Animated Solution for Chemistry - Organic Chemistry: Ammonolysis of alkyl halides followed by the treatment with NaOH solution can be used to prepare primary, secondary and tertiary amines. The purpose of NaOH in the reaction is

The ammonolysis of alkyl halides is a fundamental reaction in organic chemistry used to synthesize amines. However, it comes with a built-in challenge that must be addressed to ensure the reaction proceeds smoothly. Let's break down the process and understand the crucial role of sodium hydroxide ($\text{NaOH}$) in this synthesis.

When an alkyl halide ($R-X$) is treated with ammonia (${\text{NH}}_3$), a nucleophilic substitution reaction occurs. The lone pair of electrons on the nitrogen atom of ammonia attacks the electron-deficient carbon of the alkyl halide, displacing the halide ion.

This initial step forms a primary amine ($R-{\text{NH}}_2$) and a molecule of hydrogen halide ($HX$).

The reaction doesn't stop at the primary amine. The newly formed primary amine still possesses a lone pair of electrons on its nitrogen atom, making it a capable nucleophile. It can attack another molecule of the alkyl halide, leading to the formation of a secondary amine ($R_2\text{NH}$) and releasing another molecule of $HX$.

This successive alkylation continues, producing tertiary amines ($R_3\text{N}$) and eventually quaternary ammonium salts ($R_4{\text{N}}^{+}{X}^{-}$). In every single step of this sequence, an acid ($HX$) is generated as a byproduct.

Here lies the catch: amines are basic in nature. The acidic byproduct ($HX$) formed during the reaction will readily react with the basic amines to form ammonium salts. Once an amine is protonated, its lone pair is no longer available, and it loses its nucleophilicity. This would effectively halt the successive alkylation process.

To prevent this, a strong base like sodium hydroxide ($\text{NaOH}$) is added to the reaction mixture. The $\text{NaOH}$ serves a specific and vital purpose: it neutralizes the acidic $HX$ as soon as it is formed.

By scavenging the acidic impurities, $\text{NaOH}$ ensures that the amines remain in their free, unprotonated state. This maintains their nucleophilicity, allowing the reaction to proceed and driving the equilibrium forward to maximize the yield of the desired amines. Therefore, the purpose of $\text{NaOH}$ is to remove the acidic impurities generated during the reaction.