Question

Select the correct set of reagents required for conversion of the following chemical reactions

• A. (i). \(\text{NaNO}_2/\text{HCl}, 273\text{--}278\text{ K}\) (ii). \(\text{H}_3\text{PO}_4, \text{H}_2\text{O}\) (iii). \(\text{KMnO}_4\) and \(\text{KOH}\)
• B. (i). \(\text{NaNO}_2/\text{HCl}\) (ii). \(\text{H}_3\text{PO}_4\) (iii). \(\text{NaOH}/\text{H}_2\text{O}\)
• C. (i). \(\text{HNO}_3/\text{H}_2\text{SO}_4\) (ii). \(\text{H}_3\text{PO}_4\) (iii). \(\text{KMnO}_4\) and \(\text{KOH}\)
• D. (i). \(\text{NaNO}_2/\text{HCl}\) (ii). \(\text{Sn}/\text{HCl}\) (iii). \(\text{KMnO}_4\) and \(\text{KOH}\)

Hint:

To remove an \(-\text{NH}_2\) group from a benzene ring:
1. \(\text{NaNO}_2/\text{HCl}\) at \(0\text{--}5^\circ\text{C} \rightarrow\) Diazonium salt.
2. \(\text{H}_3\text{PO}_2/\text{H}_2\text{O}\) or \(\text{CH}_3\text{CH}_2\text{OH} \rightarrow\) Reduction to \(-\text{H}\).
To oxidize alkyl side chains to \(-\text{COOH}\): Alkaline \(\text{KMnO}_4\).

Answer 1

The Correct Option is A

Concept:
Transforming 2-bromo-4-methylaniline into 2-bromobenzoic acid requires two key synthetic modifications:
1. Deamination: complete removal of the aromatic primary amino group (\(-\text{NH}_2 \rightarrow -\text{H}\)),
2. Side-chain oxidation: vigorous oxidation of the methyl group to a carboxylic acid group (\(-\text{CH}_3 \rightarrow -\text{COOH}\)).

Step 1: Diazotization of the Amino Group:
Treating 2-bromo-4-methylaniline with sodium nitrite in aqueous hydrochloric acid at \(273\text{--}278\text{ K}\) quantitatively converts the primary aromatic amino group into a diazonium salt:
\[ \text{Ar}-\text{NH}_2 + \text{NaNO}_2 + 2\text{HCl} \xrightarrow{273\text{--}278\text{ K}} \text{Ar}-\text{N}_2^+\text{Cl}^- + \text{NaCl} + 2\text{H}_2\text{O} \]

Step 2: Reductive Deamination Using Hypophosphorous Acid:
The diazonium intermediate is treated with hypophosphorous acid (\(\text{H}_3\text{PO}_4\)) in water, reducing the diazonium group to a hydrogen atom and evolving nitrogen gas:
\[ \text{Ar}-\text{N}_2^+\text{Cl}^- + \text{H}_3\text{PO}_2 + \text{H}_2\text{O} \rightarrow \text{Ar}-\text{H} + \text{N}_2 + \text{H}_3\text{PO}_3 + \text{HCl} \] This removes the functional group at position 4 relative to methyl, isolating 2-bromotoluene.

Step 3: Oxidation of Alkyl Side Chain:
Heating 2-bromotoluene with alkaline potassium permanganate (\(\text{KMnO}_4\) and \(\text{KOH}\)) followed by acidic workup oxidizes the methyl group to a carboxylic acid:
\[ \text{C}_6\text{H}_4(\text{Br})(\text{CH}_3) \xrightarrow[\text{(ii) }\text{H}_3\text{O}^+]{\text{(i) }\text{KMnO}_4, \, \text{KOH}, \, \Delta} \text{C}_6\text{H}_4(\text{Br})(\text{COOH}) \] This sequence yields 2-bromobenzoic acid.
Final Answer:
The correct reagent sequence is (i). \(\text{NaNO}_2/\text{HCl}, 273\text{--}278\text{ K}\), (ii). \(\text{H}_3\text{PO}_4, \text{H}_2\text{O}\), (iii). \(\text{KMnO}_4\) and \(\text{KOH}\), corresponding to option (A).