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  • Finding R and S for Chiral Centers

Key Questions

It is a stereochemical label to indicate the relative spatial orientation of each atom in a molecule with a non-superimposable mirror image .

R indicates that a clockwise circular arrow that goes from higher priority to lower priority crosses over the lowest priority substituent and that lowest-priority substituent is in the back.

The R and S stereoisomers are non-superimposable mirror images , which means if you reflect them on a mirror plane, they do not become the exact same molecule when you overlay them.

When you label a molecule as R or S , you consider the priorities of each substituent on the chiral carbon (connected to four different functional groups).

Let's take this chiral amino acid for example:

http://www.nobelprize.org/

Some general ways you could determine the priorities are:

  • HIgher atomic number of the directly-attached atom gives higher priority
  • Atomic number of the atom attached to the one is considered in step 1 if two substituents have the same first atom
  • Higher number of same-atom branches determines greater priority if the overall substituents are too similar (e.g. isopropyl has higher priority than ethyl)

With ( R )-alanine:

  • #"NH"_2# has priority 1 due to highest atomic number for #"N"# .
  • #"COOH"# has priority 2 due to the higher atomic number of #"O"# vs. #"H"# in #"CH"_3#
  • #"CH"_3# has priority 3 as a result.
  • #"H"# has priority 4 .

Now, if you draw a circular arrow starting at #"NH"_2# , going to #"COOH"# , crossing over #"H"# since it is in the back, and to #"CH"_3# , then you would have gone clockwise.

https://upload.wikimedia.org/

Since the lowest priority atom is in the back, the clockwise arrow corresponds to the R configuration.

If you had started from the same R configuration but oriented #"H"# in the front and #"CH"_3# in the back , it would have been S configuration. Let's call this S configuration A, where you just nudge two substituents to flip them from front/back to back/front.

If you reflect the same R configuration over a mirror plane, keeping the orientations of #"H"# in the back and #"CH"_3# in the front after the flip, the configuration is also S . Let's call this S configuration B, where you've actually done a reflection.

If you start from S configuration B, and flipped it over a vertical axis (literally rotating #180^o# in space), you would get S configuration A.

assign r and s configuration to the following compound

  • How many stereoisomers of a heptulose are possible? How many are D and how many are L sugars? How many names will be needed for all the isomers?
  • What does R configuration mean?
  • What does S configuration mean?
  • How can I identify and draw the optical isomers for the isomers of #[Cr(H_2O)_3Cl_3 ]^+#?
  • How can I identify and draw the geometric isomers for the isomers of #[Ni(CN)_2Cl_2]^(2-)#?
  • How can I identify and draw the optical isomers for all the isomers of #[Co(NH_3 )_2Cl_2 ]^-#
  • Determine whether the cis or trans isomers in #[Cr(H_2O)_3Cl_3]^+#?
  • Is ( R )-lactic acid dextrorotatory or levorotatory?
  • Is ( R )-sodium lactate dextrorotatory or levorotatory?
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  • When is a compound optically active?
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  • How does the Cahn-Ingold-Prelog system work?
  • How can I assign relative priorities to the groups or atoms in each of the following: #-CH_2OH#, #-CH_3#, #-H# and #-CH_2 CH_2OH#?
  • How can I show the R-configuration of the molecule bromochlorofluoroiodomethane?
  • Question #27b9d
  • How do structural isomers differ from stereoisomers?
  • Question #16b44
  • Question #255f6
  • What are the rules for defining E-Z configuration?
  • How do we determine #D# and #L# terms, and how do they relate to absolute configuration?

Stereochemistry (R and S), Isomers, and Optical Activity

  • Introduction to Chirality and Chiral Centers
  • Chiral and Achiral Molecules
  • Finding R and S for Tricky Examples
  • Enantiomers
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Home / Assigning R/S To Newman Projections (And Converting Newman To Line Diagrams)

Stereochemistry and Chirality

By James Ashenhurst

  • Assigning R/S To Newman Projections (And Converting Newman To Line Diagrams)

Last updated: November 6th, 2023 |

Determining R and S Configurations of Newman Projections

How do you determine  R and  S configurations on Newman projections?

The key is to be able to quickly convert Newman projections into line diagrams , and then use the familiar CIP rules to determine R/S on the line diagrams.

So how do you convert Newman projections to line diagrams? That’s what we’re going to cover in this article.

