Chapter 5.11, Problem 15P

Interpretation Introduction

Interpretation:

The structural formulas of the two isomeric bromides formed in the reaction of $\text{(2R,4R)-4-methyl-2-hexanol}$ with hydrogen bromide are to be written, identifying the configurations of all chirality centers. The major product is to be identified.

Concept introduction:

Secondary alcohols undergo nucleophilic substitution via a modified ${\text{S}}_{\text{N}}\text{1}$ reaction mechanism.

An ${\text{sp}}^{\text{2}}$ carbocation intermediate is formed in the first step of the ${\text{S}}_{\text{N}}\text{1}$ reaction mechanism.

The nucleophile can then attack the planar carbocation from below the plane as well as from above the plane, leading to a racemic mixture of products.

The product with inverted symmetry is generally favored because the proximity of the water molecule and the carbocation formed in the first step hampers the attack of nucleophile from that face.

For an optically active alcohol with more than one chirality centers, only the absolute configuration at the carbon atom bearing the hydroxyl group changes while the absolute configuration at the other chiral center remains the same.