Some alkyl halides undergo substitution 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.

(N/A) The competition between substitution and elimination reactions in alkyl halides depends on several structural and reaction conditions:
$1$. Nature of Alkyl Halide: Primary $(1^{\circ})$ alkyl halides prefer substitution $(S_N2)$ due to less steric hindrance. Tertiary $(3^{\circ})$ alkyl halides prefer elimination $(E2)$ due to high steric hindrance around the electrophilic carbon,which hinders nucleophilic attack but facilitates the removal of a $\beta$-hydrogen by a base.
$2$. Nature of Base: Strong and bulky bases favor elimination,while weaker or less hindered bases favor substitution.
$3$. Solvent and Temperature: High temperatures generally favor elimination (as it is entropy-favored),while lower temperatures favor substitution.
Examples:
- Substitution: $CH_3CH_2Cl + KOH(aq) \rightarrow CH_3CH_2OH + KCl$.
- Elimination: $CH_3CH_2Cl + KOH(alc) \rightarrow CH_2=CH_2 + KCl + H_2O$.