Chapter 5, Problem 36P

Interpretation Introduction

Interpretation:

It is to be explained why the reaction of one of the trimethylcyclohexanols shown with hydrogen chloride gives a single product while the other gives a mixture of two stereoisomers.

Concept introduction:

Secondary and tertiary alcohols react with hydrogen halide by an ${\text{S}}_{\text{N}}\text{1}$ mechanism that involves formation of a carbocation intermediate.

Tertiary carbocations are stable and allow both the faces to become equivalent with respect to their surroundings. However, sterically less hindered face is favorable.

In case of alcohols, the first step in an ${\text{S}}_{\text{N}}\text{1}$ mechanism is proton transfer. A proton transfer to an alcohol gives an alkyloxonium ion.

The alkyloxonium ion dissociates to a water molecule in the second step, and a carbocation is formed.

The third step is the reaction of a carbocation with the nucleophile. The nucleophile uses an unshared electron pair that can be used to form a covalent bond between the carbon atom of a carbocation and the halide ion.

For sterically hindered cyclohexanols, the sterically less hindered sites are favorable to form a product.