Answer :
In the reaction between Grignard reagents and carboxylic acids, two equivalents of the Grignard reagent are necessary. Let's understand why:
Proton Transfer: The first equivalent of the Grignard reagent is used to deprotonate the carboxylic acid. Grignard reagents are strong bases, which means they will preferentially engage in acid-base reactions before nucleophilic additions when an acidic proton is available. Carboxylic acids have an acidic hydrogen atom in the hydroxyl group, and the Grignard reagent will abstract this proton, forming a carboxylate ion.
Nucleophilic Attack: After the carboxylic acid has been deprotonated, the carboxylate ion is not susceptible to nucleophilic attack. Thus, after deprotonation, a second equivalent of the Grignard reagent is needed. However, note that the carboxylate generally does not react further with another equivalent of Grignard. Instead, if you started with an ester, this second equivalent allows the Grignard reagent to perform a nucleophilic attack on the carbonyl carbon of an ester or ketone derived from further reactions, leading to the formation of a tertiary alcohol after hydrolysis in the subsequent step.
Explanation of Options:
- A (Correct Answer): One equivalent of the Grignard reagent will deprotonate the carboxylic acid first. This statement accurately describes the initial reaction step where Grignard reagent acts as a base.
- B: One equivalent of the Grignard reagent is needed for the nucleophilic attack at carbonyl. While true for many reactions with carbonyl compounds, in this specific context (direct reaction with a carboxylic acid), it's more about deprotonation first.
- C: The proton transfer reaction is faster than the nucleophilic attack. This is correct but more so an outcome of the chemistry principles rather than the focus on why two equivalents are needed in this specific context.
In summary, option A is the correct choice, as it most directly explains why a Grignard reagent and a carboxylic acid require one equivalent to deprotonate.
