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JEE Advanced levelSingle correct optionDifficulty 3 of 3o2154

Toluene must be converted mainly into 2-nitrotoluene while avoiding the para isomer. Which sequence uses the sulfonic acid group as a blocking group?

  1. A(i) conc. (para sulfonation); (ii) , ; (iii) dilute , steam, heat
  2. B(i) , only
  3. C(i) conc. (para sulfonation); (ii) dilute , steam, heat; (iii) ,
  4. D(i) , ; (ii) fuming ; (iii) dilute , steam, heat

Answer: A

Sulfonation of toluene gives mainly the para sulfonic acid because of sterics, which occupies the para position. Nitration then occurs ortho to the methyl group (meta to ). Finally the reversible sulfonation is undone by hot dilute acid, leaving 2-nitrotoluene. Nitrating alone gives an ortho/para mixture, and removing the blocking group before nitration regenerates toluene.
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