Stereochemistry in Chemical Reactions and Resolution

Assigning Absolute Configuration (R/S)

To communicate the exact spatial arrangement of a molecule, chemists use the Cahn-Ingold-Prelog (CIP) priority rules to assign an absolute configuration of either $R$ (rectus, right) or $S$ (sinister, left).

Using Projections

Different ways of drawing molecules require different approaches to assigning $R/S$:

1. Wedge-Dash Projections: Standard 3D representation.
2. Newman Projections: Used to view molecules along a specific carbon-carbon bond.
3. Fischer Projections: A 2D representation where vertical lines represent bonds going away from you (into the page) and horizontal lines represent bonds coming toward you (out of the page).

Crucial Rule for Fischer Projections: When assigning $R/S$ in a Fischer projection, if the lowest priority group is on a horizontal line, you must reverse your final answer (if it looks like $R$, it is actually $S$, and vice versa).

Stereochemistry in Radical Reactions

In reactions like free radical substitution (e.g., the monobromination of an alkane), the mechanism proceeds through a series of steps: initiation, propagation, and termination.

The Radical Intermediate

During propagation, a hydrogen atom is abstracted to form a free radical intermediate. This radical is typically planar or undergoes rapid inversion, making it achiral. Because the radical is flat, a reagent (like $\text{Br}_2$) can attack from either the top or bottom face with equal probability. This results in a racemic mixture—a 50:50 ratio of enantiomers.

Stability and Selectivity

The stability of the radical intermediate dictates the major product. The stability trend is: $3^\circ > 2^\circ > 1^\circ > \text{methyl}$ Bromination is generally more selective than chlorination, meaning it is more likely to target the most stable radical (the tertiary position).

Nucleophilic Substitution: $\text{S}_{\text{N}}1$ vs $\text{S}_{\text{N}}2$

The stereochemical outcome of nucleophilic substitution depends entirely on the mechanism.

$\text{S}_{\text{N}}2$ Mechanism

The $\text{S}_{\text{N}}2$ reaction is a one-step concerted mechanism. The nucleophile attacks the substrate from the side opposite to the leaving group (backside attack). This leads to a characteristic inversion of configuration. The rate is dependent on both the concentration of the substrate and the nucleophile.

$\text{S}_{\text{N}}1$ Mechanism

The $\text{S}_{\text{N}}1$ reaction is a multi-step process where the rate-limiting step is the formation of a carbocation. Because the carbocation intermediate is planar, the nucleophile can attack from either side, often leading to racemization (a mixture of enantiomers). The stability of the carbocation ($3^\circ > 2^\circ > 1^\circ$) is the primary driver of this pathway.

Resolution of Enantiomers

Because enantiomers have identical physical properties like boiling points, they cannot be separated by standard methods like distillation. To obtain a single pure enantiomer from a racemic mixture, chemists use a process called resolution.

The Process of Resolution

1. Reaction with a Chiral Agent: The racemic mixture is reacted with an enantiomerically pure chiral agent.
2. Formation of Diastereomers: This reaction converts the pair of enantiomers into a pair of diastereomers.
3. Separation: Unlike enantiomers, diastereomers have different physical properties (solubility, boiling point, etc.), allowing them to be separated by ordinary physical methods like recrystallization or chromatography.
4. Recovery: Once separated, the original compound is recovered by breaking the bond to the chiral agent.