups:

assign r and s configuration to the following compound 1 cooh

Notice that the aldehyde group has a higher priority than the alcohol because the C=O double is counted as if the carbon is connected to two oxygen atoms.

Step 3. Determine the direction of the arrow; if the lowest priority is pointing away from you (vertical position), then the configuration is as it should be: clockwise (CW)- R , counterclockwise (CCW)- S:

assign r and s configuration to the following compound 1 cooh

In this case, the CH 2 OH group is the lowest priority and pointing away from us therefore, the configuration is based on the direction of the arrow.

Let’s now consider an example where the lowest priority is on a horizontal position (wedge line):

assign r and s configuration to the following compound 1 cooh

Keep in mind that the lowest priority is pointing towards as.

Step 2. Assign the priorities:

assign r and s configuration to the following compound 1 cooh

Step 3. Determine the direction not the arrow and change the result ( R to S or S to R ) because the lowest priority is pointing towards as:

assign r and s configuration to the following compound 1 cooh

Fischer Projections with More Than One Chiral Center

When there is more than one chiral center in the Fischer projection, it gets a little more complicated as they need to be assigned separately.

For example, determine the absolute configuration of each chiral carbon in the following Fischer projection:

assign r and s configuration to the following compound 1 cooh

Let’s start with the top carbon treating the second one simply as a priority group for the R and S designation. The methyl group is the lowest priority and therefore, the counterclockwise indicating an S configuration must be switched to R :

assign r and s configuration to the following compound 1 cooh

And now, we can concentrate on the second carbon atom by treating the first one as a priority group to assign the configuration:

assign r and s configuration to the following compound 1 cooh

The NH 2 group is the highest priority followed by the carbon connected to Cl and then the methyl group on the bottom.

The horizontal hydrogen indicates that the chirality should be changed from R to S .

In the end, let’s put all these steps in a little summary for determining the R and S configuration in Fischer projections:

assign r and s configuration to the following compound 1 cooh

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Practicing R and S is never too much. This 1.5-hour video is a collection of examples taken from the multiple choice quizzes determining the R and S configuration in the context of naming compounds, determining the relationship between compounds, and chemical reactions. 

  • How to Determine the R and S configuration
  • The R and S Configuration Practice Problems
  • Chirality and Enantiomers
  • Diastereomers-Introduction and Practice Problems
  • Cis and Trans Stereoisomerism in Alkenes
  • E and Z Alkene Configuration with Practice Problems
  • Enantiomers Diastereomers the Same or Constitutional Isomers with Practice Problems
  • Optical Activity
  • Enantiomeric Excess (ee): Percentage of Enantiomers from Specific Rotation with Practice Problems
  • Calculating Enantiomeric Excess from Optical Activity
  • Symmetry and Chirality. Meso Compounds
  • Fischer Projections with Practice Problems
  • R and S Configuration in the Fischer Projection
  • Converting Bond-Line, Newman Projection, and Fischer Projections
  • Resolution of Enantiomers: Separate Enantiomers by Converting to Diastereomers

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assign r and s configuration to the following compound 1 cooh

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0. (e) CH,CH(Br)CH(Br)COOH ) Assign R or S configuration to each of the following compounds: OH COOH 12. HOOC-C-H Got H2N-C-H CH CHZ 13 CH, (c) H,C-C-CH CH, OH (d) CH3-C-C, Hs H 14

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Chemistry LibreTexts

3.3: Configurations

  • Last updated
  • Save as PDF
  • Page ID 408937

  • Muhammad Arif Malik
  • Hampton University, Hampton, VA

Learning Objectives

  • understand the meaning of absolute configuration, chiral centers, and draw perspective drawings and Fisher projections to represent chiral compounds.
  • Apply rules to assigning D/L or R/S stereodescriptors to the chiral compounds containing one or two chiral centers.
  • Understand optical activity as a physical characteristic of chiral compounds and the factors that affect it.
  • Recognize stereochemical relationships between organic compounds, including enantiomers, diastereomers, and meso.

