Which acid could not be prepared by treating a Grignard reagent with CO2? a. p-methylbenzoic acid.
b. cyclopentane carboxylic acid.
c. 4-oxopentanoic acid.
d. 3-methylbutanoic acid.

4-oxopentanoic acid.

Explanation:

In this case, we must remember that the Grignard reaction is a reaction in which carbanions are produced. Carboanions have the ability to react with CO2 to generate a new C-C bond and a carboxylate ion. Finally, the acid medium will protonate the carboxylate to produce the carboxylic acid group.

The molecules that can follow the mechanism described above are the molecules: p-methylbenzoic acid, cyclopentane carboxylic acid and 3-methylbutanoic acid. (See figure 1)

In the case of 4-oxopentanoic acid, the possible carbanion will attack the carbonyl group to generate a cyclic structure and an alcohol group (1-methylcyclopropan-1-ol). Therefore, this molecule cannot be produced by this reaction. (See figure 2)