Aromatic Reactions: Nitration using HNO₃/H₂SO₄

Mixed acid is simply concentrated nitric acid dissolved in concentrated sulfuric acid. The strong dehydration by H₂SO₄ protonates HNO₃, ejects water, and liberates nitronium ion (NO₂⁺). Every electrophilic aromatic substitution (EAS) text shows the same choreography: the arene π bond attacks NO₂⁺, the σ-complex (Wheland intermediate) captures the positive charge, and HSO₄⁻ (or H₂O) removes the benzylic proton to re-form the aromatic π system. The nitro group that appears is powerfully deactivating and meta-directing, so nitration is both a synthesis tool (gateway to anilines) and a directing-pattern teaching moment.

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Key Emphasis (Teaching Pivots)

• Electrophile identity: HNO₃ + H₂SO₄ (“mixed acid”) rapidly generates nitronium ion (NO₂⁺); never draw “HNO₃ attacks the ring.”
• Canonical EAS flow: π-attack → σ-complex (Wheland intermediate with explicit benzylic H) → deprotonation restores aromaticity and regenerates acid.
• Directing logic: EDG/activators give ortho/para, EWG/deactivators give meta, halogens remain the classic o/p-but-deactivating exception. Once –NO₂ is installed it enforces meta logic for subsequent EAS.
• Temperature control: Cool (0–55 °C) runs limit polynitration; hotter or fuming conditions increase [NO₂⁺] and risk dinitration.
• Aniline trap: Free –NH₂ is protonated to –NH₃⁺ under mixed acid, flipping to meta-directing. Protect as acetanilide to retain o/p nitration, then hydrolyze.
• Assets/tests baseline: Default mechanism frames and tests use benzene; router overlays then swap in substituted arenes (toluene, anisole, nitrobenzene, acetanilide) for directing scenarios.

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Quick Summary

• Reagents/conditions: Concentrated HNO₃ in concentrated H₂SO₄ (“mixed acid”), typically 0–55 °C; forcing temperatures or fuming acid increase NO₂⁺ concentration.
• Electrophile: NO₂⁺ generated via H₂SO₄-protonated nitric acid followed by water loss (A → B in the mechanism art).
• Outcome: Ar–H → Ar–NO₂; the nitro group is strongly deactivating, meta-directing, and readily reduced to –NH₂ downstream.
• Mechanism: Hydration-free nitronium formation → π attack to give σ-complex → HSO₄⁻ (or H₂O) removes the benzylic proton → aromaticity restored.
• Directing control: EDG (–OR/–NR₂/alkyl) favor para > ortho when sterics block ortho; EWG (–NO₂, –SO₃H, –C=O families) enforce meta; halogens = o/p but slow.
• Process control: Keep mixtures cold, add arene to acid (not vice versa), and quench quickly to avoid poly-nitration, oxidation, or dark tars.

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Mechanism — Mixed-Acid Nitration (4 Frames; arrows A–F)

Each frame uses benzene as the baseline substrate.

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Mechanistic Checklist (Exam Focus)

• Always show nitronium (NO₂⁺) generation before the ring attack—no “HNO₃ arrow directly into benzene.”
• Rate-determining step = π-attack forming the σ-complex.
• The σ-complex should include the benzylic H and the adjacent carbocation so the deprotonation arrows make sense.
• Aniline is protonated (–NH₃⁺) under mixed acid → meta; acetanilide stays o/p. State this explicitly when analyzing directing conflicts.
• –NO₂ is strongly deactivating and meta-directing; halogens are the only o/p-but-deactivating exception.
• Temperature and acid strength modulate poly-nitration—draw attention to conditions when explaining selectivity.

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Worked Examples

All reagent badges reuse the exact PNG that appears in the Reagent Search grid. Each figure shows Reactant → Reagent button → Product (single-stage process).

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Scope & Limitations

• Smooth substrates: Benzene, alkylbenzenes, anisoles, phenols (run cold), and mildly activated rings nitration readily.
• Challenging substrates: Strongly deactivated rings (–CF₃, –NO₂, –SO₃H, carbonyl clusters) require hotter or more concentrated acid; expect slower rates and lower yields.
• Functional-group sensitivity: Acid-sensitive protecting groups or oxidizable side chains may not survive mixed acid. Consider using sulfonation/desulfonation blocking strategies or milder nitrating mixtures when possible.
• Polynitration risk: Elevated temperature, long dwell times, or highly activated rings invite di/trinitration. Keep the bath cold and quench promptly.
• Metal coordination: Strong donor groups (e.g., thiols) can bind or quench nitronium; mask them before nitration.

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Practical Tips

• Charge nitric acid into sulfuric acid (never reverse) while cooling to keep the exotherm under control.
• Use glass or PTFE tools—mixed acid attacks many metals and most elastomers.
• Add the arene slowly into the cooled mixed acid, monitor temperature, then quench by pouring onto crushed ice and neutralizing carefully.
• Limit exposure time once nitration is complete; prolonged hot residence invites oxidation, tar, or over-nitration.
• Plan downstream: Nitro groups reduce smoothly (Fe/HCl, Sn/HCl, catalytic hydrogenation), so nitration is a standard entry point to anilines.

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Exam-Style Summary

HNO₃/H₂SO₄ (mixed acid) generates nitronium ion (NO₂⁺). The arene’s π bond attacks NO₂⁺ to form a σ-complex; HSO₄⁻ (or H₂O) deprotonates to restore aromaticity and deliver Ar–NO₂. The nitro group is strongly deactivating and meta-directing—protect amines (acetanilide) when you need o/p nitration, and keep temperatures low to avoid polynitration.

Exam cues / pitfalls

• Forgetting to show NO₂⁺ formation or drawing “HNO₃ attacks benzene.”
• Neglecting the σ-complex frame, which leaves nowhere for the proton-abstraction arrow to land.
• Claiming anilines direct o/p under mixed acid without protecting the amine—state the protonation issue.
• Ignoring temperature/time when asked how to avoid di- or trinitration of activated rings.

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Interactive Toolbox

• Mechanism Solver — Step through the HNO₃/H₂SO₄ nitration mechanism with narrated explanations, highlighting nitronium formation and σ-complex frames.
• Reaction Solver — Predict ortho/meta/para outcomes for any arene under mixed acid and surface warnings when conditions risk polynitration.
• IUPAC Namer — Practice naming nitroaromatic products (e.g., 4-nitrotoluene, m-dinitrobenzene) without exposing SMILES in the learner copy.

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