What is a configuration?

A molecule's permanent geometry that results from its bonds' spatial arrangement is called configuration . Stereoisomers have different configurations for the same set of atoms and the same group of bonds. For example, cis-but-2-ene has a different configuration than its stereoisomer trans-2-butene, shown in Figure 3.1.1.

Representing a configurations

clipboard_eaab5c2c33fb173576a260d8f8844c450.png

Perspective drawing

Since the tetrahedral geometry is two V's perpendicular to each other, one V is placed in the plane of the screen (or page) and represented by solid lines. The other V is represented by a solid wedge representing the bonding coming out of the screen towards the viewer and a hashed wedge representing the bond going beyond the screen away from the viewer, as illustrated below for the case of two configurations of 2-chlorobutane. The view angle is from the direction of the top-left corner. The V in the plane of the screen appears larger and the other appears smaller. Usually, the solid wedge is near the viewer relative to the hashed wedge.

clipboard_e2cc58806730d82342d0312d67fa4dad2.png

If the configuration of two or more connected \(\ce{C's}\) needs to be shown, the V's in the plane of the screen is usually drawn pointing downwards or upwards, making a zig-zag line as illustrated below for the case of two configurations of 2-chlorobutane.

clipboard_e0973c620c3d49474b127f5e1c083ffed.png

In the case of skeletal formulas, \(\ce{C's}\) and \(\ce{H's}\) are not shown. However, the configuration is still readable correctly from the bonds shown, as illustrated below for the case of two configurations of 2-chlorobutane.

clipboard_e703443e55e773fa3039402f67df4a4e9.png

Fisher projections

Fisher projections represent the configuration as a projection of the molecule in 2D with four bonds in the shape of a cross, as explained in Figure \(\PageIndex{1}\).

clipboard_eb74e9bdfb8c4aa91a670a30c7b473cf0.png

Examples of Fisher projections of the two configurations of 2-chlorobutane are shown below, along with the perspective drawings.

clipboard_e46652aef77bf8e2167ec43d896d167d3.png

When more than one consecutive \(\ce{C's}\) needs to be shown by a Fisher projection, the \(\ce{C}\) chain, i.e., the skeleton, is shown vertically. For this purpose, \(\ce{C-C}\) single bond is rotated by 180 o resulting in a semi-cyclic conformation. Then the \(\ce{C's}\) of interest are viewed from the edge-on so that the horizontal bonds are pointing towards the viewer and drawn vertically from top to bottom, as illustrated Figure \(\PageIndex{2}\).

clipboard_e07923a2c0650a08a11306d43edae10aa.png

Chiral center

A compound with sp-, sp 2 -hybridized central atom, or a sp3-hybridized central atom with two or more same groups have a superimposable mirror image, as illustrated in Figure \(\PageIndex{3}\). Since the compound and its mirror image are identical in these cases, these compounds are achiral, i.e., not chiral.

clipboard_e91b0aaa83c2a8d54506ab2358aeccc04.png

A compound with four different groups attached to an sp 3 -hybridized central atom has two distinct configurations, which are related as non-superimposable mirror images of each other, i.e., chiral. Since the two configurations represent different compounds that are stereoisomers, making and breaking of bonds, i.e., a chemical change, is needed to inter-convert them, as illustrated in Figure \(\PageIndex{3}\).

An atom, often a \(\ce{C}\) with four different groups attached to it, is called a chiral center .

A chiral center is often a cause of the chirality of the molecule. A stereochemical descriptor, e.g., D or L and R or S, is used to distinguish the two configurations of a chiral center.

An absolute configuration is the spatial arrangement of the atoms of a chiral molecular entity specified by a stereochemical descriptor .

