Question

True about alkyl halides is/are-

• A. Tertiary alkyl halides undergo ${S_N}^2$ substitutions
• B. Alkyl iodides on exposure to sunlight gradually darken
• C. Alkyl chlorides do not give beilstein test
• D. A nucleophilic substitution is most difficult in alkyl iodides

Answer 1

The correct option is B Alkyl iodides on exposure to sunlight gradually darken

Tertiary alkyl halides undergo ${S_N}^1$ and not ${S_N}^2$ substitutions. It is because tertiary carbocations formed as an intermediate is stable(good for $S_{N}1$) and also tertairy alkyl halides are sterically crowded, so it hinders attack of a nucleophile(not favourable for $S_{N}2$).

Alkyl iodides on exposure to sunlight gradually darkens because the bond enthalpy of $C-I$ is low and on exposure to sunlight, it breaks and free iodine start forming which show dark colour.
Alkyl fluorides do not give beilstein test positive.
Nucleophilic substitution is most frequent in alkyl iodides because of its low bond enthalpy.

Hence, (b) is correct.