Table of Contents

  • Determining ( R) and ( S) On Newman Projections: A Good Thing To Know For A Stereochemistry Exam
  • Most People Are Actually OK At Visualizing Familiar Things In 3-D. The Problem For Beginners In Organic Chemistry Is That Molecules Are Not Familiar
  • Cats Are Familiar. So Let’s Draw The “Newman Projection” Of A Cat
  • The Newman Projection: Eclipsed and Staggered Conformations
  • Eclipsed And Staggered Conformations Are Interconverted By Rotation Of 60° Along The Central C-C Bond
  • How To Convert A Newman Projection  To A Line Diagram
  • A Cheat Sheet For Going From A Newman To A Line Diagram: There Are Only 4 Templates To Consider
  • Determining R and S Configuration On Newman Projections: Example #1
  • Determining R and S Configuration On Newman Projections: Example #2
  • Determining R and S On A Newman With An Eclipsed Conformation: Example #3.
  • Conclusion: Determining R and S Configurations In Newman Projections

1. Determining ( R) and ( S) On Newman Projections

In two recent posts we discussed how to use the Cahn-Ingold-Prelog (CIP) rules to assign (R/S) to configurations of chiral carbons in a variety of situations, both simple and more complex .

So far, all the questions have asked you to assign (R/S) on molecules drawn as bond-line diagrams, such as the molecule shown bottom left.

This is fine. But every once in awhile – like on an exam, for instance, hint hint – you might find yourself thrown for a loop. For example, how do you determine R/S when the molecule is drawn as a Newman? (bottom right)

The trick is to convert the Newman projection to the bond-line diagram and then assign R/S.

This post explains how to do that.

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

2. Most People Are Actually Pretty Good At Visualizing Familiar Things In 3-D. The Problem For People Starting Organic Chemistry Is That Molecules Are Not Familiar

One common thing I hear from students about why organic chemistry is hard is that they say they have “a hard time visualizing things in 3D”.

I actually don’t think this is true.

I think most people are fine visualizing things in 3D.

The problem is that visualizing molecules is unfamiliar. 

Given this hypothesis, let’s take something that is familiar and do some visualization exercises.

Here’s a picture of a hungry Jerusalem street cat.

Image-of-a-jerusalem-street-cat

Could you visualize what it would look like from the side?

Almost certainly, because you are very familiar with how cats look from most angles.

If you had to make a drawing (stick figures are fine) it would probably look something like this:

Note we took some liberties. The legs facing us are drawn as wedges and the ones pointing away are dashes .

[Here, I drew the two wedges on the “inside” relative to the dashes, but drawing them on the outside (or even alternating) is OK, since it amounts to the same thing]

3. The “Newman Projection” Of A Cat

Now let’s do the same kind of exercise, but in reverse.

Let’s take that the stick figure we just drew and try to picture what it would look like from the front (i.e. look from the left) and from the back (look from the right).

For reasons that will soon become apparent, we’ll add a bit of detail: let’s give the cat some colored “socks” (orange and blue).

[You might ask: what’s that weird looking symbol?  It’s the Side-Eye of the Illuminanti, the symbol of the underground secret society of chemists that rules the world  just a symbol that says,  “imagine looking at this thing from this direction”]

Because you likely have a  very good 3-D mental model of a cat , you shouldn’t have found exercise this too hard.

Hopefully you got something like this, below. For simplicity, I omitted drawing in the eyes [2 in the front view, 1 in the back view (heh)]

The circle represents the cat’s body, since the front and back hips block each other.

Maybe you noticed this helpful correspondence:

  • When we looked at the cat from the  left (i.e. front view) the groups on wedges ( orange ) ended up on the  right  side.
  • When we looked at the cat from the  right  (i.e. back view) the groups on  wedges  ( orange ) ended up on the  left side  

4. The Newman Projection: Eclipsed and Staggered Conformations

Of course this has all just been a roundabout way of reviewing the Newman projection, as well as an exercise in  trying to help you realize that you are better at visualizing molecules in 3-D than you previously may have thought.

It helps that cats map on to molecules pretty well!

Recall that Newman projections are a convenient way of showing conformations in molecules. For example, the cat we just drew was in the “eclipsed” conformation, where the head and tail both line up with each other like the hour and minute hands on a clock striking midnight. The front and back legs line up as well.

The other significant conformation of note is the “staggered” conformation, where the front three groups are offset by 60 degrees with respect to the back three groups.

[Despite several attempts, I was unable to obtain a good photo of a Jerusalem street cat in a staggered conformation. They really don’t like being twisted. So we’ll have to work with models.]

5. Eclipsed And Staggered Conformations Are Interconverted By Rotation Of 60° Along The Central C-C Bond

In the example below, we’ll rotate the back carbon 60 degrees clockwise (CW) with respect to the front carbon, along the central carbon-carbon bond.  After this is done, note how the green hydrogens have moved from 12:00 to 2:00, 4:00 to 6:00, and 8:00 to 10:00 respectively.