Stereocenter

A stereocenter is an atom, axis, or plane with at least three different groups attached to it, so interchanging any two groups creates a new stereoisomer. The chiral center is a sub-class stereocenter, as illustrated in Figure \(\PageIndex{3}\). The \(\ce{C's}\) of the \(\ce{C=C}\)-bond of cis-but-2-ene and trans-but-2-ene shown in Figure 3.1.1, have three different groups and they are sterocenters. For example, swapping \(\ce{-CH3}\) and \(\ce{-H}\) on any of the two \(\ce{C's}\) converts cis-but-2-ene to its stereoisomer trans-but-2-ene and vice versa.

Stereochemical descriptors

Two sets of stereochemical descriptors, i.e., D or L and R or S, are in common use and described next.

D/L Stereochemical descriptors

The D and L stereochemical descriptors were developed for monosaccharides, i.e., sugars. The monosaccharides have an aldehyde or a ketone group placed on the top or near the top in Fisher projections. Every other \(\ce{C}\) usually has an alcohol (\(\ce{-OH}\)) group on them, and they are chiral except the terminal \(\ce{C's}\), which are achiral.

Looking at the bottom-most chiral \(\ce{C}\) in the Fisher projection of a monosaccharide, if the \(\ce{-OH}\) group is towards the right of the Fisher projection, it is designated D and if it is towards the left, it is designated L, as illustrated in Figure \(\PageIndex{4}\).

A hyphen separates the name of the monosaccharide from the D or L stereodescrptor. The D or L defines the absolute configuration of the bottom-most chiral \(\ce{C}\) and the name that follows it defines the absolute configuration of all other chiral \(\ce{C's}\) in a monosaccharide.

Amino acids have a chiral (\ce{C}\) with an amine (\(\ce{-NH2}\)) and a carboxylic acid (\(\ce{-COOH}\)) attached to it. The \(\ce{-COOH}\) group is placed on the top and the \(\ce{-NH2}\) group is either on the left or right side of the Fisher projection.

If the \(\ce{-NH2}\) group is towards the right of the Fisher projection of an amino acid it is designated D. If it is towards the left, it is designated L, as illustrated in Figure \(\PageIndex{4}\).

  • Natural monosaccharides (sugars) have D-configurations, and natural amino acids have L-configurations with few exceptions.
  • Although the common names of monosaccharides and amino acids with a D or L descriptor are not IUPAC names, these are commonly used in biochemistry.

R/S Stereochemical descriptors

The R and S stereochemical descriptors are part of the IUPAC nomenclature. For this purpose, the four groups attached to the chiral center are assigned priority 1, 2, 3, and 4, where 1 is the highest priority and 4 is the lowest. Then R or S is assigned based on the groups' relative orientations, considering the priority order.

Sequence rules

The following is the sequence rules, also called Cahn-Ingold-Prelog (CIP) sequence rules for assigning priority.

clipboard_ebad8045f938ebd639b52111364beb9b5.png

  • If the tie does not break at the distance of two bonds, move to atoms at the distance of three bonds from the chiral center and repeat the process until the first point of difference arrives.

CIP rule for atoms with double or triple bond

Atom bonded by a double bond is treated as connected twice, and by a triple bond is treated as connected three times. For example, \(\ce{-CH=CH2}\) and \(\ce{-C#CH}\) are the same for the atom directly attached to the chiral center. For comparing atoms two bonds away, the groups are treated as explained in the illustration below, with the first point of difference highlighted in red.

clipboard_e6db1a4959cfcecb97ba5039317cc70fd.png

The rule is to stop at the first point of difference, even if the order may change later, as illustrated below with the first point of difference highlighted in red.

clipboard_e0911e731785d521f1282ba0edf2a8007.png

Assigning R or S to the chiral center

After all four groups on the chiral center have been assigned priority numbers 1, 2, 3, and 4, two situations arise:

