Animated Solution for Chemistry - Organic Chemistry: What is A in the following reaction?

Imagine you are in a laboratory, and your goal is to synthesize a pure primary amine. Your first instinct might be to take an alkyl halide and react it directly with ammonia. It seems simple enough, right? However, there is a catch here. Ammonia is a good nucleophile, but once it reacts to form a primary amine, that newly formed primary amine is also a good nucleophile—often even better than ammonia itself!

This leads to a frustrating chain reaction known as overalkylation. Your primary amine reacts with another molecule of the alkyl halide to form a secondary amine, which then forms a tertiary amine, and eventually, you end up with a messy soup containing quaternary ammonium salts. To solve this elegant problem, chemists use a brilliant workaround: the Gabriel Phthalimide Synthesis.

In this problem, we are given benzyl bromide and potassium phthalimide. Potassium phthalimide is a salt that provides the phthalimide anion. If you look closely at its structure, the nitrogen atom carries a negative charge, making it a fantastic nucleophile. However, because it is flanked by two bulky, electron-withdrawing carbonyl groups, it is strictly limited to reacting just once. It cannot undergo overalkylation.

On the other side of the ring, we have benzyl bromide. The carbon atom attached to the bromine is highly electrophilic and relatively unhindered, making it a perfect target for a nucleophilic attack.

The $S_{N}2$ Attack

The reaction kicks off with a classic $S_{N}2$ mechanism. The nitrogen atom of the phthalimide anion uses its lone pair to attack the electrophilic benzylic carbon. Simultaneously, the bromide ion—an excellent leaving group—departs.

This single-step, concerted reaction yields our stable intermediate: N-benzylphthalimide. At this stage, the benzyl group is firmly locked onto the nitrogen atom, safely protected from any further alkylation.

Setting the Amine Free

Now that we have successfully attached our alkyl group to the nitrogen, we need to remove the bulky phthalimide protecting group. This is achieved through alkaline hydrolysis.

We introduce hydroxide ions (${}^{\ominus }\text{OH}$) in an aqueous medium. These hydroxide ions act as nucleophiles, attacking the electrophilic carbonyl carbons of the phthalimide moiety. Through a series of addition-elimination steps, both carbon-nitrogen bonds are cleaved.

This process breaks down the imide ring, freeing the nitrogen atom along with its attached benzyl group. The nitrogen picks up protons from the water, yielding our final, pure major product: benzylamine, a primary amine. The remainder of the protecting group is washed away as a phthalate by-product.

The Gabriel synthesis is a masterclass in chemical control, allowing us to synthesize primary amines exclusively. By identifying the final product as benzylamine, we can confidently conclude that the correct option is (d).