Electrophilic aromatic substitution
Nitration of benzene
How the Nitration of benzene works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: conc. HNO₃, conc. H₂SO₄ (mixed acid). Conditions: 0–50 °C. Bonds formed: C–N (Ar–NO₂); H₂O; bonds broken: Ar C–H; N–OH (HNO₃). Stereochemistry: Not applicable (planar arene). Rate and selectivity: Attack on NO₂⁺ is rate-limiting; NO₂ deactivates the ring toward further nitration.
At a glance
Step 1: Generate the nitronium ion
Sulfuric acid protonates nitric acid, and loss of water generates the nitronium ion (NO₂⁺) — a small, potent electrophile, generated in situ specifically because NO₂⁺ itself is too unstable to store or handle directly.
Step 2: π electrons attack NO₂⁺
The ring's π electrons attack this nitronium ion, forming the same resonance-stabilized arenium ion intermediate seen in every EAS reaction in this library.
Step 3: Deprotonation restores aromaticity
Loss of a proton restores aromaticity, giving nitrobenzene — this reaction is the standard entry point into nitroaromatic chemistry, and the nitro group installed here is itself a strong deactivator, so a second nitration runs far more slowly and meta-selectively.
Abbreviations and terms
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
- EAS
- Electrophilic aromatic substitution. An electrophile replaces H on an arene via a Wheland intermediate.
- Wheland
- Wheland intermediate (arenium ion). The cyclohexadienyl cation formed when an electrophile adds to a benzene ring.
How it compares with related reactions
| Nitration of benzene | Bromination of benzene (FeBr₃-catalyzed) | Duff reaction | |
|---|---|---|---|
| Reagents | conc. HNO₃, conc. H₂SO₄ (mixed acid) | Br₂ with FeBr₃ (Lewis-acid catalyst) | Hexamethylenetetramine (HMTA), acid (AcOH/glycerol or TFA); then H₃O⁺ |
| Conditions | 0–50 °C | Anhydrous, 25 °C, dark | Heat, 100–150 °C; aqueous hydrolysis |
| Stereochemistry | Not applicable (planar arene) | None (planar, achiral) | Ortho-selective (OH directs; H-bonded iminium) |
| Rate / selectivity | Attack on NO₂⁺ is rate-limiting; NO₂ deactivates the ring toward further nitration | Slow step = arenium formation; deactivated rings react poorly | Moderate; electron-rich phenols only |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Nitration of benzene?
conc. HNO₃, conc. H₂SO₄ (mixed acid). Conditions: 0–50 °C.
Which bonds are formed and which are broken?
Formed: C–N (Ar–NO₂); H₂O. Broken: Ar C–H; N–OH (HNO₃).
What is the stereochemical outcome?
Not applicable (planar arene)
What controls the rate or the selectivity?
Attack on NO₂⁺ is rate-limiting; NO₂ deactivates the ring toward further nitration
Study guides that help with this reaction
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go
- How to answer mechanism questions in an exam