Answer :
The reaction of a Grignard reagent with an ester involves nucleophilic acyl substitution followed by nucleophilic addition, resulting in a tertiary alcohol. The initial attack replaces the OR' group with an R group from the Grignard to form a ketone, which upon further reaction with Grignard, leads to the alcohol.
Reaction Mechanism of an Ester with a Grignard Reagent
The reaction of a Grignard reagent with a ketone or ester is a key transformation used to synthesize tertiary alcohols. The mechanism involves two main steps. In the nucleophilic acyl substitution, the Grignard reagent acts as a nucleophile and attacks the electrophilic carbon atom of the ester's carbonyl group. The OR' group leaves, and an R group from the Grignard reagent takes its place, leading to the formation of a ketone intermediate. This intermediate does not accumulate because ketones are more reactive than esters and can quickly undergo a second reaction.
In the second step of the mechanism, another molecule of the Grignard reagent adds to the newly-formed ketone in a process known as nucleophilic addition. This results in an alkoxide intermediate, which, upon acid work-up, is protonated to yield the desired tertiary alcohol.
To summarize, the first step is the nucleophilic attack by the Grignard reagent on the carbonyl carbon of the ester. The OR' group of the ester is replaced with the alkyl group from the Grignard, forming a ketone. A second nucleophilic attack by another Grignard molecule results in the tertiary alcohol, where two R groups attached to the central carbon are from the Grignard reagent, and the third is from the original ester.
