JEE Advanced 2021 Paper 2 Q01 Chemistry Organic Chemistry Hydrocarbons Hard

JEE Advanced 2021 Paper 2 · Q01 · Hydrocarbons

The reaction sequence(s) that would lead to o-xylene as the major product is(are)

(A) Toluene (Me on benzene) bearing -NH$_2$ at the ortho position: (i) NaNO$_2$/HCl, 273 K; (ii) CuCN; then (iii) DIBAL-H then H$_3$O$^+$; (iv) N$_2$H$_4$, KOH, heat. (B) Toluene (Me on benzene) bearing -Br at the ortho position: (i) Mg, CO$_2$, H$_3$O$^+$; (ii) SOCl$_2$; then (iii) H$_2$, Pd-BaSO$_4$; (iv) Zn-Hg, HCl. (C) o-Methylstyrene (ortho-methyl vinyl benzene): (i) i. BH$_3$ ii. H$_2$O$_2$, NaOH; (ii) PBr$_3$; then (iii) Zn, dil. HCl. (D) Indene (bicyclic, fused cyclopentene-benzene): (i) O$_3$, Zn/H$_2$O; (ii) N$_2$H$_4$, KOH, heat.

  1. A. Sequence (A): ortho-methylaniline $\xrightarrow{\text{(i) NaNO}_2\text{/HCl, 273 K; (ii) CuCN}}$ ortho-methyl benzonitrile $\xrightarrow{\text{(iii) DIBAL-H then H}_3\text{O}^+;\ \text{(iv) N}_2\text{H}_4\text{, KOH, heat (Wolff-Kishner)}}$ o-xylene
  2. B. Sequence (B): ortho-bromotoluene $\xrightarrow{\text{(i) Mg, CO}_2\text{, H}_3\text{O}^+;\ \text{(ii) SOCl}_2}$ ortho-methyl benzoyl chloride $\xrightarrow{\text{(iii) H}_2\text{, Pd-BaSO}_4 \text{ (Rosenmund)};\ \text{(iv) Zn-Hg, HCl (Clemmensen)}}$ o-xylene
  3. C. Sequence (C): ortho-methyl styrene $\xrightarrow{\text{(i) BH}_3\text{; H}_2\text{O}_2\text{, NaOH (anti-Markovnikov)}}$ 2-(ortho-tolyl)ethanol $\xrightarrow{\text{(ii) PBr}_3;\ \text{(iii) Zn, dil. HCl}}$ ortho-ethyl toluene (NOT o-xylene)
  4. D. Sequence (D): Indene $\xrightarrow{\text{(i) O}_3\text{, Zn/H}_2\text{O (ozonolysis cleaves only the C=C of the five-membered ring)}}$ ortho-phthalaldehyde-like dialdehyde $\xrightarrow{\text{(ii) N}_2\text{H}_4\text{, KOH, heat}}$ a dimethylbenzene (the aromatic ring is untouched; gives o-xylene only if the cyclopentene ring opens to leave two methyl groups ortho)
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Reveal answer + step-by-step solution

Correct answer:A, B, D

Solution

Trace each sequence to the substitution pattern on the benzene ring.

(A) o-toluidine $\xrightarrow{\text{NaNO}_2/\text{HCl,\ 273\ K}}$ o-methylbenzenediazonium chloride $\xrightarrow{\text{CuCN (Sandmeyer)}}$ o-methylbenzonitrile $\xrightarrow{\text{DIBAL-H, then H}_3\text{O}^+}$ o-methylbenzaldehyde $\xrightarrow{\text{N}_2\text{H}_4,\text{KOH, heat (Wolff-Kishner)}}$ o-xylene. CORRECT.

(B) o-bromotoluene $\xrightarrow{\text{Mg, CO}_2, \text{H}_3\text{O}^+}$ o-methylbenzoic acid $\xrightarrow{\text{SOCl}_2}$ o-methylbenzoyl chloride $\xrightarrow{\text{H}_2/\text{Pd-BaSO}_4 (\text{Rosenmund})}$ o-methylbenzaldehyde $\xrightarrow{\text{Zn-Hg, HCl (Clemmensen)}}$ o-xylene. CORRECT.

(C) o-methylstyrene $\xrightarrow{\text{BH}_3; \text{H}_2\text{O}_2/\text{OH}^-}$ 2-(o-tolyl)ethan-1-ol (anti-Markovnikov hydration) $\xrightarrow{\text{PBr}_3}$ 2-(o-tolyl)ethyl bromide $\xrightarrow{\text{Zn/dil. HCl}}$ o-ethyltoluene, NOT o-xylene.

(D) Indene $\xrightarrow{\text{O}_3,\ \text{Zn/H}_2\text{O}}$ a dialdehyde on the benzene ring with a -CH$_2$CHO arm; Wolff-Kishner of both carbonyls would give an o-disubstituted benzene with a methyl and an ethyl group (o-ethyl tolu … [truncated]

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