Question

Match for the given reaction sequence: \(\frac{\mathrm{HNO}_{3}}{\mathrm{H}_{2} \mathrm{SO}_{4}}{\longrightarrow} \mathrm{K} \stackrel{\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S}}{\longrightarrow} \mathrm{L} \frac{\mathrm{NaNO}_{2} / \mathrm{HCl}}{\left(0-5^{\circ} \mathrm{C}\right)}{\longrightarrow} \mathrm{M} \frac{\mathrm{H}_{2} \mathrm{O}}{\triangle} \mathrm{N}\) Column-I (A) \(K\) (B) \(\mathrm{L}\) (C) \(\mathrm{M}\) (D) \(\mathrm{N}\) Column-II

Short answer

Match the following: - (A) \(K\): Nitro compound - (B) \(L\): Amino compound - (C) \(M\): Diazonium salt - (D) \(N\): Phenolic compound (containing the hydroxyl group)

Step-by-step solution

Identify the reaction from HNO3/H2SO4 to K

In the first step, the reaction occurs in the presence of HNO3/H2SO4, which is a nitrating mixture that introduces the nitro (-NO2) group in aromatic compounds. This is a nitration reaction in which a hydrogen atom is replaced by a nitro group in an aromatic compound. So, compound K should be a nitro compound.

Identify the reaction from K to L with (NH4)2S

In this step, compound K reacts with (NH4)2S, which is a reducing agent that can convert nitro groups into amino (-NH2) groups. This reaction is a reduction process, so compound L should be an amino compound.

Identify the reaction from L to M with NaNO2/HCl at 0-5°C

In this step, compound L (an amine) reacts with NaNO2 and HCl at 0-5°C. Under these cold conditions, the amino group (-NH2) in compound L is converted into a diazonium ion (-N2+) by forming a salt with the chloride anion. This is called diazotization reaction. Compound M should be a diazonium salt.

Identify the reaction from M to N with H2O/Δ (heat)

In this step, compound M reacts with water (H2O) at high temperature (heated). The diazonium salt (N2+Cl-) in M reacts with water typically undergoes deamination, which means removal of the diazonium group and its replacement by a hydroxyl (-OH) group. This process is called hydrolysis. So, compound N should be a phenolic compound with a hydroxyl group. Now that we understand the reaction sequence and the nature of each compound, let's match the compounds from Column-I with their properties or products in Column-II. Match the following: - (A) \(K\): Nitro compound - (B) \(L\): Amino compound - (C) \(M\): Diazonium salt - (D) \(N\): Phenolic compound (containing the hydroxyl group)

Key concepts

Nitration Reaction

The nitration reaction is a fundamental process in organic chemistry where a nitro group (-NO2) is introduced onto an aromatic ring. The most common nitrating agents are a mixture of nitric acid (HNO3) and sulfuric acid (H2SO4). A classic example is the nitration of benzene, where one of the hydrogen atoms on the benzene ring is replaced by a nitro group.

This reaction involves an electrophilic aromatic substitution mechanism, wherein the nitronium ion (NO2+), generated from the acid mixture, acts as the electrophile. The presence of the sulfuric acid helps to create a more powerful nitrating agent and to absorb water produced during the reaction, which otherwise would decrease the reaction's efficiency.

Nitration is not only pivotal for creating compounds used in explosives like TNT but also serves as a key step in synthesizing pharmaceuticals and dyes. In the context of the exercise, the nitration reaction transforms an aromatic compound to form compound K, which now contains a nitro group.

Reduction of Nitro Groups

The reduction of nitro groups to amino groups is a significant transformation in the chemistry of aromatic compounds. The amino group (-NH2) is highly versatile, allowing the molecule to further partake in a variety of chemical reactions. The reduction process can use different reducing agents; in the case of the exercise, (NH4)2S, ammonium sulfide, is used.

This reduction is typically achieved by various reducing agents such as metal/hydride combinations, catalytic hydrogenation, or sulfur-containing compounds like (NH4)2S. Each method necessitates specific experimental conditions and offers different benefits. For example, sulfur-containing reducing agents like (NH4)2S work well at reducing nitro groups selectively in the presence of other functionalities that might be sensitive to more potent reducing conditions.

Once the nitro group on compound K is reduced, it becomes compound L, which now features an amino group, setting the stage for further reactions.

Diazotization Reaction

The diazotization reaction is a key transformation in organic synthesis, particularly for creating diazonium salts from amines. This reaction typically involves treating a primary aromatic amine with nitrous acid (HNO2), which is commonly generated in situ by reacting sodium nitrite (NaNO2) with hydrochloric acid (HCl).

The reaction is temperature sensitive and is usually performed at cold temperatures (0-5°C) to ensure stability of the diazonium salt formed. During this reaction, the nitrous acid converts the amino group (-NH2) on the aromatic amine into a diazonium group (-N2+), with the release of a water molecule.

Diazonium compounds are highly reactive and can be used to form a variety of other functional groups or participate in coupling reactions to form azo compounds, which are dyes. In the exercise, this reaction converts the amino compound L into the diazonium salt M.

Hydrolysis of Diazonium Salts

Hydrolysis of diazonium salts plays a crucial role in the synthesis of various aromatic compounds. Diazonium salts are particularly interesting in organic chemistry due to their high reactivity and versatility. Hydrolysis, as the term suggests, involves the reaction of water with another compound.

When a diazonium salt undergoes hydrolysis, it typically involves heating with water, which results in the decomposition of the salt to yield a phenolic compound. Specifically, the diazonium group (-N2+) is replaced by a hydroxyl group (-OH), a reaction that takes advantage of the instability of diazonium salts at elevated temperatures.

These phenolic compounds are important as they contribute to the structure of many natural products, pharmaceuticals, and are also precursors to various polymers. In the exercise scenario, compound M, a diazonium salt, is converted through hydrolysis to compound N, which is a phenol with a hydroxyl group in place of the diazonium moiety.