Haloalkanes and Haloarenes
Questions here turn on substitution versus elimination, SN1 versus SN2, and the special inertness of aryl halides.
Substitution mechanisms
- SN2: backside attack, second order, inversion of configuration, favoured by 1° halides and polar aprotic solvents
- SN1: carbocation intermediate, first order, racemisation, favoured by 3° halides and polar protic solvents
- SN1 reactivity: 3° > 2° > 1°; SN2 reactivity: 1° > 2° > 3°
- allyl and benzyl halides are fast by both routes; vinyl and aryl halides are inert
Named reactions
- Finkelstein: R–I from R–X with NaI in dry acetone
- Swarts: R–F with AgF or Hg2F2
- Wurtz: R–R using Na in dry ether (symmetrical alkanes only)
- Sandmeyer: Ar–Cl/Br from diazonium salt with Cu2Cl2/Cu2Br2 and HX
- Grignard formation requires dry ether
Haloarenes
- nucleophilic substitution is difficult because of partial double-bond character, but is enabled by o/p electron-withdrawing groups such as NO2, or by harsh conditions through a benzyne intermediate
Worked example
2-Bromo-2-methylpropane with aqueous KOH reacts quickly by SN1: the tertiary carbocation is well stabilised, so the rate is first order and the product can be racemic if the carbon becomes chiral. 1-Bromobutane needs SN2, is much slower, and gives inversion of configuration.
Traps that cost marks
- Using alcoholic KOH and expecting substitution; it favours elimination giving the more substituted alkene (Saytzeff).
- Writing SN1 for a primary halide.
- Claiming chlorobenzene undergoes SN2 readily.