Question

Propose a mechanism for each of the following reactions:

Short answer

a)

b)

Step-by-step solution

Grignard reagent

Nucleophilic addition of the Grignard reagent to the carbonyl carbon forms a tetrahedral intermediate that is unstable because it has a group that can be eliminated. The tetrahedral intermediate eliminates methoxide ions, forming a ketone. The ketone reacts with a second molecule of the Grignard reagent, forming an alkoxide ion that forms tertiary alcohol when it is protonated.

Mechanism for the given reaction

The nucleophilic attacks the carbonyl carbon, so the pi bond cleaves and forms a tetrahedral intermediate. The alkoxide ion attacks the carbonyl carbon of the ester group to form an intermediate. The leaving group, the ethoxide ion expels out by the reformation of the carbonyl group to form the desired product.

Mechanism for the formation of a

b)

Protonation of the carbonyl oxygen forms an oxonium ion. The -OH group of carboxylic acid attacks the carbonyl carbon and forms a cyclic ring followed by the proton transfer forming a good leaving group. The carbocation is formed by expelling out the leaving group. The methanol attacks the cation followed by the loss of proton forms the desired product.

Mechanism for the formation of b