Q27E - Organic Chemistry (Mcmurry)

Question

Naturally occurring compounds called cyanogenic glycosides, such as lotaustralin, release hydrogen cyanide, HCN, when treated with aqueous acid. The reaction occurs by hydrolysis of the acetal linkage to form a cyanohydrin, which then expels HCN and gives a carbonyl compound

a.  Show the mechanism of the acetal hydrolysis and the structure of the cyanohydrins that results

b.  Propose a mechanism for the loss of HCN, and show the structure of the carbonyl compound that forms

Step-by-Step Solution

Answer

a.  The mechanism of the acetal hydrolysis when lotaustralin is treated with aqueous acid is

 The structure of the cyanohydrin formed is

b. The mechanism of the loss of HCN from the acetal and the structure of the carbonyl compound formed, when lotaustralin is treated with aqueous acid is

Step 1: Definition of structure

The arrangement of chemical bonds between atoms in a molecule is defined as structure

Step 2: Explanation of product formation

a.  Initially the aqueous acid protonates lotaustralin initially. The protonated lotaustralin eliminates a molecule of cyanohydrin to yield an intermediate. The nucleophilic attack of water on the intermediate and the subsequent deprotonation yields the acetal. Elimination of a molecule of HCN from the cyanohydrins yields 2-butanone.The mechanism of the acetal hydrolysis when lotaustralin is treated with aqueous acid is

The structure of the cyanohydrin formed is

b.  The mechanism of the loss of HCN from the acetal and the structure of the carbonyl compound formed, when lotaustralin is treated with aqueous acid is