When we look at this “staggered” molecule from the side, we obtain a bond-line diagram where the bonds in the plane of the page have a zig-zag configuration (bottom right).

If we look at this “staggered” bond-line diagram from the left, we obtain the “staggered” Newman, drawn top right.

6. How To Convert A Newman Projection  To A Line Diagram

So how do we convert a Newman diagram to a bond-line diagram? This section will walk through all the steps.

The first thing to recognize is that in bond-line diagrams there are only 4 possible patterns that the bonds in the plane of the page will follow.

There are two possible “zig-zag” shapes, corresponding to the “staggered” conformation, and there are also two possible “C-shapes” corresponding to the “eclipsed” conformation. [Note that line diagrams are often tilted 30° from these directions, but for simplicity we’re going to keep the central C-C bond strictly horizontal]. 

If we look from the left on each of those 4 line diagram patterns, we can see that each one generates a different Newman projection pattern.

There are 4 Newman projection patterns:

  • front down/back up,
  • front up/back down,
  • front up/back up,
  • and front down/back down.

Now that we’ve seen how the patterns work in the forward direction, let’s now apply these patterns in the  reverse direction. 

7. A Cheat Sheet For Going From A Newman To A Line Diagram: There Are Only 4 Templates To Consider

Using these templates, we can take any Newman projection and work backwards to get the corresponding bond-line template, and then draw in the dashes and wedges.

One important thing to note. As we saw with the cat, when we look from the left side of the molecule:

  • all groups on the right (R) become wedges, and
  • all groups on the left (L) become dashes

If you follow through with the pattern of looking at the molecule from the left perspective, then all you need to remember is to draw the wedges on the right side of the Newman diagram.

8.  Determining R and S Configuration On Newman Projections: Example #1

Let’s apply this to a few specific examples.

First, let’s assign R/S to a Newman drawn in a staggered conformation with a single stereocenter.

This one is drawn as (front up, back down) so we will use Template #2 from above.

In this example we drew the (front up, back down) staggered template, and then filled in the bonds. Note that the groups on the right of the Newman (Br and CH3) became attached to wedges in the line diagram.

You should obtain (R) as the configuration.

9.  Determining R and S Configuration On Newman Projections: Example #2

Next, let’s go back and do our original example (2-bromo-3,4-dimethyl pentane).

It is also drawn in the staggered conformation (front down, back up). So here we will use  Template #1. 

Using the same method, you should obtain (R) for the stereocenter containing Br and (S) for the stereocenter on carbon #3. For details on how this was done,

To see the full details hover here or click on this link .

10. Determining R and S On A Newman With An Eclipsed Conformation: Example #3.

What if the molecule is in an eclipsed conformation? Try this one.

This follows the (front down, back down) pattern, so follow Template #4

You should obtain (3R, 4R). To see details of how it was done, hover here or click on this link .

11. Conclusion: Determining R and S Configurations In Newman Projections

If you can visualize what a cat would look like from the front and from the side, then you should be able to convert a Newman projection to a line diagram. This is the first step in determining R/S on a Newman projection.

Knowing that there are only a few templates makes it easier.

Once you do it enough times, you won’t even need the templates, and you might find that it’s easier to just do it in your head.

Comments or questions? Please ask!

In the next post, we’ll look at the Fischer projection.

Thanks again to Matt for helping with this post.  Hire Matt as your tutor! 

Related Articles

  • How To Determine R and S Configurations On A Fischer Projection
  • Enantiomers vs Diastereomers vs The Same? Two Methods For Solving Problems
  • Newman Projection of Butane (and Gauche Conformation)
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  • Staggered vs Eclipsed Conformations of Ethane
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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

  • 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

  • Types of Isomers: Constitutional Isomers, Stereoisomers, Enantiomers, and Diastereomers
  • How To Draw The Enantiomer Of A Chiral Molecule
  • Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules
  • Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) - The Method of Dots
  • 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
  • On Cats, Part 4: Enantiocats
  • On Cats, Part 6: Stereocenters
  • 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

12 thoughts on “ assigning r/s to newman projections (and converting newman to line diagrams) ”.

This is the clearest and most delightful explanation of the Newman projection I have ever encountered. I’m sharing it with all my classmates!

Thanks. Glad you find it helpful Quill!

Hi, does it matter whether ‘the dashed bond is drawn between the wedges and planar bond’ ,or, ‘the wedged bond is drawn between the dashed and in-plane bond’ in the line diagram(I am referring to tetrahedral bonds around an atom)? If yes, how? Thanks!

This was amazing.Cleared all my doubts.Thanks!

Thank you very very much 😀😀

This is beautiful. Works every time. Thanks.

