Ésteres em Álcoois: Reação de Grignard

Grignard reagents, organomagnesium halides, function as carbanions that attack the carbonyl carbon of esters through nucleophilic acyl substitution. Two equivalents of the Grignard reagent are required to reduce esters to tertiary alcohols, with each equivalent introducing identical alkyl groups bonded to the hydroxyl-bearing carbon of the final alcohol product.

The ketone intermediate does not accumulate because ketones are more reactive toward nucleophilic attack than esters. Since excess Grignard reagent is present, the second equivalent immediately attacks the ketone intermediate, preventing its isolation and driving the reaction forward to form the tertiary alkoxide ion.

The mechanism begins with nucleophilic attack by the carbanion at the carbonyl carbon, forming a tetrahedral intermediate. The carbonyl group then reconstructs with departure of the alkoxide ion as a leaving group, yielding a ketone. A second Grignard equivalent attacks the ketone to form an alkoxide intermediate, which is finally protonated in aqueous acid to yield the tertiary alcohol.

Regular esters are reduced to tertiary alcohols by Grignard reagents, while formate esters are reduced to secondary alcohols. This difference arises because formate esters lack a substituent on the carbonyl carbon, resulting in only one alkyl group from the Grignard reagent being incorporated into the final alcohol product.

Aqueous acid serves as the proton source in the final step of the reaction. After the second Grignard equivalent attacks the ketone to form a magnesium alkoxide salt, aqueous acid protonates the alkoxide intermediate, yielding the tertiary alcohol as the final product and regenerating the hydroxyl group.

Two identical alkyl groups derived from the Grignard reagent are incorporated into the final alcohol product. This occurs because two equivalents of the Grignard reagent are required: the first generates the ketone intermediate, and the second attacks the ketone to introduce the second alkyl group at the hydroxyl-bearing carbon.

The tetrahedral intermediate forms when the carbanion from the Grignard reagent attacks the carbonyl carbon of the ester. This four-coordinate carbon intermediate is unstable and quickly collapses by expelling the alkoxide ion as a leaving group, regenerating the carbonyl group and forming the ketone intermediate that proceeds to the next reaction step.