  • Case 1: The lowest priority, i.e., #4, is pointing away from the viewer, i.e., it is on a hashed line in the perspective drawing or on a vertical line in Fisher projection. In this case, R ( rectus , Latin for right) is assigned if an arrow drawn from #1 to #2 to #3 is clockwise or directed to the right, and S ( sinister , Latin for left) is assigned if it is directed to the left or counterclockwise, as illustrated in Figure \(\PageIndex{5}\).
  • swap #4 with the group pointing away from the viewers,
  • since a swap of any two groups changes the configuration, reverse the assignments, i.e., assign R if an arrow drawn from #1 to #2 to #3 is clockwise or directed to the right and S if it is directed to the left or counterclockwise, as illustrated in Figure \(\PageIndex{5}\).

clipboard_ec44c23434572e7cc604f2790204e2740.png

The stereochemical descriptor R or S describes the absolute configuration of the chiral center. They are placed within small brackets at the beginning of the IUPAC name, separated by a hyphen, as shown in the examples below. If there is more than one chiral center in a compound, the locant number precedes each chiral center's R or S descriptor, as explained in the examples below.

clipboard_ef1846e530302063d5d7e07dccacf7250.png

Example \(\PageIndex{1}\)

clipboard_e0c8e4d8d128503009352456d1180cff5.png

  • Assign priority based on the atomic number of atoms attached to the chiral center: \(\ce{-Br}\) > \(\ce{-NH2}\) > \(\ce{-CH3}\) > \(\ce{-H}\).

clipboard_ef7e3227753aa3c8943078fda8fd5f635.png

  • Draw an arrow from #1 to #2 to #3. The arrow is counterclockwise, i.e., the stereodescriptor is S.

Example \(\PageIndex{2}\)

clipboard_ed20883f483fbed85a079a9f7b9f931cb.png

  • Tiebreaker is \(\ce{O}\) two bonds away \(\overset{\large{O, O, O}}{\small{H, H, H}}\), assigning #2 to \(\ce{-COOH}\) and #3 to \(\ce{-CH3}\).

clipboard_e6f36177b1b6e656149e156bf5b9af5f3.png

  • Draw an arrow from #1 to #2 to #3. The arrow is counterclockwise. Since two groups were swapped, reverse the assignment i.e., counterclockwise is R.

Trick for assigning R and S

clipboard_e28ef1a15479bdbba73d75f407fa3bf95.png

Example \(\PageIndex{3}\)

clipboard_e494baebbf4b2c6a366e9695e1ed3c5a3.png

  • Tiebreaker is \(\ce{C}\) two bonds away \(\overset{\large{C, H, H}}{\small{H, H, H}}\), assigning #2 to \(\ce{-CH2CH3}\) and #3 to \(\ce{-CH3}\).

clipboard_e7220f3e3f784060acc0e6ece9046f242.png

  • Draw an arrow from #1 to #2 to #3. The arrow is clockwise. Since the lowest priority group is on vertical line, reverse the assignment i.e., clockwise is S.

Example \(\PageIndex{4}\)

clipboard_e6de1ac8a4b55c484b1ee14cd205e28e5.png

  • Tiebreaker is \(\ce{S}\) two bonds away \(\overset{\large{S, C, C}}{\small{O, O, O}}\), assigning #2 to \(\ce{-C(CH3)2SH}\) and #3 to \(\ce{-COOH}\).

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  • Draw an arrow from #1 to #2 to #3. The arrow is clockwise. Since the lowest priority group is on the solid wedge, reverse the assignment i.e., clockwise is S.

Answer: S (This compound is (S)-penicillamine)

Example \(\PageIndex{5}\)

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  • Draw an arrow from #1 to #2 to #3. The arrow is clockwise. Clockwise is R.

Answer: R (This compound is (R)-penicillamine)

Compounds with one chiral center

Compounds containing one chiral center are chiral. They have a stereoisomer called enantiomer, which is their non-superimposable mirror image. Some examples of compounds containing one chiral center, their enantiomers with their absolute configuration assigned by D/L system and R/S system, and their tetetrahederal geometry illustrated are shown in Figure \(\PageIndex{6}\).

