Question
Benzene reacts with a mixture of concentrated HNO₃ and concentrated H₂SO₄ at 55°C. Write the mechanism of the reaction and explain why toluene gives predominantly ortho and para products, while nitrobenzene gives the meta product when further nitrated.
(JEE Main 2023 — this exact directing group comparison has appeared 4 times in the last 6 years.)
Solution — Step by Step
Concentrated H₂SO₄ protonates HNO₃, which then loses water to form the nitronium ion, NO₂⁺.
This is why we need H₂SO₄ — it’s not just a solvent. It generates the actual electrophile.
NO₂⁺ attacks the π electron cloud of benzene, forming a carbocation intermediate called the arenium ion (or Wheland intermediate / sigma complex).
The ring loses aromaticity at this point — the carbon attacked becomes sp³. This is the slow, rate-determining step.
HSO₄⁻ (the base present) removes a proton from the sp³ carbon, restoring the aromatic ring.
Aromaticity is the driving force here — the ring wants to get back to the stable 6π system.
The —CH₃ group is an electron-donating group (EDG) via hyperconjugation and induction. It pushes electron density into the ring, making it more reactive overall.
More importantly, when we draw the arenium ion intermediate, the positive charge is most stable at the ortho and para positions relative to —CH₃. The methyl group directly stabilises the carbocation at those positions through hyperconjugation.
The —NO₂ group is an electron-withdrawing group (EWG) via resonance. It pulls electrons out of the ring through the oxygen lone pairs going toward nitrogen.
When the electrophile attacks ortho or para to —NO₂, the arenium ion places a positive charge directly on the carbon bearing —NO₂ — that’s two adjacent positive centres. Extremely destabilising.
Meta attack avoids this — the positive charge never sits on the carbon attached to —NO₂. So meta product forms even though the ring is deactivated overall.
Why This Works
EAS comes down to one question: where is the ring most electron-rich? Electrophiles go to the position with the highest electron density, and the resonance structures of the intermediate tell us exactly where that is.
EDGs (—OH, —NH₂, —CH₃, —OR) donate electrons by resonance or hyperconjugation. They push density to ortho and para positions — those positions get attacked. The ring reacts faster than benzene itself.
EWGs (—NO₂, —CHO, —COOH, —CN) withdraw electrons by resonance. They drain the ortho and para positions hardest, leaving meta as the “least bad” option. The ring reacts slower than benzene — that’s why getting dinitrobenzene requires harsher conditions.
Quick memory trick for the board exam: EDG = activating = o/p directing. EWG = deactivating = m directing. The only exception to memorise is the halogens — they are deactivating (EWG by induction) but still o/p directing (EDG by resonance). This exact exception is a favourite 1-mark question in CBSE boards.
Alternative Method — Using Resonance Structures Directly
Instead of remembering rules, draw the three arenium intermediates for each position and see which is most stable.
For toluene, attack at para gives an intermediate where one resonance structure places the positive charge on the carbon bearing —CH₃. Hyperconjugation from the three C—H bonds of the methyl group stabilises this directly.
For nitrobenzene, attack at ortho or para gives an intermediate with a resonance structure placing + on the carbon bonded to —NO₂. Since —NO₂ itself is already electron-deficient, this is extremely unstable. Meta avoids this entirely.
This method takes longer in an exam but is zero-memory — you derive the answer from first principles every time.
Common Mistake
Students often write that halogens are meta directors because they are deactivating. This is wrong. Halogens (—Cl, —Br, —F, —I) are deactivating (they slow the reaction via induction) but they are ortho/para directors (via resonance donation of lone pairs into the ring). Marking a halogen as meta director in JEE or NEET will cost you the mark. Always separate the two questions: “Does it activate/deactivate the ring?” and “Where does it direct?”
| Group Type | Example | Ring Activity | Direction |
|---|---|---|---|
| EDG (resonance) | —OH, —NH₂ | Strongly activating | o/p |
| EDG (hyperconj.) | —CH₃ | Weakly activating | o/p |
| Halogens | —Cl, —Br | Deactivating | o/p |
| EWG (resonance) | —NO₂, —CHO | Strongly deactivating | meta |
| EWG (induction) | —CF₃ | Deactivating | meta |