Some alkyl halides undergo substitution reaction whereas some undergo elimination reaction on treatment with bases. Discuss the structural features of alkyl halides with the help of examples which are responsible for this difference.

Some alkyl halides undergo substitution reaction whereas some undergo

1.	Some alkyl halides undergo substitution reaction whereas some undergo elimination reaction on treatment with bases. Discuss the structural features of alkyl halides with the help of examples which are responsible for this difference.
Answer» Solution :A chemical reaction is the result of COMPETITION. An alkyl HALIDE, with hydrogen atom on carbons neighbouring to halogen carrying carbon, has two COMPETING routes: substitution and elimination. Which route will be taken depends upon the nature of alkyl halide, strength and size of base/nucleophile and reaction conditions. Thus a bulkier nucleophile like `C_(4)H_(9)O^(-)`will prefer to act as a base. It will abstract a proton rather than approach a tetravalent carbon atom for steric reasons. On the contrary, a smaller nucleophile will attach itself to the carbon carrying the halogen. A primary alkyl halide will prefer a `S_(N)2`reaction, a secondary halide will tend to give `S_(N)2`or elimination reaction depending upon the strength of the base/nucleophile and a tertiary alkyl halide will give `S_(N)1`or elimination reaction depending upon the stability of the carbocation or the more substituted alkene.