Hi! How would you know which template to use? For instance, when I’m given the molecular formula and I’m required to draw staggered, how would I know if i should be using front up back down or front down back up? Thank you! :)

You could use either! they are equally valid.

I like it!!! Thank you so much!!! :))

Glad you like it Hannah!

Why is cis/trans notation is not respected when rotating the newman projection to most/least stable conformation, for example?

cis and trans is only used when the configuration is locked, such as in double bonds and cycloalkanes. syn and anti can be used for conformations.

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5.3 Chirality and the R/S Naming System

Other than geometric isomers, there is another type of stereoisomer that is related to a special property called chirality. We will start with the basic concepts of chirality, then expand the topic further from there.

5.3.1 Chiral and Chirality

T o talk about chirality, let’s first take a closer look at our left hand and our right hand. The left hand can be regarded as a mirror image of the right hand, and vice versa. Now, let’s try to superimpose (overlay) the left hand on the right hand. Can you do that?

No ! No matter how hard you try, the left hand can not be superimposed on the right hand. This is because of the special property of the hand that is called chirality . Both the left and right hand are chiral (ky-ral) and show chirality . Chiral is derived from the Greek word cheir , which means “hand”, and chirality means “handedness”.

""

The definition of chirality is the property of any object (molecule) being non-superimposable on its mirror image . The left and right hand are mirror images of each other, and they are not superimposable, so both the left hand and right hand are chiral. You can also find many other objects in daily life that show chirality as well.

""

If an object is superimposable on its mirror image (in such a case, the object and its mirror image are exactly identical), then this object is not chiral, and it is referred to as achiral .

""

In organic chemistry, we are interested in organic molecules that are chiral. Let’s see the following molecular models that represent a molecule and its mirror image.

""

In the models here, the four balls with different colors represent four different substituents, and the two structures are mirror images of each other. The effort of trying to superimpose one structure on the other does not work. Therefore, according to the definition of chiral/chirality, both molecules are non-superimposable on the mirror image, so they are both chiral and show chirality.

""

The chirality of the molecule results from the structure of the central carbon. When the central carbon is sp3 carbon and bonded with four different groups (represented by four different colors in the model), the molecule is chiral. The central carbon is called the chirality center (or asymmetric center). A molecule with one chirality center must be chiral. The chirality center can also be called the asymmetric center. We will use the term chirality center in this book.

In summary, a chirality (asymmetric) center should meet two requirements:

  • sp 3 carbon;
  • bonded with four different groups. 

For following compounds, label each of the chirality center with a star.

""

  • The carbons in CH 3 or CH 2 are NEVER chirality centers. The chirality center must be the carbon bonded with a branch (or branches).
  • sp 2 double bond carbon is NEVER a chirality center.
  • Carbon in a ring can also be chirality center as long as it meet the two requirements.
  • Not all the above compounds have a chirality center.

""

Exercises 5.2

  • Draw the structure of following compounds, determine which one has an chirality center and label it with a star.

a) 1-bromobutane,

b) 1-pentanol,

c) 2-pentanol,

d) 3-pentanol,

e) 2-bromopropanoic acid

f) 2-methyl cyclohexanone

      2.  Label all the chirality centers in the following molecules.

Nicotine & cholesterol

Answers to Chapter 5 Practice Questions

5.3.2 Stereoisomer with One Chirality Center — Enantiomers

For 2-butanol, we are able to recognize that C2 is the chirality center.

""

The perspective formula shows the 3D structure of 2-butanol in two different ways, and they are non-superimposable mirror images of each other.

""

The two mirror images are different molecules. They have the same bonding but differ in the way the atoms arranged in space. So, the two molecules are stereoisomers . This specific type of stereoisomer is defined as an enantiomer . Molecules that are a pair of non-superimposable mirror images of each other are called  enantiomers .

Important Properties of Enantiomers:

  • Enantiomers are a pair of non-superimposable mirror images.
  • Enantiomers are a pair of molecules that are both chiral and show chirality ( Enantiomers must be chiral ).
  • For any chiral molecule, it must have its enantiomer, that is, the mirror image of the molecule.
  • Achiral molecules do not have enantiomers. The mirror image of an achiral molecule is an identical molecule to itself.

To draw the 3D structure of any enantiomer, we need to use perspective formula  with solid and dashed wedges to show the tetrahedral arrangements of groups around the sp 3 carbon (refer to  section 2.11 ). Out of the four bonds on tetrahedral carbon, two bonds lie within the paper plane are shown as ordinary lines, the solid wedge represent a bond that point out of the paper plane, and the dashed wedge represent a bond that point behind the paper plane. For the first enantiomer, you can draw the four groups with any arrangement, then draw the other enantiomer by drawing the mirror image  of the first one. Please note, although it seems there are different ways to show the enantiomers, there are only total two  enantiomers, we will learn in next section how to identify and designate each of them.