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Enantiomers have the same physical and chemical properties, except when they react with or interact with other chiral objects or molecules.

For example, hands and gloves are chiral objects. The right-hand glove fits on the right hand, and the left-hand fits on the left. Similarly, enantiomer molecules react equally and when they are produced in a chemical reaction they are produced equally, except when one of the reagents is chiral. For example, enzymes are chiral reagents and usually react with one of the two enantiomers in the reactants and/or produce one of the two enantiomers.

Optical activity

The light has an electric field oscillating perpendicular to the direction of propagation of the light. Ordinary light has an electric field oscillating in all places perpendicular to the direction of propagation. When it passes through a polarizer, it transmits only the waves oscillating in one plane, i.e., plane polarized light, as illustrated in Figure \(\PageIndex{7}\). The plane polarized light is composed of left-handed and right-handed circularly polarized light, which are non-superimposable mirror images of each other, i.e., chiral, as shown in Figure \(\PageIndex{7}\).

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When the plane-polarized light passes through a solution of chiral molecules, one enantiomer absorbs more right-handed and the other more left-handed component. Therefore, if the solution is of a pure enantiomer, the plane of the plane-polarized light rotates clockwise or counterclockwise. The enantiomer that rotates the plane polarized clockwise is called dextrorotatory (abbreviated as d or (+)), and the other that rotates it counterclockwise is called levorotatory (abbreviated as l or (-)).

  • Chiral compounds are optically active, and achiral compounds are optically inactive .
  • An instrument used to measure the ability of a compound to rotate the plane of a polarized light is called a polarimeter , illustrated in Figure \(\PageIndex{8}\).

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The rotation of the plane of plane-polarized light by an enantiomer is called observed rotation , measured in degrees. The observed rotation depends on the concentration of the sample, the path length of the light through the sample, the wavelength of the light used, and temperature.

Specific rotation

When the concentration is fixed to 1 g/mL (or density of the pure sample in g/mL) and path length to 1 dm, the observed rotation is specific rotation [\(\alpha\)].

Usually, the wavelength (\(\lambda\)) is 589 nm, i.e., sodium D light and temperature (T) is room temperature and shown as subscript and superscrpt: \([\alpha]^{T}_{\lambda}\), as in the following formula.

\[[\alpha]^{T}_{\lambda} = \frac{\text{Observed rotation (in degree)}}{\text{Path length (dm)}\times\text{Concentration (g/mL)}} \nonumber \]

The specific rotation is a characteristic physical property of chiral compounds. If one enantiomer is dextrorotatory, the other is leavorotatory to the same degree, as shown in the following examples.

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S and R define the absolute configuration of the chiral center, and dextrorotatory (+) or levorotatory (-) is an experimental property of the compound. A compound with S configuration may be dextrorotatory (+) or levorotatory (-), the same for R configuration. For example, (S)-lactic acid is dextrorotatory (+), and its salt (S)-sodium lactate is levorotatory (-), as shown below.

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Racemic mixture

Mixing a chiral compound with its enantiomer diminishes its optical activity. A 50:50 mixture of enantiomers is optically inactive called a racemic mixture .

Compounds with more than one chiral centers

The maximum number of stereoisomers possible for chiral compounds is given by the formula 2 n , where n is the number of chiral centers. For example, glyceraldehyde shown in Figure 3.1.1 has one chiral center and (2 1 = 2) two stereoisomers: D-glyceraldehyde and L-glyceraldehyde. Erythrose, i.e., another monosaccharide, has two chiral centers and (2 2 = 4) four stereoisomers, as shown in Figure \(\PageIndex{9}\).

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D-erythrose and L-erythrose are enantiomers, and D-threose and L-threose are also enantiomers, but erythrose is not an enantiomer of threose.

Diastereomers

Stereoisomers that are not enantiomers are diastereomers of each other. For example, D-erythrose is a diastereomer of D-threose and L-threose, as illustrated in Figure \(\PageIndex{9}\).

