Reaction Sequence of 4 Nitrotoluene to Benzoic Acid

To determine the correct option(s), let us analyze the given reaction sequence step-by-step:

1. Step-by-Step Reaction Sequence Analysis

• Starting Material: 4-nitrotoluene ($p\text{-nitrotoluene}$, $\text{CH}_3\text{-C}_6\text{H}_4\text{-NO}_2$).

• Step 1 ($\text{4-nitrotoluene} \xrightarrow{\mathbf{P}} \mathbf{Q}$):

  • Reduction of the nitro group ($-\text{NO}_2$) to an amino group ($-\text{NH}_2$) can be achieved using reducing agents such as $\mathbf{P} = \text{H}_2/\text{Pd, ethanol}$ or $\mathbf{P} = \text{Sn/HCl}$.
  • This yields 4-methylaniline ($p$-toluidine, $\mathbf{Q} = p\text{-CH}_3\text{-C}_6\text{H}_4\text{-NH}_2$).

• Step 2 ($\mathbf{Q} \xrightarrow{\mathbf{R}} \mathbf{S}$):

  • Diazotization of 4-methylaniline ($\mathbf{Q}$) using $\mathbf{R} = \text{NaNO}_2/\text{HCl}$ or $\text{HNO}_2$ at low temperatures ($0-5^\circ\text{C}$) yields 4-methylbenzenediazonium chloride ($\mathbf{S} = p\text{-CH}_3\text{-C}_6\text{H}_4\text{-N}_2^+\text{Cl}^-$).

• Step 3 ($\mathbf{S} \xrightarrow{\text{H}_2\text{O}} \mathbf{T}$):

  • Hydrolysis of the diazonium salt ($\mathbf{S}$) with warm water replaces the $-\text{N}_2^+\text{Cl}^-$ group with a hydroxyl group ($-\text{OH}$), producing 4-methylphenol ($p$-cresol, $\mathbf{T} = p\text{-CH}_3\text{-C}_6\text{H}_4\text{-OH}$).

• Step 4 ($\mathbf{S} \xrightarrow{\mathbf{U}} \text{Benzoic acid}$):

  • To convert 4-methylbenzenediazonium chloride ($\mathbf{S}$) to benzoic acid ($\text{C}_6\text{H}_5\text{COOH}$), two transformations are required:
    1. Deamination: The diazonium group ($-\text{N}_2^+\text{Cl}^-$) is reduced to a hydrogen atom ($-\text{H}$) using either hypophosphorous acid ($\text{H}_3\text{PO}_2$) or ethanol ($\text{CH}_3\text{CH}_2\text{OH}$), forming toluene ($\text{C}_6\text{H}_5\text{CH}_3$).
    2. Oxidation: The methyl group ($-\text{CH}_3$) of toluene is oxidized to a carboxylic acid group ($-\text{COOH}$) using alkaline potassium permanganate ($\text{KMnO}_4 - \text{KOH, heat}$) followed by acidification.

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2. Verification of Options

• Option (A):

  • $\mathbf{P} = \text{H}_2/\text{Pd, ethanol}$ reduces $-\text{NO}_2$ to $-\text{NH}_2$.
  • $\mathbf{R} = \text{NaNO}_2/\text{HCl}$ converts $-\text{NH}_2$ to $-\text{N}_2^+\text{Cl}^-$.
  • $\mathbf{U} = 1.\ \text{H}_3\text{PO}_2 \quad 2.\ \text{KMnO}_4 - \text{KOH, heat}$ first reduces the diazonium group to yield toluene, then oxidizes toluene to benzoic acid.
  • Option (A) is CORRECT.

• Option (B):

  • $\mathbf{P} = \text{Sn/HCl}$ reduces 4-nitrotoluene to 4-methylaniline ($\mathbf{Q}$).
  • $\mathbf{R} = \text{HNO}_2$ diazotizes 4-methylaniline.
  • $\mathbf{S} = p\text{-CH}_3\text{-C}_6\text{H}_4\text{-N}_2^+\text{Cl}^-$ is correctly identified as the diazonium salt.
  • Option (B) is CORRECT.

• Option (C):

  • $\mathbf{S} = p\text{-CH}_3\text{-C}_6\text{H}_4\text{-N}_2^+\text{Cl}^-$ is the diazonium salt.
  • $\mathbf{T} = p\text{-CH}_3\text{-C}_6\text{H}_4\text{-OH}$ ($p$-cresol) is formed by heating $\mathbf{S}$ with water.
  • $\mathbf{U} = 1.\ \text{CH}_3\text{CH}_2\text{OH} \quad 2.\ \text{KMnO}_4 - \text{KOH, heat}$ reduces $\mathbf{S}$ to toluene using ethanol, followed by oxidation to benzoic acid.
  • Option (C) is CORRECT.

• Option (D):

  • If $\mathbf{Q} = 4\text{-nitrobenzoic acid}$, reduction with $\mathbf{R} = \text{H}_2/\text{Pd, ethanol}$ gives 4-aminobenzoic acid, which on reaction with $\text{H}_2\text{O}$ does not give $p$-cresol ($\mathbf{T}$). Thus, this path is incorrect.
  • Option (D) is INCORRECT.

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Conclusion

The correct options are A, B, and C.