To identify compounds A and B in the reaction sequence, each step must be analyzed.
The starting material is bromobenzene. The reaction proceeds in two primary stages:
Nitration: Bromobenzene is treated with concentrated nitric acid (\(\text{HNO}_3\)) to introduce a nitro group. Aromatic nitration typically uses \(\text{HNO}_3\) and concentrated sulfuric acid. When only \(\text{HNO}_3\) is considered, the nitro group attaches to an ortho or para position relative to bromine due to its electron-withdrawing nature.
Due to steric hindrance in this mechanism, the nitro group preferentially attaches at the para position. Consequently, compound A is para-bromonitrobenzene.
Diazonium Formation and Sandmeyer Reaction: The subsequent step involves reaction with NaOH followed by HCl. The combined action of NaOH and HCl typically converts the intermediate amine to a diazonium salt via the Sandmeyer Reaction. This process also replaces the nitro group with a hydroxyl group through saponification and subsequent acidification.
Thus, compound B is phenol.
The correct answer is depicted in the accompanying image.
Accordingly, compounds A and B in the reaction sequence are para-bromonitrobenzene and phenol, respectively.