Several possible ways to show the structures are included in the answer here. However, your answer can be different to any of them, as long as a pair of mirror images are shown.

""

Exercises 5.3

Draw the pair of enantiomers of  2-chloro-1-propanol .

5.3.3 R/S Naming System of the Chirality Center

The two enantiomers are different compounds, though they are very similar; therefore, we need a nomenclature system to distinguish between them, to give each one a different designation so that we know which one we are talking about. That is the R/S naming system defined in IUPAC. The R/S designation can be determined by following the Cahn-Ingold-Prelog rule, the rule devised by R. S Cahn, C. Ingold and V. Prelog.

Cahn-Ingold-Prelog Rule:

  • Assign priorities of the groups (or atoms) bonded to the chirality center by following the same priority rules as for the E/Z system ( section 5.2 ).  The highest priority group is labelled as #1, and lowest priority group is labelled as #4 in this book.
  • Orient the molecule in a way that the lowest  priority group  (#4) is pointing away  from you.
  • Look at the direction in which the priority decreases for the other three groups, that is 1→ 2 → 3. 

For the co unterclockwise direction, the designation is S – ,  sinister , which means “left” in Latin.

Let’s take the following molecule as an example to practice the rule:

C in the center and Cl, OH,H, & CH3 around

Step 1: The priorities are assigned.

Cl (1), OH (2), CH3 (3), & H (4)

Step 2: Re-orient the molecule, so H (#4, lowest priority) is on the position away from us. Then, the other three groups will be arranged in this way:

Cl (1) then OH (2), then CH3 (3)

Step 3: Go along the direction from #1→#2→#3; it is in the clockwise direction, so this enantiomer is assigned an R configuration, and the complete name of the molecule is ( R )-1-chloroethanol .

Now, let’s assign the configuration of the other enantiomer:

""

Following the same steps, put H away from us, and the arrangement of the other three groups is:

""

The counterclockwise direction gives the S configuration, and the complete name of the molecule is ( S )-1-chloroethanol .

Examples:  Assign R/S configuration of the chirality center.

C center, CH3, F, H, & CH2CH2Cl around

More practical hints  about R/S assignment with Cahn-Ingold-Prelog rule:

  • Assigning priority is the first possible challenge for applying the C.I.P. rule. Review and practice the guidelines in section 5.2 .
  • The second challenge is to re-orient the molecule (to arrange the #4 group away from you). The molecule model will be very helpful for this purpose . Assemble a molecular model with four different colors connected on the carbon. Compare your model to the given structure and match the assigned priority to each color; for example, red is #1, blue is #2, etc. Then, rotate the model to arrange the lowest (#4) group away from you and see how the other groups locate to get the answer.

For the perspective formula of enantiomers, it is important to know the following properties:

  • One (odd number of) switch (interchanging) for a pair of groups inverts the configuration of the chirality center.
  • Two (even number of) switches get the original configuration back.

""

For the structures above:

  • One switch of A  leads to B , A is R and B is S , so A and B are enantiomers.
  • One switch of B  leads to C , B is S and C is R , so  B and C are enantiomers.
  • Two switches of C  lead to A , and both C and A are R , so  C and A are identical .

Exercises 5.4

Determine the R/S configuration of the chirality center in following compounds.

""

Answers to Chapter 5 Practice Questions 

Exercises 5.5

Determine the relationship for each pair of molecules: enantiomers, identical, constitutional isomers, non-isomer:

""

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Chemistry Steps

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

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

Step 1. Draw the horizontal bonds as wedge lines:

assign r and s configuration to the following compound

Step 2. Assign the priorities of the four groups:

assign r and s configuration to the following compound

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

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

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

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

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

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

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

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

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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) or (S) designations to each of the following compounds.

A) primary order of $$hc \equiv c - > -c(ch_3)_3> ch_3->h- $$ make two interchanges so that the lowest priority (h) is the dotted line..

assign r and s configuration to the following compound

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5.3: Chirality and R/S Naming System

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Other than geometric isomers, there is another type of stereoisomer that is related to a special property called chirality. We will start with the basic concepts of chirality, then expand the topic further from there.

5.3.1 Chiral and Chirality

To talk about chirality, let’s first take a closer look at our hands, left hand and right hand. The left hand can be regarded as the mirror image of the right hand, and vice versa. Now let’s try to superimpose (overlay) the left hand on right hand, can you do that?

No! No matter how hard you try, the left hand can not be superimposed on the right hand. This is because of the special property of hand, that is called chirality . Both left and right hand are chiral (ky-ral), and show chirality . Chiral derived from the Greek word cheir , that means “hand”, and chirality means “handedness”.

