Question

\(\mathrm{p}-\) Nitroaniline is obtained by (a) 1,4 dinitrobenzene \(\stackrel{\mathrm{NH}_{4} \mathrm{HS}}{\longrightarrow}\) (b) Benzene sulphonic acid \(\stackrel{\mathrm{HNO}_{3} / \mathrm{H}_{2} \mathrm{SO}_{4}}{\longrightarrow}\) (c) Aniline \(\frac{1 . \text { Acetylation } 2 . \mathrm{HNO}_{3} / \mathrm{H}_{2} \mathrm{SO}_{4}}{\text { 3. aq. } \mathrm{NaOH}, \Delta}\) (d) Aniline \(\frac{1 . \mathrm{HNO}_{3} / \mathrm{H}_{2} \mathrm{SO}_{4}}{2 . \mathrm{aq} \cdot \mathrm{NaOH}}\)

Short answer

c. Aniline undergoes acetylation, nitration, and deacetylation to yield p-nitroaniline.

Step-by-step solution

Analyze Option (a)

For option (a), when 1,4 dinitrobenzene is treated with ammonium hydrogen sulfide (\( \text{NH}_4\text{HS} \)), it leads to the reduction of one nitro group to an amine group, producing \( \text{p-nitroaniline} \). This reaction is suitable for obtaining \( \text{p-nitroaniline} \).

Analyze Option (b)

In option (b), benzene sulphonic acid is subjected to nitration using \( \text{HNO}_3/\text{H}_2\text{SO}_4 \), which typically results in the formation of nitro groups on the benzene ring. However, this does not directly yield p-nitroaniline. The nitro group would attach to the benzene ring, resulting in a nitrobenzene derivative instead.

Analyze Option (c)

Option (c) describes sequential steps beginning with aniline. First, aniline is acetylated to protect the amine group, which is then nitrated to add a nitro group. The final step of deacetylation in alkaline conditions (with aqueous NaOH, \( \Delta \)) forms \( \text{p-nitroaniline} \). This pathway successfully leads to the desired product.

Analyze Option (d)

In option (d), the direct nitration of aniline using \( \text{HNO}_3/\text{H}_2\text{SO}_4 \) suffers from over-nitration issues due to deactivation of the aromatic ring by electron donation. This results in a mixture of products and does not efficiently lead to \( \text{p-nitroaniline} \). Post-nitration treatment with \( \text{aq. NaOH} \) does not correct this issue efficiently.

Conclusion

Comparing all options, options (a) and (c) effectively lead to \( \text{p-nitroaniline} \), but (c) is more reliable as a standard synthetic route. Thus, for the cleanest approach to obtain \( \text{p-nitroaniline} \), option (c) is preferred.

Key concepts

Nitration

Nitration is a crucial step in many organic synthesis processes, especially when introducing a nitro group into an aromatic compound, like benzene or its derivatives. The process typically involves a nitrating mixture, often concentrated nitric acid (\( \text{HNO}_3 \)) and sulfuric acid (\( \text{H}_2\text{SO}_4 \)), forming a nitronium ion (\( \text{NO}_2^+ \)). This ion is a powerful electrophile, which can attack the electron-rich aromatic ring, substituting one of the hydrogen atoms for a nitro group (\( \text{NO}_2 \)).
It's important to control the reaction conditions, such as temperature and concentration, to ensure the addition of the nitro group occurs at the desired position on the aromatic ring. For example, electrophilic aromatic nitration tends to place the nitro group at the para position when adjacent groups direct this orientation.
In compound production such as p-nitroaniline, controlled nitration under specified conditions is essential to avoid multiple nitro groups forming, which can lead to by-products and a less efficient synthesis.

Acetylation

Acetylation is a method used in organic chemistry to protect functional groups during reactions. For example, the introduction of an acetyl group (\( \text{CH}_3\text{CO} \)) can convert an amine group into an amide, shielding it from further reactions. This is especially useful during nitration of aniline, which can otherwise lead to undesired products.
In the synthesis of p-nitroaniline, aniline undergoes acetylation to form acetanilide. This transformation reduces the reactivity of the amine group, allowing nitration to proceed without directly affecting the amine. After nitration, the amide can be reverted to the original amine with basic conditions, such as (\( \text{aq. NaOH} \)).
The ability to selectively protect and later regenerate functional groups makes acetylation a pivotal step in synthetic routes, as seen in the successful production of p-nitroaniline by first acetylating aniline before nitration.

Aromatic Substitution

Aromatic substitution refers to the replacement of a hydrogen atom on an aromatic ring with another atom or group of atoms. It's a distinctive characteristic of aromatic compounds because of the stability of the aromatic ring structure. This stability means reactions often involve an electrophile attacking the aromatic system.
Electrophilic aromatic substitution involves an electrophile, such as the nitronium ion (\( \text{NO}_2^+ \)), replacing a hydrogen atom on the benzene ring. This type of substitution is seen in the nitration and acetylation processes used for chemical productions like p-nitroaniline. The reaction must be carefully controlled to direct the substitution to the most suitable position on the aromatic ring, often influenced by existing substituents.
The nature of these substituents determines the directing effect, either ortho, meta, or para, impacting where any additional groups are placed during the reaction processes.

Synthetic Route in Organic Chemistry

Choosing a synthetic route is pivotal in organic chemistry as it determines the efficiency, yield, and purity of the final product. A well-designed synthetic route considers the reactivity of functional groups, reaction conditions, and possible side reactions.
In synthesizing p-nitroaniline, the route involving acetylation of aniline followed by nitration and deacetylation highlights a good synthetic pathway. Each step is tailored to protect functional groups, guide the nitration, and ultimately yield a clean product.
A synthetic route must minimize by-products and use easily accessible and cost-effective reagents. It needs to be safe, scalable, and environmentally benign wherever possible, achieving the target compound with the least number of steps with high atom economy.