Tutorial 6 — Stereochemistry

Learning Outcomes

Identify stereocenters and chiral molecules
Understand R/S configuration
Interpret and draw Fischer projections
Recognize enantiomers, diastereomers, and meso compounds
Understand optical activity

Part A: Chirality and Stereocenters

Question 1

a) Define the following terms:

Extracted definition: A point (location) in a molecule where the exchange of substituents (atoms or groups of atoms) leads to the formation of different stereoisomers.

b) What is a chiral center (stereocenter)?

Extracted definition: An atom (typically carbon) bonded to four (4) different substituents (atoms or groups of atoms), in a tetrahedral arrangement.

c) Identify the number of stereocenters in each of the following molecules:

2-bromobutane
1,2-dichlorocyclohexane
2,3-dihydroxybutanedioic acid (tartaric acid)
3-methylhexane

Question 2

a) Explain why all stereoisomers of 2-bromobutane are optically active.

b) How many stereoisomers are possible for a molecule with n chiral centers?

c) Draw all stereoisomers of 2,3-dichlorobutane and identify which are optically active.

Part B: R/S Configuration

Question 3

a) State the Cahn-Ingold-Prelog (CIP) priority rules for assigning R/S configuration.

b) Assign R/S configuration to the chiral center in each of the following:

(R)-2-butanol
(S)-2-bromobutane
(2R,3S)-2,3-dibromobutane

c) Draw the enantiomer of (R)-lactic acid.

Question 4

a) Define:

Enantiomers
Diastereomers
Meso compound

b) Classify the following pairs as enantiomers, diastereomers, or identical molecules:

(2R,3R)-2,3-dichlorobutane and (2S,3S)-2,3-dichlorobutane
(2R,3S)-2,3-dichlorobutane and (2S,3R)-2,3-dichlorobutane
(2R,3R)-tartaric acid and meso-tartaric acid

Part C: Fischer Projections

Question 5

a) Explain how to convert a 3D structure to a Fischer projection.

Extracted instructions: To accurately convert a 3D image into a Fischer projection, it's essential to understand the spatial orientation of the bonds and view the molecule from the correct angles. The first atom/group (based on IUPAC numbering) is observed at the top, while the last atom/group is observed at the bottom. Both the top and bottom atoms/groups should be positioned away from the observer, indicating that they are oriented into the plane. The remaining two atoms/groups should be positioned near to the observer, indicating that they are oriented out of the plane.

b) Convert the following wedge-dash structures to Fischer projections:

(R)-2-butanol
(2S,3R)-2,3-dihydroxybutanoic acid

c) Convert the following Fischer projections to wedge-dash structures:

    COOH
     |
H — C — OH
     |
    CH3

Question 6

a) Determine the R/S configuration of each chiral center in the following Fischer projection:

    CHO
     |
H — C — OH
     |
HO — C — H
     |
    CH2OH

b) Is this D- or L-erythrose?

c) Draw the enantiomer using Fischer projection.

Part D: Optical Activity

Question 7

a) Define specific rotation $[\alpha]$.

b) A solution containing 2.0 g of an optically active compound in 10.0 mL of solution gives an observed rotation of +6.5° in a 10 cm polarimeter tube. Calculate the specific rotation.

c) A mixture of (+)- and (-)-2-butanol shows a specific rotation of -4.2°. If the specific rotation of pure (+)-2-butanol is +13.5°, calculate the enantiomeric excess (ee) and the percentage composition of the mixture.

Key Concepts

Chirality — Property of non-superimposability on mirror image
Stereocenter — Atom where exchange of substituents gives stereoisomers
Enantiomers — Non-superimposable mirror images
Diastereomers — Stereoisomers that are not mirror images
Fischer Projection — 2D representation of 3D molecules
R/S Configuration — Absolute configuration nomenclature
Optical Activity — Rotation of plane-polarized light

FAD1018 Tutorial 6 — Stereochemistry