""

The definition of the chirality is the property of any object (molecule) of being non-superimposable on its mirror image . The left and right hand are mirror image to each other, and they are not superimposable, so both left hand and right hand are chiral. Other than that, you can also find lots other objects in daily life that show chirality as well.

""

If an object is superimposable on its mirror image (for such case the object and its mirror image are exact identical), then this object is not chiral, that can be said as achiral .

""

In organic chemistry, we are interested in organic molecules that are chiral. Let’s see the following molecular models that represent a molecule and its mirror image.

""

In the models here, the four balls with different colors represent four different substituents, and the two structures are mirror image to each other. The effort of trying to superimpose one structure to the other does not work. Therefore, according to the definition of chiral/chirality, both molecules are non-superimposable on the mirror image, so they both chiral and show chirality.

""

The chirality of the molecule results from the structure of the central carbon. When the central carbon is sp 3 carbon, and bonded with four different groups (represented by four different colors in the model), the molecule is chiral. The central carbon is called chirality center (or asymmetric center ). The molecule with one chirality center must be chiral. Chirality center can also be called asymmetric center. We will use the term chirality center in this book.

As a summary, a chirality (asymmetric) center should meet two requirements:

  • sp 3 carbon;
  • bonded with four different groups.

For following compounds, label each of the chirality center with a star.

""

  • The carbons in CH 3 or CH 2 are NEVER chirality centers. The chirality center must be the carbon bonded with a branch (or branches).
  • sp 2 double bond carbon is NEVER a chirality center.
  • Carbon in a ring can also be chirality center as long as it meet the two requirements.
  • Not all the above compounds have a chirality center.

""

Exercises 5.2

  • Draw the structure of following compounds, determine which one has an chirality center and label it with a star.

a) 1-bromobutane,

b) 1-pentanol,

c) 2-pentanol,

d) 3-pentanol,

e) 2-bromopropanoic acid

f) 2-methyl cyclohexanone

2. Label all the chirality centers in the following molecules.

Nicotine & cholesterol

Answers to Practice Questions Chapter 5

5.3.2 Stereoisomer with One Chirality Center — Enantiomers

For 2-butanol, we are able to recognize that C2 is the chirality center.

""

The perspective formula show the 3D structure of 2-butanol in two different ways, and they are non-superimposable mirror images to each other.

""

The two mirror images are different molecules. They have the same bonding, but differ in the way that the atoms arranged in space. So the two molecules are stereoisomers . This specific type of stereoisomer here is defined as enantiomers . Molecules that are a pair of non-superimposable mirror images of each other are called enantiomers .

Important Properties about Enantiomers:

  • Enantiomers are a pair of non-superimposable mirror images.
  • Enantiomers are a pair of molecules, they both chiral and show chirality. ( Enantiomer must be chiral ).
  • For any chiral molecule, it must has its enantiomer, that is the mirror image to the molecule.
  • Achiral molecule does not have enantiomer. The mirror image of an achiral molecule is the identical molecule to itself.

Examples: Draw the pair of enantiomers of 2-bromopropanoic acid.

To draw the 3D structure of any enantiomer, we need to use perspective formula with solid and dashed wedges to show the tetrahedral arrangements of groups around the sp 3 carbon (refer to section 2.11 ). Out of the four bonds on tetrahedral carbon, two bonds lie within the paper plane are shown as ordinary lines, the solid wedge represent a bond that point out of the paper plane, and the dashed wedge represent a bond that point behind the paper plane. For the first enantiomer, you can draw the four groups with any arrangement, then draw the other enantiomer by drawing the mirror image of the first one. Please note, although it seems there are different ways to show the enantiomers, there are only total two enantiomers, we will learn in next section how to identify and designate each of them.

Several possible ways to show the structures are included in the answer here. However, your answer can be different to any of them, as long as a pair of mirror images are shown.

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Exercises 5.3

Draw the pair of enantiomers of 2-chloro-1-propanol .

5.3.3 R/S Naming System of Chirality Center

The two enantiomers are different compounds, although they are very similar. Therefore we need a nomenclature system to distinguish between them, to give each one a different designation so that we know which one we are talking about. That is the R/S naming system defined in IUPAC. The R/S designation can be determined by following the Cahn-Ingold-Prelog rule, the rule devised by R. S Cahn, C. Ingold and V. Prelog.

Cahn-Ingold-Prelog Rule:

  • Assign priorities of the groups (or atoms) bonded to the chirality center by following the same priority rules as for E/Z system ( section 5.2 ). The highest priority group is labelled as #1, and lowest priority group labelled as #4 in this book.
  • Orient the molecule in the way that the lowest priority group (#4) pointing away from you.
  • Look at the direction in which the priority decrease for the other three groups, that is 1→ 2 → 3.

