Question

A compound 'A' when treated with \(\mathrm{HNO}_{3}\) (in the presence of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) ) gives compound 'B', which is then reduced with \(\mathrm{Sn}\) and \(\mathrm{HCl}\) to aniline? The compound 'A' is (a) acetamide (b) ethane (c) benzene (d) toluene

Short answer

The compound 'A' is benzene.

Step-by-step solution

Identify Reagents and Reaction Types

The problem involves two main chemical reactions. The first reaction is the nitration of compound 'A' using \(\mathrm{HNO}_{3}\) and \(\mathrm{H}_{2}\mathrm{SO}_{4}\). This typically indicates electrophilic aromatic substitution, which commonly occurs with aromatic compounds like benzene or toluene. The second reaction is the reduction of a nitro compound with \(\mathrm{Sn}\) and \(\mathrm{HCl}\) to form aniline, suggesting an amine formation from a nitro group.

Analyze Final Product Formation

The formation of aniline, \(\mathrm{C_6H_5NH_2}\), suggests that the initial compound 'A' must be an aromatic compound capable of forming a nitro compound that can be reduced to an amine. Aniline is derived from nitrobenzene (\(\mathrm{C_6H_5NO_2}\)), thus the precursor compound is likely benzene or an aromatic ring carrying groups that can produce a similar outcome after reactions.

Evaluate the Options

Option (a) acetamide and (b) ethane are not aromatic compounds, thus not suitable for nitration to form an aniline precursor. Option (c) benzene can be nitrated to nitrobenzene, while option (d) toluene can be nitrated to nitrotoluene, which on reduction could also lead to formation of aniline. However, the simplicity of forming aniline from benzene's nitration and subsequent reduction makes benzene the most straightforward answer.

Confirm with Chemical Knowledge

Benzene \(\mathrm{(C_6H_6)}\), when treated with \(\mathrm{HNO}_{3}\) and \(\mathrm{H}_{2}\mathrm{SO}_{4}\), undergoes nitration to form nitrobenzene \(\mathrm{(C_6H_5NO_2)}\). Subsequently, nitrobenzene is reduced with \(\mathrm{Sn}\) and \(\mathrm{HCl}\) to aniline \(\mathrm{(C_6H_5NH_2)}\). This aligns perfectly with the problem’s final product, confirming benzene as compound 'A'.

Key concepts

Nitration Reaction

Nitration is an essential chemical reaction mainly using a mixture of concentrated nitric acid \( \mathrm{HNO}_3 \) and sulfuric acid \( \mathrm{H}_2\mathrm{SO}_4 \). This reaction is used to introduce a nitro group \( \mathrm{-NO_2} \) into an aromatic ring, such as benzene. This type of reaction is known as Electrophilic Aromatic Substitution.
During the nitration process:

• The nitric acid acts as the nitronium ion \( \mathrm{NO_2^+} \) donor, which is the active electrophile.
• Sulfuric acid acts as a catalyst, helping generate the nitronium ion from nitric acid by protonation and dehydration.
• The aromatic ring, with its delocalized electrons, is a nucleophile that attacks the nitronium ion, forming a sigma complex.
• This intermediate loses a proton to restore the aromaticity of the ring, resulting in the formation of a nitrobenzene compound \( \mathrm{C_6H_5NO_2} \).

This nitration reaction is crucial because the nitro group is a key functional group for further chemical transformations, such as in the production of aniline.

Aniline Formation

Aniline is a widely used aromatic amine, which can be synthesized in a multi-step chemical process starting from benzene. The significance of aniline formation in chemistry stems from its role as a precursor for dyes, drugs, and polymers.
To form aniline, these steps are followed:

• Begin with the nitration of benzene to introduce a nitro group, forming nitrobenzene \( \mathrm{C_6H_5NO_2} \).
• The nitro group acts as a substrate for reduction reactions. Specifically, nitrobenzene undergoes a reduction process to convert the nitro group \( \mathrm{-NO_2} \) to an amine group \( \mathrm{-NH_2} \).
• Reduction can be achieved using metal and acid combinations, where materials such as tin \( \mathrm{Sn} \) and hydrochloric acid \( \mathrm{HCl} \) are commonly used.
• This chemical reduction results in the formation of aniline \( \mathrm{C_6H_5NH_2} \).

Aniline's formation through such transformations exemplifies the versatility of functional group interconversions in aromatic chemistry.

Reduction of Nitro Compounds

Reduction reactions play a crucial role in converting nitro compounds into amines. This process is pivotal in the chemical industry, enabling the synthesis of various chemicals, including pharmaceuticals and dyes.
When reducing nitro compounds:

• Metals such as tin \( \mathrm{Sn} \), with an acid like hydrochloric acid \( \mathrm{HCl} \), serve as reducing agents.
• These agents donate electrons to the nitro group \( \mathrm{-NO_2} \), leading to its conversion into an amino group \( \mathrm{-NH_2} \).
• This process typically involves multiple steps, where the nitro group is incrementally reduced via intermediates such as nitroso and hydroxylamine, finally forming the desired amine.

Reduction of nitrobenzene to aniline \( \mathrm{C_6H_5NH_2} \) showcases how functional groups on aromatic rings can be transformed, highlighting the chemical pathway from an environmentally persistent group to a more reactive and versatile functional group.