Haloalkanes and Haloarenes
Haloalkanes and Haloarenes tests substitution versus elimination, the SN1/SN2 distinction, and why aryl halides are so unreactive.
Core rules
- SN2: one step, backside attack, inversion of configuration, second order; favoured by CH₃ > 1° > 2° > 3°, strong nucleophile, polar aprotic solvent.
- SN1: two steps through a carbocation, racemisation, first order; favoured by 3° > 2° > 1°, polar protic solvent.
- C–X bond strength falls I < Br < Cl < F, so reactivity in substitution follows R–I > R–Br > R–Cl > R–F.
- Aryl halides resist nucleophilic substitution: the carbon is sp², the C–X bond has partial double-bond character by resonance, the phenyl cation is unstable, and the electron-rich ring repels nucleophiles. Chlorobenzene needs NaOH at 623 K and 300 atm with copper.
- Chlorobenzene is deactivating but o,p-directing in electrophilic substitution.
- Named reactions: Finkelstein (NaI in dry acetone, SN2), Swarts (AgF or Hg₂F₂), Wurtz (symmetrical alkanes only), Grignard (RMgX from R–X and dry Mg in ether), Saytzeff elimination (more substituted alkene).
Worked example
2-Bromobutane with alcoholic KOH gives but-2-ene as the major product. The base removes a β-hydrogen from the more substituted neighbouring carbon, and the more alkylated alkene is the more stable (Saytzeff rule). With aqueous KOH, substitution dominates instead and butan-2-ol forms, so the solvent decides the outcome.
Traps that cost marks
- Saying SN1 gives complete inversion instead of racemisation with inversion bias.
- Expecting a single pure product from Wurtz coupling of two different halides.
- Ranking aryl halides as more reactive than alkyl halides toward AgNO₃.
- Ignoring stereochemistry in SN2 answers.
- Forgetting allylic and benzylic halides are exceptionally reactive by both mechanisms.