Haloalkanes and Haloarenes
Haloalkanes and Haloarenes
This domain tests substitution and elimination mechanisms, reactivity order and the special behaviour of aryl halides.
Core rules
- SN2: one step, backside attack, inversion, favoured by 1° halides and polar aprotic solvents.
- SN1: carbocation intermediate, racemisation, favoured by 3° halides and polar protic solvents.
- Reactivity to SN1: 3° > 2° > 1°; to SN2: 1° > 2° > 3°.
- Elimination with alcoholic KOH follows Saytzeff's rule: the more substituted alkene predominates.
- Haloarenes resist nucleophilic substitution because of partial double-bond character and sp² carbon; they undergo electrophilic substitution.
- Named reactions: Finkelstein (NaI/acetone), Swarts (AgF), Wurtz, Sandmeyer.
Worked example 2-Bromobutane with aqueous KOH gives butan-2-ol by substitution. With alcoholic KOH, elimination gives but-2-ene as the major product and but-1-ene as minor. The reagent, not the substrate alone, decides substitution versus elimination.
Traps
- Saying haloarenes undergo SN2 readily.
- Forgetting that SN1 at a chiral centre gives racemisation, not complete inversion.
- Writing the major Saytzeff product as the less substituted alkene.