Animated Solution for Chemistry - Organic Compounds Containing Nitrogen: In the above chemical reaction, intermediate X and reagent / condition A are

Aniline

Imagine you are in a chemistry lab, and you are tasked with converting aniline, a primary aromatic amine, into phenol. This transformation is a classic two-step journey that relies on one of the most versatile intermediates in organic chemistry: the diazonium salt. Let's break down the reaction sequence step by step.

The Diazotization Reaction

The first step involves treating aniline with a mixture of sodium nitrite ($NaN{O}_2$) and hydrochloric acid ($HCl$). When these two reagents mix, they generate nitrous acid ($HN{O}_2$) in situ. Nitrous acid is highly reactive and immediately attacks the amine group of aniline.

This reaction is famously known as diazotization. The $-{\text{NH}}_2$ group is transformed into a diazonium group ($-{\text{N}}_2^{+}{\text{Cl}}^{-}$). Thus, our intermediate $X$ is Benzene diazonium chloride.

You might have noticed a very specific condition written over the arrow: $273-278\text{ K}$ (which is $0-5^{\circ }\text{C}$). Why such a chilling requirement?

There is a catch here. Diazonium salts are notoriously unstable. The $-{\text{N}}_2^{+}$ group is desperate to leave as stable nitrogen gas ($N_2$). If the temperature rises even slightly above $5^{\circ }\text{C}$, the diazonium salt will spontaneously decompose, reacting with any moisture present to form phenol prematurely. To control the reaction and actually isolate or use the diazonium salt, we must keep the reaction mixture ice-cold.

Hydrolysis to Phenol

Now that we have our intermediate $X$ safely prepared in an ice bath, we move to the second step. Our goal is to replace the diazonium group with a hydroxyl ($-\text{OH}$) group to form phenol.

Because the diazonium group is such a spectacular leaving group, we don't need harsh reagents. Simply warming the aqueous solution of the diazonium salt is enough. The thermal energy provides the necessary push for the nitrogen gas to bubble out, allowing a water molecule to attack the benzene ring.

Therefore, the reagent/condition $A$ required for this transformation is simply ${\text{H}}_2\text{O}/\Delta$ (water and heat).

By understanding the delicate balance of temperature and the nature of leaving groups, we can flawlessly navigate this classic organic synthesis pathway.