Question

Identify A and B in the following reaction sequence.

• A.
• B.
• C.
• D.

Answer 1

The Correct Option is A

To identify compounds A and B in the reaction sequence, we need to analyze each step of the given reactions.

The initial compound is bromobenzene. The reaction sequence involves two main steps:

1. Nitration: Bromobenzene reacts with concentrated nitric acid (\(\text{HNO}_3\)) to introduce a nitro group. The usual condition for aromatic nitration involves \(\text{HNO}_3\) and concentrated sulfuric acid, but considering only \(\text{HNO}_3\) here, the nitro group gets added to one of the ortho or para positions relative to the bromine due to electron-withdrawing properties.

In this mechanism, due to steric hindrance, the nitro group preferably occupies a para position. Therefore, compound A is para-bromonitrobenzene.

1. Diazonium Formation and Sandmeyer Reaction: The next step involves reacting with NaOH and then HCl. The combination of NaOH and HCl typically facilitates the conversion to a diazonium salt by first forming the intermediate amine which is then converted using the Sandmeyer Reaction. This conversion is also known to replace the nitro group with a hydroxyl group (saponification followed by acidification).

Thereby, compound B is phenol.

From the options, the correct answer is shown in the image below:

Thus, the compounds A and B in the reaction sequence are para-bromonitrobenzene and phenol, respectively.

Answer 2

The reaction mechanism involves nitration followed by hydrolysis:

\[ \text{Bromobenzene} \xrightarrow{\text{Conc. HNO}_3} \text{Bromonitrobenzene (A)} \]

Then:

\[ \text{Bromonitrobenzene (A)} \xrightarrow{\text{NaOH, HCl}} \text{p-Bromophenol (B)} \]