Chapter 10: Q18P (page 519)

A formate ester, such as ethyl formate, reacts with an excess of a Grignard reagent to give (after protonation) secondary alcohols with two identical alkyl groups.
a. Propose a mechanism to show how the reaction of ethyl formate with an excess of allylmagnesium bromide gives, after protonation, hepta-1,6-dien-4-ol.

b. Show how you would use reactions of Grignard reagents with ethyl formate to synthesize the following secondary alcohols.
i. pentan-3-ol
ii. diphenylmethanol
iii. trans, trans-nona-2,7-dien-5-ol

Short Answer

Expert verified

a. The mechanism of the given reaction of ethyl formate with excess allylmagnesium bromide is shown as.

i. Synthesis of pentan-3-ol

ii.Synthesis of diphenylmethanol

iii. Synthesis of diphenylmethanol

Step by step solution

Grignard reagent with ethyl formate

When added to ethyl formate, Grignard reagents act as a nucleophile and attack the carbonyl group of ethyl formate. Ethoxide ion is a leaving group, and an aldehyde intermediate is formed during this reaction mechanism.

a. Mechanism for the reaction between ethyl formate and an excess of allylmagnesium bromide

This is a nucleophilic substitution reaction, where an attack at carbonyl carbon by Grignard reagent occurs, and ethoxide ion () is a leaving group. An aldehyde intermediate is formed, again attacked by Grignard reagent at carbonyl carbon. At last, after protonation, secondary alcohol is formed.

Mechanism of a reaction between ethyl formate and Grignard reagent:

Step 3:b. Reactions of Grignard reagents with ethyl formate

When an excess of Grignard reagent () is added to ethyl formate, the first equivalent of Grignard reagent is added to ethyl formate, and ethoxide ion is expelled out. Now, the second equivalent of Grignard reagent is added and followed by protonation forms secondary alcohol (pentan-3-ol) formed is shown as,

The reaction between the Grignard reagent and ethyl formate: