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Assam ASSEB HS Chemistry study materials
Assam ASSEB HS Chemistry study resources

Assam ASSEB HS Chemistry: Haloalkanes and Haloarenes

Assam ASSEB HS Chemistry · Work through your exam, one topic at a time.

All guide sections

Haloalkanes and Haloarenes

What this domain tests

Preparation methods, classification, SN1 and SN2 mechanisms and their stereochemistry, elimination, Grignard reagents, and the distinctive behaviour of aryl halides.

Core rules

  • Preparation: alcohols with HX, PCl₅ or SOCl₂; alkanes by free-radical halogenation; alkenes by addition of HX or X₂; Sandmeyer reaction for aryl halides; Finkelstein (NaI in acetone) and Swarts (AgF) exchange reactions.
  • SN2 is a single concerted step with backside attack, giving inversion of configuration. It is favoured by primary substrates, strong nucleophiles and polar aprotic solvents such as acetone or DMSO.
  • SN1 proceeds through a carbocation, giving racemisation with some inversion. It is favoured by tertiary substrates, weak nucleophiles and polar protic solvents.
  • For a given alkyl group, reactivity is RI > RBr > RCl > RF, because the C–I bond is weakest.
  • Alcoholic KOH causes elimination; the Saytzeff rule predicts the more substituted alkene as the major product.
  • Grignard reagents (RMgX) require dry ether. Water gives an alkane, HCHO gives a primary alcohol, other aldehydes give secondary alcohols, ketones give tertiary alcohols, and CO₂ followed by acid work-up gives a carboxylic acid.
  • Aryl halides resist SN1 and SN2 because of partial double-bond character in the C–X bond, the sp² carbon and the unstable phenyl cation; they react instead by the benzyne mechanism under extreme conditions.

Worked example

2-Bromobutane treated with aqueous KOH undergoes mainly SN1 because the secondary carbocation is reasonably stable; the product is butan-2-ol, obtained as a nearly racemic mixture. The same substrate with alcoholic KOH undergoes E2 elimination, and Saytzeff's rule makes but-2-ene the major product rather than but-1-ene.

Traps that cost marks

Choose the reagent by solvent, not by name alone: aqueous KOH substitutes, alcoholic KOH eliminates. SN1 of a chiral substrate gives racemisation, not pure inversion. Chloroform must be stored in a dark, closed bottle with a little ethanol to prevent phosgene formation.