Haloalkanes and Haloarenes
Haloalkanes and haloarenes tests preparation, nucleophilic substitution, stereochemistry and the environmental behaviour of halogenated compounds.
Preparation and reactivity
Haloalkanes are prepared from alcohols with HX, PX3 or SOCl2, and by halogen exchange (Finkelstein, Swarts). The C–X bond is polar, so carbon is electrophilic. Reactivity toward SN follows 1° > 2° > 3° for SN2 and 3° > 2° > 1° for SN1.
SN1 and SN2
SN2 is a one-step bimolecular reaction with backside attack, inversion of configuration, and rate = k[substrate][nucleophile]. SN1 is two-step, via a carbocation, with racemisation and rate = k[substrate]; rearrangements are possible. Allylic and benzylic halides are unusually reactive; aryl and vinyl halides are inert toward SN because of partial double-bond character and sp2 hybridisation.
Reactions and environment
Haloalkanes undergo elimination with alcoholic KOH, Wurtz coupling with Na, and Grignard formation with Mg in dry ether. Haloarenes undergo electrophilic substitution; the halogen is ortho/para directing. Chloroform, DDT and CFCs are environmental concerns: CFCs deplete ozone, and DDT is persistent and bioaccumulates.
Worked example
2-bromobutane with aqueous KOH gives butan-2-ol. With alcoholic KOH it gives but-2-ene. Aqueous KOH favours substitution; alcoholic KOH favours elimination. SN2 at the chiral carbon would invert configuration; SN1 would give a racemic mixture because the planar carbocation is attacked from both faces.
Mark-losing traps
Writing SN1 for a primary halide, forgetting that SN2 needs a strong nucleophile and aprotic solvent, claiming aryl halides undergo easy SN, and omitting stereochemical consequences are common. Also distinguish Finkelstein (NaI, acetone) from Swarts (AgF) reactions.