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

Explanation

Primary and secondary alkyl halides prefer to undergo substitution reaction by `S_(N)2` mechanism. Thus n-butyl chloride and sec-butyl chloride react with aqueous NaOH to form the corresponding alcohols as the substitution product. On the other hand, tert-alkyl halides prefer to undergo elimination reaction (mainly by E2 mechanism). Thus, tert-butyl bromide reacts with ethanolic KOH to form isobutene as the predominant product. `underset("n-butyl chlorid")(CH_(3)CH_(2)CH_(2)CH_(2))-Clunderset(S_(N)2)overset(aq.NaOH)(rarr)CH_(3)CH_(2)CH_(2)CH_(2)OH`
`underset("sec-butyl chloride")(CH_(3)CH_(2)underset(CH_3)underset(|)(C)H-Cl)underset(S_(N)2)overset("aq.NaOH)(rarr)CH_(3)CH_(2)underset(CH_3)underset(|)(C)H-OH`