The formula 2 n tells the maximum possible number of stereoisomers of compounds having n number of chiral centers. The actual number may be less than the maximum. For example, tartaric acid has two chiral centers and three stereoisomers: (+) tartaric acid and (-) tartaric acid, which are enantiomers and achiral stereoisomers that is diastereomers of the first two, as illustrated in Figure \(\PageIndex{10}\). The third isomer is achiral because its mirror image is identical, simply rotated by a 180 o .

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Meso compound

A meso compound is an achiral compound with two or more chiral centers. A meso compound has a plane of symmetry that divides into two mirror-image halves, as illustrated in Figure \(\PageIndex{10}\).

Enantiomers have the same physical and chemical properties, except when interacting with chiral objects or reagents. Diastereomers have different physical and chemical properties, as can be observed in Table 1.

Chirality in life

A large number of compounds in living things are chiral. For example, amino acids and carbohydrates are chiral. Macro-molecules like proteins and nucleic acids are also chiral. Chirality is also observed on a macro-scale, e.g., our hands, foot, and ears are chiral. Similarly, horns and spots in axis deer are chiral, as shown in Figure \(\PageIndex{11}\).

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Stereospecific reactions in living things

Most chemical reactions in living things are stereospecific, i.e., one stereoisomer selectively reacts, and one is selectively produced. For example, the human digestion enzyme chymotrypsin has 268 chiral centers and 2 268 possible stereoisomers, but only one isomer is produced. This is because amino acids that constitute chymotrypsin and other enzymes have L-configuration. The enzyme-catalyzed reactions are often stereospecific because their binding sites are chiral, and only one enantiomer reactant fits in them nicely for the reaction, as illustrated in Figure \(\PageIndex{12}\). Similarly, one specific surface of enzyme-bound substrates is exposed for the reaction that results in the selective production of one enantiomer.

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Chirality manifests in taste, flavor, odor, and drug action as their receptors have chiral binding sites, as illustrated in Figure \(\PageIndex{12}\). Examples include:

  • L-aspartame tastes sweet and is used as an artificial sweetener but D-aspartame is tasteless,
  • R-(-)-carvone smells like spearmint, and S-(+)-carvone smells like caraway,
  • R-(+)-limonene smells like orange and lemon, and S-(-)-limonene smells like a spruce tree,
  • (S)-penicillamine has antiarthritic activity, and (R)-penicillamine is toxic, and
  • S-ibuprofen has anti-inflammatory action, and R-ibuprofen is inactive.

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IMAGES

  1. How to Determine the R and S configuration

    assign r and s configuration to the following compound 1 cooh

  2. Absolute Configurations Assigning R and S

    assign r and s configuration to the following compound 1 cooh

  3. How to assign R and S configuration

    assign r and s configuration to the following compound 1 cooh

  4. Solved 1. Assign R/S configuration to the following

    assign r and s configuration to the following compound 1 cooh

  5. Assign `R` and `S` configuration of the following compound.

    assign r and s configuration to the following compound 1 cooh

  6. How To Determine R and S Configurations On A Fischer Projection

    assign r and s configuration to the following compound 1 cooh

VIDEO

  1. Assign R/S configuration in the following compounds • Stereoisomerism • Organic Chemistry

  2. #R/S nomenclature Or R/S Configuration

  3. Sequence Rule!R&S Configuration #bedkdian #bsc1stsemester

  4. R S configuration

  5. #R/S nomenclature Or R/S Configuration

  6. R S Configuration in amino acids

COMMENTS

  1. 6.3: Absolute Configuration and the (R) and (S) System

    Rules for assigning an R/S designation to a chiral center. 1: Assign priorities to the four substituents, with #1 being the highest priority and #4 the lowest. Priorities are based on the atomic number. 2: Trace a circle from #1 to #2 to #3. 3: Determine the orientation of the #4 priority group.

