Electrophilic aromatic substitution
Benzene's ring is very stable because its electrons are spread around the ring (aromatic). It reacts by substitution, not addition, so the ring survives. An electrophile attacks, a cation forms, then H⁺ is lost to restore the ring.
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Analogy
Swapping one tile in a mosaic while keeping the whole pattern.
Example
HNO₃ + H₂SO₄ → NO₂⁺ (electrophile)NO₂⁺ attacks the ringLose H⁺, ring restoredProduct: nitrobenzene
In practice
Nitration of benzene, with a mixture of nitric and sulfuric acids, is the first step to making dyes and many medicines.
Practice
Flashcards
- What is the product when benzene reacts with NO₂⁺?Nitrobenzene (substitution, ring kept).
Questions people ask
Why doesn't benzene react like an alkene?
Addition to benzene would destroy the aromatic ring, losing its extra stability of about 150 kJ/mol. Substitution keeps the ring intact, so it is the favoured path. Benzene is also less reactive than alkenes, which is why it needs a strong electrophile and a catalyst.
What does an activating or directing group do?
Groups on the ring change both the speed and the position of the next substitution. Electron-donating groups such as OH and CH₃ speed up the reaction and direct new groups to the ortho and para positions. Electron-withdrawing groups such as NO₂ slow it and direct to meta.