Why is carbon-nitrogen bond length in aromatic amines shorter than in aliphatic amines?

Explanation

Due to +R effect of `-NH_2` group, the carbon-nitrogen bond in aromatic amines (refer to resonating structures of aniline on page 15/43 acquires some partial double bond character. On the other hand, in aliphatic amines, the carbonnitrogen bond has only single bond character. Therefore, carbon- nitrogen bond length in aromatic amines is shorter than in aliphatic amines.