# Why can't the reaction of grignard reagent with carboxylic acid give a ketone?

I understand that a Grignard reagent can act both as a base and a nucleophile. It acts as a base in the presence of a proton source (eg: alcohols, amine, water, etc). But when reacting with a ketone or an aldehyde, it acts as a nucleophile and uses a specific reaction mechanism. Why isn't that same reaction mechanism not viable for reaction with carboxylic acid?

Why is this reaction mechanism wrong? Is the formation of a diol not possible due to repulsions or is there some other reason?

• Grignard reagent is a superbase. It would rather take the acid's hydrogen rather than attack electrophilic carbon. – Huy Ngo Feb 16 '17 at 10:42
• Not sure if I would call a Grignard a superbase, but it will react with the proton in a carboxylic acid rather than reacting as a ncuelophile on the carbonyl carbon – Nuclear Chemist Jun 6 '18 at 7:52
• Also worth noting that an organolithium will deprotonate the carboxylic acid, and then attack the carboxylate ion to give a doubly-deprotonated gem-diol, which then reverts to a ketone upon work-up. This is because organolithium compounds are more nucleophilic than Grignards. This is an alternative to the answer about using a Grignard + a nitrile. – Bryan Hanson Jul 8 '18 at 23:13

$\ce{CH_3MgI + CH_3COOH -> }$ just a salt
$\ce{CH_3MgI + CH_3CN -> (CH3)2C=N-M ->[H2O] (CH3)2C=O}$.