Question

I don't understand how 2 methyl groups can be added to an ester to form a tertiary alcohol when I only have 1 equivalent of MeMgBr. Doesn't 1 equivalent mean that I only have 1 Methyl group that I can add (so only a ketone with 1 methyl group would be formed)?

Answer 1

When an ester reacts with a Grignard reagent, the ester carbon-oxygen double bond is broken and the oxygen atom becomes nucleophilic. Adding 2 methyl groups to an ester to form a tertiary alcohol requires using 2 equivalents of MeMgBr. With 1 equivalent, only 1 methyl group will be added.

Step-by-step explanation:

When an ester reacts with a Grignard reagent, such as MeMgBr, the ester carbon-oxygen double bond is broken and the oxygen atom becomes nucleophilic.

The MeMgBr then adds to the oxygen atom, resulting in an alkoxide ion. Next, a protonation step occurs, in which the alkoxide ion is protonated to form an alcohol.

In this case, since you have 1 equivalent of MeMgBr, only 1 methyl group will be added.

However, if the reaction is repeated with another equivalent of MeMgBr, another methyl group can be added to the ester to form a tertiary alcohol.