Haloalkanes and Haloarenes
Haloalkanes and Haloarenes tests substitution and elimination mechanisms, reactivity trends and named reactions. The key is to classify the substrate and the reagent.
Mechanisms
SN2 is concerted, one-step, favoured by primary halides and polar aprotic solvents; it gives inversion of configuration. SN1 is two-step via a carbocation, favoured by tertiary halides and polar protic solvents; it gives racemisation. Elimination with alcoholic KOH follows Zaitsev's rule, giving the more substituted alkene as the major product. Aqueous KOH favours substitution.
Haloarenes
Aryl halides are much less reactive toward nucleophilic substitution because the C-X bond has partial double-bond character, the ring is sp2 hybridised, and a phenyl cation is unstable. They undergo electrophilic substitution at ortho and para positions. Named reactions include Finkelstein (RI + NaI in acetone), Swarts (RCl + AgF), Wurtz coupling and Sandmeyer conversion of diazonium salts.
Worked example
2-bromobutane with alcoholic KOH undergoes elimination. Removing H from C3 gives but-2-ene; removing H from C1 gives but-1-ene. But-2-ene is more substituted and is the major product by Zaitsev's rule. With aqueous KOH, substitution dominates and butan-2-ol is formed instead.
Traps
Using SN1 for a primary halide, expecting SN2 at an aryl carbon, and reversing the major alkene under Zaitsev's rule are common mistakes.