JEE Advanced 2022 Paper 1 Q14 Chemistry Organic Reactions & Mechanisms Amines & Diazonium Hard

JEE Advanced 2022 Paper 1 · Q14 · Amines & Diazonium

Considering the following reaction sequence,

4-nitrotoluene (H$_3$C-C$_6$H$_4$-NO$_2$, para) $\xrightarrow{P}$ Q $\xrightarrow{R}$ S $\xrightarrow{\text{H}_2\text{O}}$ T

S $\xrightarrow{U}$ benzoic acid (C$_6$H$_5$-COOH)

the correct option(s) is(are)

  1. A. P = H$_2$/Pd, ethanol; R = NaNO$_2$/HCl; U = 1. H$_3$PO$_2$; 2. KMnO$_4$ - KOH, heat
  2. B. P = Sn/HCl; R = HNO$_2$; S = 4-methylbenzenediazonium chloride (H$_3$C-C$_6$H$_4$-N$_2^+$Cl$^-$)
  3. C. S = 4-methylbenzenediazonium chloride (H$_3$C-C$_6$H$_4$-N$_2^+$Cl$^-$); T = 4-methylphenol (p-cresol, H$_3$C-C$_6$H$_4$-OH); U = 1. CH$_3$CH$_2$OH; 2. KMnO$_4$ - KOH, heat
  4. D. Q = 4-nitrobenzoic acid (HOOC-C$_6$H$_4$-NO$_2$, para); R = H$_2$/Pd, ethanol; T = 4-methylphenol (p-cresol, H$_3$C-C$_6$H$_4$-OH)
JEE Advanced multi-correct — pick every correct option, then check.
Reveal answer + step-by-step solution

Correct answer:A, B, C

Solution

Starting material: 4-nitrotoluene (p-nitrotoluene). The branch leading to benzoic acid requires the methyl to be oxidised to -COOH; the linear chain S $\xrightarrow{\text{H}_2\text{O}}$ T is characteristic of hydrolysis of a diazonium salt to a phenol.

Forward sequence: p-nitrotoluene $\xrightarrow{P}$ Q (reduction of -NO$_2$ to -NH$_2$): Q = 4-methylaniline (p-toluidine). $\xrightarrow{R}$ S (diazotisation with NaNO$_2$/HCl at 0–5 $^\circ$C): S = 4-methylbenzenediazonium chloride. $\xrightarrow{\text{H}_2\text{O}}$ T (hydrolysis of the aryldiazonium salt): T = 4-methylphenol (p-cresol).

Branch S $\xrightarrow{U}$ benzoic acid: U must (i) remove the -N$_2^+$Cl$^-$ (replace by H) and (ii) oxidise the methyl to -COOH. Step (i) uses H$_3$PO$_2$ (hypophosphorous acid, the classic deaminative reduction of aryldiazonium salts to ArH) giving toluene, and step (ii) oxidises toluene to benzoic acid by alkaline KMnO$_4$/heat.

Now evaluate the four options against the canonical assignment (P = H$_2$/Pd or Sn/HCl, both being acceptable reductants for ArNO$_2 \rightarrow$ ArNH$_2$; R = NaNO$_2$/HCl or equivalently HNO$_2$ generated in situ; U = H$_3$PO$_2$ followed by KMn … [truncated]

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