Haloalkanes and Haloarenes
Domain 10: Haloalkanes and Haloarenes
This domain tests the reactivity of carbon–halogen bonds: substitution, elimination, Grignard formation, and the special stability of aryl halides.
Core rules
- Haloalkanes: the C–X bond is polar; reactivity toward nucleophilic substitution follows C–I > C–Br > C–Cl > C–F for leaving-group ability.
- SN1 proceeds through a carbocation; rate order is 3°>2°>1°>methyl, and racemisation is common at a chiral centre. SN2 is concerted, backside attack, inversion of configuration, and favoured by 1°/methyl and polar aprotic solvents.
- Elimination competes: strong bulky base and heat favour E2; Saytzeff's rule favours the more substituted alkene.
- Haloarenes are much less reactive toward nucleophilic substitution because the C–X bond has partial double-bond character and the ring resists backside attack; they undergo electrophilic aromatic substitution instead.
- Grignard reagents (RMgX) form in dry ether; they react with water, so moisture must be excluded.
Worked example
2-bromo-2-methylpropane is tertiary; in aqueous ethanol it ionises to a tertiary carbocation, so SN1 is fast and the solution becomes turbid quickly. 1-bromobutane is primary and reacts mainly by SN2, which is slow under SN1 conditions. This is why the order of SN1 reactivity is 3°>2°>1°.
Traps
- Reversing SN1 and SN2 reactivity orders.
- Forgetting inversion in SN2 and racemisation in SN1.
- Predicting the Saytzeff product when a bulky base favours the Hofmann product.
- Expecting haloarenes to undergo easy SN2.
- Using wet ether for Grignard preparation.