Q43E - Organic Chemistry (Mcmurry)

Question

When cyclohexanone is heated in the presence of a large amount of acetone cyanohydrin and a small amount of base, cyclohexanone cyanohydrin and acetone are formed. Propose a mechanism.

Step-by-Step Solution

Answer

Formation of Cyanohydrin

Step-by-Step Solution Step 1: Cyanohydrins

When hydrogen cyanide adds to the aldehydes or ketones then they give cyanohydrins. Cyanohydrins are also called hydroxyalkanenitrile.

The addition reaction is reversible and carried out in a basic medium as cyanohydrin is a weak acid so the base activates the conversion of $\text{HCN}$ to ${\text{CN}}^{\text{-}}$ .

Step 2: Mechanism of the reaction

The given reaction proceeds via two steps which are as follows;

Part (a):  Formation of 

In this step, lone pair of oxygen is delocalized and a carbon-oxygen double bond is formed and cyanide is removed from the given molecule as a leaving group.

Formation of HCN

Part (b): Formation of cyanohydrin

In this step, lone pair of oxygen abstracts the hydrogen atom of $\text{HCN}$  molecule so that the cyanide ion form will attack the carbonyl carbon to give cyanohydrin as follows;

Formation of Cyanohydrin