For counterclockwise direction, designation is S – , sinister , means “left” in Latin.

Let’s take the following molecule as an example to practice the rule:

C in the center and Cl, OH,H, & CH3 around

Step 1: The priorities are assigned.

Cl (1), OH (2), CH3 (3), & H (4)

Step 2: Re-orient the molecule, so H (#4, lowest priority) is on the position away from us. Then the other three groups will be arranged in this way:

Cl (1) then OH (2), then CH3 (3)

Step 3: Go along the direction from #1→#2→#3, it is in the clockwise direction, so this enantiomer is assigned R configuration, and the complete name of the molecule is ( R )-1-chloroethanol .

Now let’s assign the configuration of the other enantiomer:

""

Following the same steps, put H away from us, and the arrangement of the other three groups is:

""

The counterclockwise direction gives the S configuration, and the complete name of the molecule is ( S )-1-chloroethanol .

Examples: Assign R/S configuration of the chirality center.

C center, CH3, F, H, & CH2CH2Cl around

More practical hints about R/S assignment with Cahn-Ingold-Prelog rule:

  • Assigning priority is the first possible challenge for applying the C.I.P. rule. Review and practice the guidelines in section 5.2 .
  • The 2 nd challenge is to re-orient the molecule (to arrange the #4 group away from you). The molecule model will be very helpful for this purpose . Assemble a molecular model with four different colors connected on the carbon. Compare your model to the given structure and match the assigned priority to each color, for example, red is #1, blue is #2, etc. Then rotate the model to arrange the lowest (#4) group away from you and see how the other groups locate to get the answer.

For the perspective formula of enantiomers, it is important to know the following properties:

  • One (odd number of) switch (interchanging) for a pair of groups invert the configuration of the chirality centre;
  • Two (even number of) switches get the original configuration back.

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For the structures above:

  • One switch of A leads to B , A is R and B is S , so A and B are enantiomers ,
  • One switch of B leads to C , B is S and C is R , so B and C are enantiomers ,
  • Two switches of C leads to A , both C and A are R , so C and A are identical .

Exercises 5.4

Determine the R/S configuration of the chirality center in following compounds.

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Exercises 5.5

Determine the relationship for each pair of molecules: enantiomers, identical, constitutional isomers, non-isomer:

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IMAGES

  1. How to Determine the R and S configuration

    assign r and s configuration to the following compound

  2. Absolute Configurations Assigning R and S

    assign r and s configuration to the following compound

  3. How to assign R and S configuration

    assign r and s configuration to the following compound

  4. Find R and S configuration of the following compounds.\n \n \n \n \n

    assign r and s configuration to the following compound

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

    assign r and s configuration to the following compound

  6. 4.3: Absolute Configuration and the (R) and (S) System

    assign r and s configuration to the following compound

VIDEO

  1. How to assign R and S configurations in telugu

  2. R S configuration

  3. Finding R/S Configuration of a molecule

  4. How to assign R & S configuration in perspective formula / Sinhala

  5. R-S configuration || D & L || Meso compound || stereochemistry by Saugat Bhandari❤️💓

  6. Organic Spectroscopy|Structure determination of organic compounds by organic spectroscopy|NET|GATE

COMMENTS

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

    Assigning R/S configuration to glyceraldehyde: Two priorities are easy: hydrogen, with an atomic number of 1, is the lowest (#4) priority, and the hydroxyl oxygen, with atomic number 8, is priority #1. Carbon has an atomic number of 6. ... Classify the following compounds as R or S? Solution. S: I > Br > F > H. The lowest priority substituent ...

  2. 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:

  3. Absolute Configuration

    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:

  4. 7.2: Naming chiral centers: 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.

  5. Finding R and S for Chiral Centers

    R and S are used to describe the configuration of a chirality center. Chirality center meaning that there are 4 different groups attached to one carbon. To determine whether the chirality center is R or S you have to first prioritize all four groups connected to the chirality center.

  6. R and S Configuration Video Tutorial & Practice

    Name the following compounds using R,S designations: c. Do the following compounds have the R or the S configuration? a. b. What is the configuration of each of the asymmetric centers in the following compounds? e. f. Citrate synthase, one of the enzymes in the series of enzyme-catalyzed reactions known as the citric acid cycl...

  7. R & S Configuration

    Cyclic organic compounds can lead to some challenges when assigning R and S configurations for a couple of reasons. The (1R,2S) and (1S,1R) stereoisomers of 1,2 dichloromethane are the same ...

