Chemistry Reaction Schemes

Nitration of benzene: electrophilic aromatic substitution

Benzene is too stable to undergo addition, so it reacts by substitution: an electrophile replaces a hydrogen and the aromatic ring is restored. In nitration, concentrated sulfuric acid generates the very reactive nitronium ion, NO₂⁺, from nitric acid.

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Electrophilic aromatic substitution

Sulfuric acid protonates nitric acid, which loses water to form the nitronium ion (NO₂⁺). Benzene's π electrons attack NO₂⁺, giving a positively charged intermediate, and loss of H⁺ restores the aromatic ring to give nitrobenzene.

Overall reaction

benzene
conc. HNO₃, conc. H₂SO₄
below 55 °C
nitrobenzene
+
water
byproduct

Mechanism — 4 steps

  1. 1Sulfuric acid protonates nitric acidproton transfer · reversible
    nitric acid
    +
    sulfuric acid
    protonated nitric acid
    +
    hydrogensulfate ion

    Electron movement

    1. A lone pair on the OH oxygen of nitric acid takes a proton from sulfuric acid (the stronger acid).
    2. The O–H bond of sulfuric acid breaks, leaving HSO₄⁻.

    Sulfuric acid is the stronger acid, so here nitric acid acts as a base.

  2. 2Loss of water gives the nitronium ionelimination of water · reversible
    protonated nitric acid
    −H₂O
    nitronium ion
    +
    water

    Electron movement

    1. A lone pair on the O⁻ forms a second N=O bond.
    2. The N–O bond to the protonated OH breaks and water leaves.

    Water is a good leaving group. The nitronium ion, O=N⁺=O, is a strong electrophile.

  3. 3Benzene attacks the nitronium ionelectrophilic attack
    benzene
    +
    nitronium ion
    slow
    arenium ion (Wheland intermediate)

    Electron movement

    1. Two electrons from the delocalised π system attack the nitrogen of NO₂⁺.
    2. One N=O π bond breaks so nitrogen doesn't exceed its octet; the electrons go to oxygen.

    This slow step breaks the aromatic ring. The positive charge in the intermediate is delocalised over the remaining five ring carbons.

  4. 4Loss of H⁺ restores aromaticityproton transfer
    arenium ion
    +
    hydrogensulfate ion
    −H⁺
    nitrobenzene
    +
    sulfuric acid

    Electron movement

    1. HSO₄⁻ removes the hydrogen from the carbon bearing the NO₂ group.
    2. The C–H electrons return to the ring, restoring the aromatic π system.

    Losing H⁺ (rather than adding a nucleophile) gives back the very stable aromatic ring. Sulfuric acid is regenerated, so it is a catalyst.

Stereochemistry

There is no stereochemistry in this reaction; the product ring is flat.

Key concepts

  • Electrophilic aromatic substitution
  • Generating the electrophile (NO₂⁺)
  • Delocalisation and aromatic stability
  • Sulfuric acid as a catalyst

Common mistakes

  • Drawing addition instead of substitution
  • Starting the attacking arrow from a single carbon atom instead of the ring's π system
  • Drawing the intermediate with a complete delocalised ring (it's broken in the intermediate)
  • Keeping the temperature too high, which leads to multiple nitration

More mechanisms