5: Stereochemistry

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Learning Objectives

After reading this chapter and completing the exercises and homework, a student can be able to:

recognize and classify molecules as chiral or achiral and identify planes of symmetry - refer to section 6.1
draw, interpret, and convert between perspective formulae and Fischer projections for chiral compounds - refer to section 6.2
name chiral compounds using (R) & (S) nomenclature - refer to section 6.3
recognize and classify diastereomers and meso compounds - refer to section 6.4 and 6.5 respectively
explain how physical properties differ for different types of stereoisomers - refer to section ?????
distinguish and discern the structural and chemical relationships between isomeric compounds - refer to section 6.6
define and explain the lack of optical activity of racemic mixtures - refer to section 6.7
determine the percent composition of an enantiomeric mixture from polarimetry data and the for specific rotation formula - refer to section 6.7
explain how to resolve (separate) a pair of enantiomers - refer to section 6.8
interpret the stereoisomerism of compounds with three or more chiral centers - refer to section 6.9
compare and contrast absolute configuration with relative configuration - refer to section 6.10
interpret the stereoisomerism of compounds with nitrogen, phosphorus, or sulfur as chiral centers - refer to section 6.11
recognize and explain biochemical applications of chirality - refer to section 6.12
describe Jean Baptiste Biot and Louis Pasteur's contributions to the understanding of optical isomers - refer to section 6.13

5.1: Chirality
Chiral carbons are tetrahedral carbons bonded to four unique groups. At first glance, many carbons may look alike, but upon closer inspection, we can discern their differences.
5.2: Fischer Projections to communicate Chirality
Converting between perspective formula structures (wedges and dashes) and Fischer projections can be useful when evaluating stereochemistry, especially for carbohydrate chemistry.
5.3: Absolute Configuration and the (R) and (S) System
The absolute configuration of chiral centers as R or S is determined by applying the Cahn-Ingold-Prelog rules.
5.4: The E/Z System for alkenes
Some alkenes cannot be unambiguously named using the cis/trans system. The Cahn-Ingold-Prelog (CIP) rules were used to develop the E/Z system for naming the stereoisomers of alkenes.
5.5: Diastereomers - more than one chiral center
Diastereomers are stereoisomers with two or more chiral centers that are not enantiomers. Diastereomers have different physical properties (melting points, boiling points, and densities). Depending on the reaction mechanism, diastereomers can produce different stereochemical products.
5.6: Meso Compounds
A meso compound is an achiral compound that has two or more chiral centers. Molecular symmetry allows the mirror images to super-impose so that they are not enantiomers.
5.7: Isomerism Summary
A simple diagram is helpful in distinguishing between the different types of isomers that are possible.
5.8: Optical Activity, Racemic Mixtures, and Separation of Chiral Compounds
Optical activity is one of the few ways to distinguish between enantiomers. A racemic mixture is a 50:50 mixture of two enantiomers. Racemic mixtures were an interesting experimental discovery because two optically active samples were combined to create an optically INACTIVE sample.
5.9: Resolution (Separation) of Enantiomers
The most commonly used procedure for separating enantiomers is to convert them to a mixture of diastereomeric salts that can be separated based on their differences in their physical properties. After separation, the isolated D or the L enantiomer can be recovered.
5.10: Biochemistry of Enantiomers
Biological activity and chirality are strongly correlated. The section explores a few examples.