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How is Carbolic Acid Prepared from Aniline?

Carbolic acid, commonly known as phenol, is an important organic compound with a variety of applications in the chemical industry. It is used in the production of plastics, resins, and as an antiseptic. One of the key methods to prepare carbolic acid involves starting with aniline, an aromatic amine. This article will provide a detailed explanation of the chemical process used to convert aniline into carbolic acid, answering the question: how is carbolic acid prepared from aniline?

Step 1: Nitration of Aniline

The first step in preparing carbolic acid from aniline is the nitration of aniline. Aniline reacts with a mixture of concentrated sulfuric acid (H₂SO₄) and nitric acid (HNO₃), leading to the formation of nitrobenzene. However, direct nitration of aniline is difficult because the amino group (-NH₂) is a strong activator and makes the aromatic ring highly reactive, potentially leading to over-nitration. To avoid this, aniline is first converted to aniline sulfate by treating it with sulfuric acid, which temporarily deactivates the amino group. This controlled nitration results in the formation of nitrobenzene:

[ \text{C}6\text{H}5\text{NH}2 \xrightarrow{\text{H}2\text{SO}4/\text{HNO}3} \text{C}6\text{H}5\text{NO}_2 ]

Step 2: Reduction of Nitrobenzene to Phenylhydroxylamine

The next step involves reducing nitrobenzene to phenylhydroxylamine, which is an intermediate in the production of carbolic acid. This reduction can be achieved using various reducing agents, such as iron filings in the presence of hydrochloric acid (Fe/HCl) or by catalytic hydrogenation. The reaction typically proceeds as follows:

[ \text{C}6\text{H}5\text{NO}2 + 3\text{H}2 \xrightarrow{\text{Fe/HCl or catalyst}} \text{C}6\text{H}5\text{NHOH} ]

Step 3: Rearrangement to Carbolic Acid

The final step in the preparation of carbolic acid from aniline is the rearrangement of phenylhydroxylamine to form carbolic acid (phenol). This rearrangement is known as the Bamberger rearrangement, where phenylhydroxylamine is treated with an acid, usually sulfuric acid, to yield phenol:

[ \text{C}6\text{H}5\text{NHOH} \xrightarrow{\text{H}2\text{SO}4} \text{C}6\text{H}5\text{OH} + \text{NH}_3 ]

The phenylhydroxylamine undergoes a rearrangement to produce phenol and ammonia. This step is crucial as it directly produces carbolic acid from the precursor.

Conclusion

In summary, the preparation of carbolic acid from aniline involves a multi-step chemical process, starting with the nitration of aniline to form nitrobenzene, followed by the reduction of nitrobenzene to phenylhydroxylamine, and finally, the rearrangement of phenylhydroxylamine to yield carbolic acid. This sequence of reactions effectively answers the question: how is carbolic acid prepared from aniline? The process is industrially significant, offering a route to produce phenol from aniline, a relatively inexpensive and readily available starting material. Understanding these steps is essential for those in the chemical industry aiming to produce phenol on a commercial scale.