Organic Chemistry Topic 5

Topic: Stereochemistry

Introduction to Stereochemistry

Stereochemistry is the branch of chemistry concerned with the three-dimensional arrangement of atoms in molecules and its effect on chemical properties and reactions. Understanding stereochemistry is essential for predicting the behavior of organic compounds, especially in biological systems.

• Stereoisomers: Compounds with the same molecular formula and connectivity but different spatial arrangements of atoms.

• Chirality: A property of a molecule that is not superimposable on its mirror image, often due to the presence of an asymmetric carbon atom.

Chirality and Stereoisomers

Chirality is a key concept in stereochemistry, referring to molecules that exist in two non-superimposable mirror image forms called enantiomers.

• Enantiomers: Stereoisomers that are mirror images of each other but not superimposable.

• Diastereomers: Stereoisomers that are not mirror images of each other.

• Assigning Configuration: The Cahn-Ingold-Prelog (CIP) priority rules are used to assign absolute configuration (R or S) to chiral centers.

Example: 2-butanol has a chiral center at the second carbon, leading to two enantiomers.

Optical Activity and Polarimetry

Chiral molecules can rotate plane-polarized light, a property known as optical activity. The direction and degree of rotation are measured using a polarimeter.

• Specific Rotation: The standardized measure of a compound's ability to rotate plane-polarized light, given by:

$[\alpha] = \frac{\alpha_{obs}}{l \cdot c}$

• where $[\alpha]$ is the specific rotation, $\alpha_{obs}$ is the observed rotation, $l$ is the path length in decimeters, and $c$ is the concentration in g/mL.

Stereoisomeric Relationships

Stereoisomers can be classified based on their relationship to each other.

• Enantiomers: Non-superimposable mirror images.

• Diastereomers: Stereoisomers that are not mirror images.

• Meso Compounds: Achiral compounds with multiple chiral centers due to an internal plane of symmetry.

Example: Tartaric acid has two chiral centers and exists as a pair of enantiomers and a meso form.

Assigning R and S Configuration

The Cahn-Ingold-Prelog rules are used to assign priorities to substituents around a chiral center and determine its configuration as R (rectus) or S (sinister).

• Assign priorities based on atomic number.

• Orient the molecule so the lowest priority group is away from you.

• If the sequence 1-2-3 is clockwise, the configuration is R; if counterclockwise, it is S.

Determining Number of Stereoisomers

The number of possible stereoisomers for a molecule with n chiral centers is generally $2^n$, unless meso forms are present.

• Formula: $\text{Number of stereoisomers} = 2^n$

• Additional info: Meso compounds reduce the total number of stereoisomers.

Fischer Projections

Fischer projections are a two-dimensional representation of three-dimensional molecules, commonly used for carbohydrates and amino acids.

• Horizontal lines represent bonds coming out of the plane.

• Vertical lines represent bonds going behind the plane.

Resolution of Enantiomers

Enantiomers can be separated (resolved) using physical or chemical methods.

• Resolving Agents: Chiral substances used to convert enantiomers into diastereomers, which can be separated.

• Chiral Column Chromatography: A technique that separates enantiomers based on their interaction with a chiral stationary phase.

Chirality vs. Achirality and Symmetry

Chiral molecules lack an internal plane of symmetry, while achiral molecules possess symmetry and are superimposable on their mirror images.

• Chiral center: Typically a carbon atom bonded to four different groups.

• Achiral molecules: May have chiral centers but possess a plane of symmetry (e.g., meso compounds).

Student Competencies

Students should be able to:

• Communicate the theory of stereochemistry and chirality.

• Identify chiral centers in complex molecules.

• Assign R/S configuration using Cahn-Ingold-Prelog rules.

• Draw and identify enantiomers and diastereomers.

• Calculate the number of possible stereoisomers.

• Identify molecules that break Bredt's rule.

• Distinguish between enantiomers, diastereomers, and meso compounds.

• Use Fischer projections to assess stereochemical relationships.

• Understand principles of enantiomeric resolution.

Summary Table: Types of Stereoisomers

Type	Definition	Relationship	Example
Enantiomers	Non-superimposable mirror images	Mirror images	2-butanol (R and S forms)
Diastereomers	Stereoisomers not related as mirror images	Not mirror images	cis- and trans-2-butene
Meso Compounds	Achiral despite having chiral centers	Internal plane of symmetry	meso-tartaric acid