Organic Chemistry: Haloalkanes, Haloarenes, Alcohols, Phenols, Ethers
This section covers haloalkanes, haloarenes, alcohols, phenols and ethers: nucleophilic substitution, elimination, Grignard reagents, aryl halide inertness, acidity of phenols, and distinguishing tests. Mechanisms and reactivity orders are central.
Core rules
- SN2 is one-step, bimolecular and gives inversion; SN1 is two-step, unimolecular and gives racemisation. SN1 reactivity: 3° > 2° > 1°, because carbocation stability increases.
- Haloarenes are less reactive toward nucleophilic substitution because the C–X bond has partial double-bond character and the ring repels nucleophiles.
- Phenols are acidic because the phenoxide ion is resonance-stabilised; alcohols are neutral. FeCl₃ gives a violet colour with phenols.
- Williamson synthesis: R–O⁻ + R′–X → R–O–R′. Primary alkyl halides work best; tertiary halides favour elimination.
Worked example
Arrange 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane in order of SN1 reactivity in aqueous ethanol. The rate depends on carbocation stability: 2-bromo-2-methylpropane forms a 3° carbocation, 2-bromobutane forms a 2° carbocation, and 1-bromobutane forms a 1° carbocation. Therefore the order is 2-bromo-2-methylpropane > 2-bromobutane > 1-bromobutane. With AgNO₃ in ethanol, the tertiary halide gives the precipitate fastest.
Traps
Do not use SN1 order for SN2; do not expect haloarenes to hydrolyse easily; do not say phenol gives a Lucas test; remember that tertiary halides in Williamson synthesis undergo elimination; distinguish alcohol and phenol tests carefully.