  8. R and S configuration in organic chemistry

    How to assign R and S configuration to a compound with two asymmetric centers? Here we can use sequence rules to rank the groups attached around each chiral ...

  9. R and S Configuration Practice Problems

    The following compound undergoes free radical bromination at the benzylic position. Give the mechanism for the monobromination of the given compound and draw the two stereoisomer products. Also, assign R and S configurations to the asymmetric carbon atoms in the products.

  10. R and S Configurations Of Newman Projections (+ Conversion To Line)

    1. Determining (R) and (S) On Newman ProjectionsIn two recent posts we discussed how to use the Cahn-Ingold-Prelog (CIP) rules to assign (R/S) to configurations of chiral carbons in a variety of situations, both simple and more complex.. So far, all the questions have asked you to assign (R/S) on molecules drawn as bond-line diagrams, such as the molecule shown bottom left.

  11. 5.3 Chirality and the R/S Naming System

    For a pair of enantiomers with one chirality center, one enantiomer has the R configuration and the other one has the S configuration. Cahn-Ingold-Prelog Rule: Assign priorities of the groups (or atoms) bonded to the chirality center by following the same priority rules as for the E/Z system ( section 5.2 ).

  12. 5.5: Simple Organic Enantiomers- R and S configurations

    About half of S enantiomers rotate light in the (+) direction and about half rotate light in the (-) direction. R and S configurations do not correlate directly with optical rotation values; these are two unrelated systems for describing enantiomers. Figure 5.5. 2: Ball-and-stick model of ( R )-bromochlorofluoromethane.

  13. Determining R and S configuration for a cyclic compound when the lowest

    Just determine the priorities on the nearest atoms (not super easy in this case), then dissect the cycle (possibly redraw it with numbers or letters instead of chemical groups), and swap any two substituents twice in order to get a comfortable structure orientation, if you have not 3D imagination to rotate the molecule or your point of view in y...

  14. 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.

  15. 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 .

  16. Assign R and S configurations for the following

    Science Chemistry Chemistry questions and answers Assign R and S configurations for the following compound. And draw enantiomers and diastereomers (12 points).Assign R and S configurations for the following compound. And draw enantiomers and diastereomers. Question: Assign R and S configurations for the following compound.

  17. DOC Assign R or S configuration to the following molecules:

    Which of the following is a meso compound? Assign R and S configurations to each stereogenic center. #2 is a meso compound. Chem 331 SI. Tuesday, March 10, 2009. Title: Assign R or S configuration to the following molecules: Author: Chelsea Last modified by: Chelsea Created Date: 3/6/2009 8:04:00 PM Company: Iowa State University

  18. R and S Configuration on Fischer Projections

    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: Step 1. Draw the horizontal bonds as wedge lines: Step 2.

  19. 5.3: Absolute Configuration: R-S Sequence Rules

    Stereocenters are labeled (R) or (S)The "right hand" and "left hand" nomenclature is used to name the enantiomers of a chiral compound. The stereocenters are labeled as (R) or (S).The Cahn-Ingold-Prelog rules of assign priorities the groups directly bonded to the chiral carbon. Having ranked the four groups attached to a chiral carbon, we describe the stereochemical configuration around the ...

  20. Solved 2. Assign R and S configurations for the following

    Chemistry Chemistry questions and answers 2. Assign R and S configurations for the following compound. And draw enantiomers and diastereomers ( 10 points). 3. What is a chiral center? What is an enantiomer? What instrument do you use to identify enantiomers? ( 4 points) This problem has been solved!

  21. Assign R and S configurations for the following compound And draw

    Assign R and S configurations for the following compound. And draw enantiomers and diastereomers (12 points). 2. Assign R and S configurations for the follow...

  22. 4.7: R and S Assignments in Compounds with Two or More Stereogenic

    The structural formula of 2-methylamino-1-phenylpropanol has two stereogenic carbons (#1 & #2). Each may assume an R or S configuration, so there are four stereoisomeric combinations possible. These are shown in the following illustration, together with the assignments that have been made on the basis of chemical interconversions.

  23. Assign (R) or (S) designations to each of the following compounds.

    Assign (R) or (S) designations to each of the following compounds. Question Assign (R) or (S) designations to each of the following compounds. Solution Verified by Toppr a) Primary order of H C ≡ C − > − C ( C H 3) 3 > C H 3 − > H − Make two interchanges so that the lowest priority (H) is the dotted line. Was this answer helpful? 0

  24. 5.3: Chirality and R/S Naming System

    For a pair of enantiomers with one chirality center, one enantiomer has the R configuration and the other one has the S configuration. Cahn-Ingold-Prelog Rule: Assign priorities of the groups (or atoms) bonded to the chirality center by following the same priority rules as for E/Z system (section 5.2).