JEE Main · 2021 · Shift-IImediumHC-117

Benzene on nitration gives nitrobenzene in presence of HNO3 and H2SO4 mixture, where:

Hydrocarbons · Class 11 · JEE Main Previous Year Question

Question

Benzene on nitration gives nitrobenzene in presence of \ceHNO3\ce{HNO3} and \ceH2SO4\ce{H2SO4} mixture, where:

Options
  1. a

    Both \ceH2SO4\ce{H2SO4} and \ceHNO3\ce{HNO3} act as bases

  2. b

    Both \ceH2SO4\ce{H2SO4} and \ceHNO3\ce{HNO3} act as acids

  3. c

    \ceHNO3\ce{HNO3} acts as a base and \ceH2SO4\ce{H2SO4} acts as an acid

  4. d

    \ceHNO3\ce{HNO3} acts as an acid and \ceH2SO4\ce{H2SO4} acts as a base

Correct Answerc

\ceHNO3\ce{HNO3} acts as a base and \ceH2SO4\ce{H2SO4} acts as an acid

Detailed Solution

Step 1: Mechanism of Nitration

In nitration, the active electrophile is \ceNO2+\ce{NO2^+} (nitronium ion): image Step 2: Acid-Base Roles

In this reaction:

  • \ceH2SO4\ce{H2SO4} (stronger acid, pKa is -9): donates a proton to \ceHNO3\ce{HNO3} (pKa is -1.4) = acts as an acid
  • \ceHNO3\ce{HNO3} (weaker acid in this context, pKa is -1.4): accepts a proton from \ceH2SO4\ce{H2SO4} = acts as a base

So: \ceHNO3\ce{HNO3} = base (proton acceptor); \ceH2SO4\ce{H2SO4} = acid (proton donor).

Key Points:

  • \ceH2SO4\ce{H2SO4} is a stronger acid than \ceHNO3\ce{HNO3}, so \ceH2SO4\ce{H2SO4} protonates \ceHNO3\ce{HNO3}
  • \ceHNO3\ce{HNO3} acts as a base relative to \ceH2SO4\ce{H2SO4}
  • The protonated nitric acid loses water to give \ceNO2+\ce{NO2^+} (electrophile)
  • This is a Bronsted acid-base interaction

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