Arrange in decreasing order of the C-N bond length: (I) CH3-NH2, (II) C6H5-NH2, (III) 4-nitroaniline (C-N bond to the ring).

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Arrange in decreasing order of the C-N bond length: (I) $CH X_{3} - NH X_{2}$ , (II) $C X_{6} H X_{5} - NH X_{2}$ , (III) 4-nitroaniline (C-N bond to the ring).

A $I > III > II$
B $II > I > III$
C $I > II > III$
D $III > II > I$

Answer: C

In methylamine the C-N bond is a pure sigma bond between sp3 carbon and nitrogen, hence the longest. In aniline the nitrogen lone pair conjugates with the ring, giving partial double-bond character and a shorter bond. In 4-nitroaniline the para nitro group pulls the lone pair further into the ring, so the C-N bond is the shortest. The order is I > II > III.

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Arrange in decreasing order of the C-N bond length: (I) CHX3​−NHX2​, (II) CX6​HX5​−NHX2​, (III) 4-nitroaniline (C-N bond to the ring).

Answer: C

More questions on Mesomeric effect and steric inhibition of resonance

Arrange in decreasing order of the C-N bond length: (I) $CH X_{3} - NH X_{2}$ , (II) $C X_{6} H X_{5} - NH X_{2}$ , (III) 4-nitroaniline (C-N bond to the ring).