Nitration (SEAr): practice and mechanism
NO₂⁺ substitutes a ring hydrogen; the existing group decides the position: activators and halogens o/p, electron-withdrawing groups meta.
Example
Sulfuric acid turns nitric acid into the nitronium ion NO₂⁺, which attacks the ring electrophilically. Activating groups and halogens direct ortho/para (here: the para product), electron-withdrawing groups such as NO₂, C=O or CN direct meta.
Mechanism step by step
- 1
Sulfuric acid protonates nitric acid and water leaves: the nitronium ion NO₂⁺.
- 2
The arene attacks NO₂⁺ with its π electrons: σ complex (arenium ion); aromaticity is briefly lost.
- 3
HSO₄⁻ removes the H at the attacked carbon and aromaticity returns: nitroarene.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- The meta position would be right for an electron-withdrawing group. This substituent pushes electrons into the ring (or is a halogen) and directs ortho/para.
- The para position would be right for an activating group. This substituent withdraws electrons; ortho and para are especially deactivated, so meta.
Often confused with: Aromatic bromination (SEAr) · Friedel–Crafts acylation
Frequently asked questions
Why do some groups direct ortho/para?
With ortho/para attack the substituent can directly stabilise the positive charge of the σ complex (+M or +I effect).
And why meta for electron-withdrawing groups?
With ortho/para the positive charge would sit right next to the electron-withdrawing group (very unfavourable); meta avoids this.