Give reasons for the following.

(i) Aniline does not undergo Friedel-Crafts reaction.

(ii) ${\left({\mathrm{CH}}_3\right)}_2\mathrm{NH}$ is more basic than ${\left({\mathrm{CH}}_3\right)}_3\mathrm{N}$ is an aqueous solution.

(iii) Primary amines have higher boiling point than tertiary amines.  

(i) Aniline is a Lewis base. It reacts with the Lewis acid AlCl₃ used in Friedel-Crafts reaction to produce a salt. Hence, it does not show Friedel-Crafts reaction.

(ii) Basic nature of amines in aqueous medium depends on + I-effect, H-bonding and steric-effect. The combined effect shows that ${\left({\mathrm{CH}}_3\right)}_2\mathrm{NH}$ is more basic than ${\left({\mathrm{CH}}_3\right)}_3\mathrm{N}.$

In ${\left({\mathrm{CH}}_3\right)}_2\mathrm{NH}$ H-bonding is more than in ${\left({\mathrm{CH}}_3\right)}_3\mathrm{N}$, which predominates over the stability due to +I effect of three $-{\mathrm{CH}}_3$ groups.

(iii) Primary amines $\left(1^0\right)$ show hydrogen bonding whereas tertiary amines $\left(3^0\right)$ do not have hydrogen bonding. The net attractive force in case of primary amines is more than that of tertiary amines. Hence, primary amines have higher boiling point than that of tertiary amines.