Chemical reactions

The essentials

Haloalkanes react mainly by nucleophilic substitution, elimination and reaction with metals. The polar C–X bond makes the carbon attractive to nucleophiles, and the halide ion is a good leaving group.

Key points

• SN2: one-step, the nucleophile attacks from the side opposite the leaving group; rate = k[RX][Nu⁻] (second order); configuration is inverted. Reactivity: CH₃X > 1° > 2° > 3° because bulky groups hinder attack.
• SN1: two steps via a carbocation; rate = k[RX] (first order); favoured in polar protic solvents; reactivity 3° > 2° > 1° > CH₃X, following carbocation stability. Allylic and benzylic halides are very reactive because their carbocations are resonance stabilised.
• For the same alkyl group, reactivity is R–I > R–Br > R–Cl > R–F.
• KCN gives alkyl cyanides (R–CN) mainly, whereas AgCN gives isocyanides (R–NC). KNO₂ gives alkyl nitrites; AgNO₂ gives nitroalkanes.
• Elimination (dehydrohalogenation): heating with alcoholic KOH gives alkenes. Saytzeff (Zaitsev) rule: the more substituted alkene is the major product. 2-Bromobutane gives mainly but-2-ene.
• With metals: Mg in dry ether gives Grignard reagents (RMgX), which react with water to give alkanes. Wurtz reaction: 2RX + 2Na (dry ether) → R–R. Wurtz–Fittig: aryl halide + alkyl halide + Na; Fittig: two aryl halides + Na.
• Haloarenes resist nucleophilic substitution because of resonance (partial double-bond character of C–Cl), sp² carbon, instability of the phenyl cation, and repulsion of nucleophile by the electron-rich ring. Chlorobenzene gives phenol only at 623 K and 300 atm with NaOH. –NO₂ groups at ortho and para positions make substitution easier.
• In electrophilic substitution, halogens are deactivating but ortho/para directing.

Common mistakes

• Using aqueous KOH when the question wants elimination; aqueous KOH mainly substitutes, alcoholic KOH eliminates.