9701 · 31.1
Halogen compounds
We'll explore why attaching a halogen to a carbon chain makes it reactive, but attaching it to a benzene ring makes it surprisingly stubborn. This difference in reactivity dictates how we use these important molecules.
Need to know
What you need to know
- **Conditions:** Warm aqueous solution of the nucleophile, e.g., NaOH(aq) or KCN(aq).
- **Mechanism:** Can be Sₙ1 (two-step, via carbocation, for tertiary/some secondary) or Sₙ2 (one-step, via transition state, for primary/some secondary).
- **Example (Hydrolysis):** $CH_3CH_2Br + OH^-(aq) \xrightarrow{Warm} CH_3CH_2OH + Br^-(aq)$
Explanation
Halogens: Attached but not Always Reactive
- Halogenoarenes have a stronger carbon-halogen (C–X) bond than halogenoalkanes, making them less reactive. This is due to the halogen's lone pair overlapping with the ring's π-system, giving the C–X bond partial double bond character.
- Because of this strong bond, nucleophilic substitution on aryl halides requires very harsh conditions (e.g., high temperature and pressure), unlike the milder conditions needed for halogenoalkanes.
- Halogenated compounds have had significant uses, such as in pesticides (DDT) and as refrigerants (CFCs). However, their stability and effect on the ozone layer have led to environmental concerns and restrictions.
- Reactivity depends on the halogen's location. A halogen on a benzene ring is unreactive to substitution but directs incoming electrophiles. A halogen on an alkyl side-chain attached to a ring behaves just like a normal halogenoalkane.