  2. Absolute Configuration

    Put the lowest priority substituent in the back (dashed line). Proceed from 1 to 2 to 3. (it is helpful to draw or imagine an arcing arrow that goes from 1--> 2-->3) Determine if the direction from 1 to 2 to 3 clockwise or counterclockwise. i) If it is clockwise it is R. ii) if it is counterclockwise it is S.

  3. How to Determine the R and S configuration

    Step 1: Give each atom connected to the chiral center a priority based on its atomic number. The higher the atomic number, the higher the priority. So, based on this, bromine gets priority one, the oxygen gets priority two, the methyl carbon is the third and the hydrogen is the lowest priority-four: Step 2:

  4. 5.3: Absolute Configuration: R-S Sequence Rules

    The molecule posed in this question has an ( S) configuration so the remaining substituents are added in a counterclockwise fashion. Exercise 5.3.1 5.3. 1. 1) Orient the following so that the least priority (4) atom is paced behind, then assign stereochemistry ( ( R) or ( S )). 2) Draw ( R )-2-bromobutan-2-ol.

  5. Solved 1. Draw R and S configurations for the following

    Draw R and S configurations for the following compound. a) (HO)C (CN) (COOH) (CH3) four different groups in the parenthesis (6 points) b) Assign either Ror S configuration for the following compound (4 points) I CHANH HC H c) For the following compound identify how many chiral carbons present and how many isomers are possible. (6 points) OH -ОН OH

  6. Solved 1. Draw R and S configurations for the following

    Chemistry Chemistry questions and answers 1. Draw R and S configurations for the following compound. a) (HO)C* (Br) (COOH) (CH3) four different groups in the parenthesis. Draw perspective 3D and Fischer projections. (8 points) b) Assign either R or S configuration for the following compound (4 points) CI "LICH2NH2 H3C H

  7. Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules

    Assigning R and S Configurations With the Cahn-Ingold-Prelog (CIP) Rules Enantiomers are stereoisomers that are non-superimposable mirror images of each other (by the way, molecules that are superimposable mirror images of each other are considered to be identical molecules).

  8. Solved Draw (R) and (S) configurations for the following

    See Answer Question: Draw (R) and (S) configurations for the following compound. a) (HO)C* (CI) (COOH) (CH3) four different groups in the parenthesis. Draw (R) and (S) configurations for perspective drawing (3D) and (R) and (S) configurations for Fischer projections. Total 4 structures.

  9. Find R and S configuration of the following compounds.

    Complete Step By Step Answer: The R and S configuration is a nomenclature used for naming enantiomers of a chiral compound. Enantiomers are the pair of compounds that are mirror images of each other. The R and S system of naming is referred to as the CIP or Cahn Ingold Prelog system.

  10. R and S Configuration on Fischer Projections

    Stereochemistry R and S Configuration on Fischer Projections To determine the R and S configuration of the chiral carbon atoms in a Fischer projection, we need first recall the concept of the Fischer projection.

  11. Answered: Assign R or S configuration to the…

    Chemistry Assign R or S configuration to the following structures. (i) COOH (ii) CH3 C C CH3 NH2 H CH2 - CH3 О-Н H (iii) Br ... но C. Assign R or S configuration to the following structures. (i) COOH (ii) CH3 C C CH3 NH2 H CH2 - CH3 О-Н H (iii) Br ... но C. Organic Chemistry 9th Edition ISBN: 9781305080485 Author: John E. McMurry

  12. Assign `R` and `S` configuration of the following compound

    Best answer Priority order at C − 2 C - 2 : Written as First assign R/ S R / S at C − 2 C - 2. Here, the lowest ligand is in the plane (i.e., on the dotted line). Priority sequence is anticlockwise, hence the configuration at C − 2 C - 2 is S S .

