Animated Solution for Chemistry - Organic Chemistry: The major product of the reaction is :

A Deceptive Diazotization

Imagine you are looking at a molecule of $(S)$-Leucine, a standard $\alpha$-amino acid. The stereochemistry is clearly defined: the amino group ($-{\text{NH}}_2$) is sitting on a dashed bond, pointing away from you into the plane of the screen.

We treat this molecule with a classic reagent mixture: ${\text{NaNO}}_2$ and aqueous $\text{HCl}$ at $0^{\circ }\text{C}$. Any seasoned organic chemist knows this is the recipe for diazotization. The nitrous acid generated in situ reacts with the primary aliphatic amine, converting it into a diazonium salt ($-{\text{N}}_2^{+}$). At this exact moment, the stereochemistry is perfectly preserved; the newly formed diazonium group is still pointing away on that dashed bond.

Carbocation vs. NGP

Here is where the trap is set. Aliphatic diazonium salts are notoriously unstable because nitrogen gas (${\text{N}}_2$) is one of the best leaving groups in the universe. The intuitive thought is that the nitrogen simply leaves, generating a planar carbocation at the $\alpha$-carbon. If that happened, water would attack from both sides, giving a racemic mixture.

But nature is more elegant than that. Right next door to our leaving group is a carboxylic acid, which in this medium exists in equilibrium with its nucleophilic carboxylate ion ($-{\text{COO}}^{-}$). This neighboring group is perfectly positioned to intervene before the nitrogen can just wander off.

The Alpha-Lactone

Instead of waiting for a carbocation to form, the carboxylate oxygen performs an intramolecular $S_{N}2$ attack on the $\alpha$-carbon. Because the leaving group ($-{\text{N}}_2^{+}$) is on a dashed bond, the nucleophile must attack from the opposite face—the front side.

This backside attack kicks out the nitrogen gas and forms a highly strained, three-membered ring known as an $\alpha$-lactone. Because the attack came from the front, the new carbon-oxygen bond is on a solid wedge. We have just witnessed our first inversion of configuration.

Water's Revenge

An $\alpha$-lactone is incredibly strained and highly reactive. It cannot survive long in an aqueous environment. Water molecules from the solvent swarm the intermediate, acting as nucleophiles.

Water attacks the $\alpha$-carbon to pop the strained ring open. Once again, the rules of $S_{N}2$ dictate a backside attack. The leaving group in this step is the lactone oxygen, which is currently sitting on a solid wedge. Therefore, the water molecule is forced to attack from the back—the dashed side.

Net Retention

The ring opens up, reforming the carboxylic acid group, and leaving a new hydroxyl group ($-\text{OH}$) attached to the $\alpha$-carbon. Because the water attacked from the dashed side, our final $-\text{OH}$ group is on a dashed bond.

Let's tally the score: one inversion to form the lactone, and a second inversion to open it. Mathematically and chemically, two inversions result in a net retention of configuration. The final $-\text{OH}$ group ends up exactly where the original $-{\text{NH}}_2$ group started. This beautiful mechanism, known as Neighboring Group Participation (NGP), leads us directly to Option (C).