Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 15.5 Optical evidence for mechanisms-Study Notes - New Syllabus

15.5 Using Optical Activity to Deduce Reaction Mechanisms

Changes in optical activity during reactions provide important evidence about the mechanism taking place. By analysing whether products are optically active or racemic, it is possible to distinguish between \( \mathrm{S_N1} \), \( \mathrm{S_N2} \), and addition reactions to carbonyl compounds.

1. Optical Activity and \( \mathrm{S_N1} \) Mechanism

Key Feature of \( \mathrm{S_N1} \)

• Reaction proceeds via a planar carbocation intermediate.

Mechanism Steps

1. Leaving group detaches.
2. Carbocation forms.
3. Nucleophile attacks from either side.

Consequence for Optical Activity

• Equal probability of attack from both sides.
• Produces both enantiomers.
• Product becomes a racemic mixture.
• Optical activity is lost.

Evidence for \( \mathrm{S_N1} \)

• Optically active reactant → racemic product.

2. Optical Activity and \( \mathrm{S_N2} \) Mechanism

Key Feature of \( \mathrm{S_N2} \)

• Nucleophile attacks from the opposite side of leaving group.
• One-step mechanism.

Consequence for Optical Activity

• Causes inversion of configuration.
• Called:

Walden inversion

• Product remains optically active if only one enantiomer forms.

Evidence for \( \mathrm{S_N2} \)

• Optically active reactant → optically active product with inverted configuration.

3. Addition to Carbonyl Compounds

Key Feature

• Carbonyl carbon is planar.
• Nucleophile can attack from either side equally.

Consequence

• If a new chiral centre forms, both enantiomers are produced.
• Product is usually a racemic mixture.

Example

Addition of \( \mathrm{HCN} \) to aldehydes/ketones.

Summary Table

Key Features

• Racemic products suggest attack from both sides.
• Inversion suggests backside attack (\( \mathrm{S_N2} \)).
• Planar intermediates often lead to racemisation.