  13. 5.5: Simple Organic Enantiomers- R and S configurations

    The tie-breaking rules are described on the next page; basically, you look at the next atoms attached to the ones that are the same. Exercise 5.5.1 5.5. 1. Explain the reasons for the assignment of configurations R and S in the models above. Assume red is bromine, bright green is chlorine and pale green is fluorine.

  14. Assign 'R' and 'S' configuration the following:

    Assign the position of an element having outer electronic configuration (n-2)f7 (n-1)d1ns2. The important principles that do not help in assigning electronic configuration to atoms are: The correct sequence of groups in assigning R, S configuration in the above-given figure is: The correct configuration assigned for compounds (I) and (I I ...

  15. 'R and S configuration Assign Rand compounds: configuration for the

    Step 1/2 To generate the R and S configuration for the following compounds, we need to use the configuration of the elements in the compounds. For the COOH compound, we need to use the configuration Step 2/2 of hydrogen and oxygen. For the CHO compound, we need to use the configuration of carbon and oxygen.

  16. Solved 1. Draw (R) and (S) configurations for the following

    Chemistry Chemistry questions and answers 1. Draw (R) and (S) configurations for the following compound. a) (HO)C (CH) (COOH) (CH3) four different groups in the parenthesis. Draw (R) and (S) configurations for perspective drawing (3D) and (R) and (S) configurations for Fischer projections.

  17. 3.6 Cahn-Ingold Prelog Rules

    Introduction. The method of unambiguously assigning the handedness of molecules was originated by three chemists: R.S. Cahn, C. Ingold, and V. Prelog and is also often called the Cahn-Ingold-Prelog rules. In addition to the Cahn-Ingold system, there are two ways of experimentally determining the absolute configuration of an enantiomer:

  18. 0. (e) CH,CH(Br)CH(Br)COOH ) Assign R or S configuration to ...

    0. (e) CH,CH(Br)CH(Br)COOH ) Assign R or S configuration to each of the following compounds: OH COOH 12. HOOC-C-H Got H2N-C-H CH CHZ 13 CH, (c) H,C-C-CH CH, OH (d) CH3-C-C, Hs H 14

  19. 5.7: Naming Enantiomers by the R,S System

    Introduction. The method of unambiguously assigning the handedness of molecules was originated by three chemists: R.S. Cahn, C. Ingold, and V. Prelog and is also often called the Cahn-Ingold-Prelog rules. In addition to the Cahn-Ingold system, there are two ways of experimentally determining the absolute configuration of an enantiomer: X-ray ...

  20. 3.3: Configurations

    Solution. Assign priority based on the atomic number of atoms attached to the chiral center: − Cl is #1, there is a tie between − CH 3 and − CH 2CH 3, and − H is #4. Tiebreaker is C two bonds away C, H, H H, H, H, assigning #2 to − CH 2CH 3 and #3 to − CH 3. Swap #4 with the group away from the viewer (hashed wedge).

  21. Solved 1. Draw R and S configurations for the following

    See Answer Question: 1. Draw R and S configurations for the following compound. a) (CH3H2C)C∗ (Br) (COOH) (CH3) four different groups in the parenthesis. Draw perspective 3D and Fischer projections. (8 points) b) Assign correct configuration for the following compound (R or S configuration). (4 points) Show transcribed image text

  22. Solved 1. Draw R and S configurations for the following

    Draw R and S configurations for the following compound. a) (C1)C (Br) (OH) (CH3) four different groups in the parenthesis. Draw (R) and (S) configurations for both perspective 3D drawing and Fischer projections (8 points) b) Assign either Ror S configuration for the following compound (4 points) Br I CHICH3 CI H 1 This problem has been solved!

  23. Solved 1. Draw R and S configurations for the following

    Step 1 1) To decide R/S configuration - i) number the groups/atoms attached to chiral carbon according to th... View the full answer Final answer Previous question Next question Transcribed image text: 1. Draw R and S configurations for the following compound. a) (CH3H2C)C∗(Br)(COOH)(CH3) four different groups in the parenthesis.