Question

Identify the product A in the following reaction.

• A.
• B.
• C.
• D.

Answer 1

The Correct Option is B

The reaction sequence is as follows:

Thus the correct answer is option 2.

Answer 2

The problem asks to identify the final product 'A' of a four-step reaction sequence starting from aniline.

Concept Used:

This is a multi-step synthesis that involves several key named reactions in organic chemistry:

1. Step (i) - Diazotization: The reaction of a primary aromatic amine (aniline) with nitrous acid (HNO₂, generated in situ from NaNO₂ and HCl) at low temperatures (0-5°C) forms a diazonium salt. This is a standard method to convert an amino group into a diazonium group (\(-N_2^+\)).
2. Step (ii) - Sandmeyer Reaction: The diazonium salt is treated with a cuprous halide (in this case, Cu₂Cl₂) to replace the diazonium group with a halogen atom. This reaction is a reliable method for introducing halogens onto an aromatic ring.
3. Steps (iii) & (iv) - Dow Process: This is an industrial method for the synthesis of phenol from chlorobenzene. It involves nucleophilic aromatic substitution under harsh conditions (high temperature and pressure), followed by an acidic workup.
   • Step (iii) is the nucleophilic substitution of the chlorine atom by a hydroxyl group from NaOH, which forms sodium phenoxide due to the basic conditions.
   • Step (iv) is the acidification of the sodium phenoxide to produce the final product, phenol.

Step-by-Step Solution:

Let's trace the transformation through each step of the reaction sequence.

Step 1: Reaction of Aniline with NaNO₂ and HCl

Aniline, a primary aromatic amine, reacts with NaNO₂ and HCl at low temperature to form benzenediazonium chloride. This is the diazotization step.

\[ \text{C}_6\text{H}_5\text{NH}_2 \xrightarrow{\text{NaNO}_2, \text{HCl}, \, 0-5^\circ\text{C}} \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \]

The product of this step is benzenediazonium chloride.

Step 2: Reaction with Cu₂Cl₂ (Sandmeyer Reaction)

The benzenediazonium chloride formed in the first step is then treated with cuprous chloride (Cu₂Cl₂). This is a Sandmeyer reaction, which replaces the diazonium group with a chlorine atom.

\[ \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{Cu}_2\text{Cl}_2} \text{C}_6\text{H}_5\text{Cl} + \text{N}_2 \]

The product after the second step is chlorobenzene.

Step 3: Reaction of Chlorobenzene with NaOH at 623 K and 300 atm

The chlorobenzene is subjected to the Dow process. It is treated with aqueous NaOH under high temperature (623 K) and high pressure (300 atm). Under these forcing conditions, a nucleophilic aromatic substitution occurs, and the chlorine atom is replaced by a hydroxyl group. Since the reaction medium is strongly basic, the initially formed phenol is deprotonated to form sodium phenoxide.

\[ \text{C}_6\text{H}_5\text{Cl} + 2\text{NaOH} \xrightarrow{623\,\text{K}, 300\,\text{atm}} \text{C}_6\text{H}_5\text{O}^-\text{Na}^+ + \text{NaCl} + \text{H}_2\text{O} \]

The product of this step is sodium phenoxide.

Step 4: Acidification with H⁺

The final step is the acidification of the reaction mixture. The proton source (H⁺) protonates the sodium phenoxide ion to yield the final product, phenol.

\[ \text{C}_6\text{H}_5\text{O}^-\text{Na}^+ \xrightarrow{\text{H}^+} \text{C}_6\text{H}_5\text{OH} \]

The final product 'A' is phenol.

Conclusion:

The final product of the entire reaction sequence is phenol. This corresponds to the structure shown in option (2).