In aromatic electrophilic substitution, deactivating groups are groups attached to the benzene ring that decrease the reactivity of the ring toward electrophiles.
Here’s a simple explanation:
What Deactivating Groups Do
- They withdraw electrons from the aromatic ring, either through inductive or resonance effects.
- This makes the ring less electron-rich, so it is less attractive to electrophiles.
- As a result, the reaction occurs more slowly compared to benzene without substituents.
Common Examples of Deactivating Groups
- Nitro group (–NO₂)
- Carbonyl groups (–CHO, –COR, –COOH)
- Cyano group (–CN)
- Sulfonic acid group (–SO₃H)
- Halogens (–Cl, –Br, –I) – special case: they deactivate but still direct to ortho/para positions.
Effect on Substitution Position
- Most deactivating groups direct new substituents to the meta position relative to themselves.
- This happens because the ortho and para positions are less stable during the intermediate step of the reaction when a deactivating group is present.
Summary
- Deactivating groups → make the ring less reactive → substitution slower → meta position usually favored (except halogens → ortho/para).