Arrange the following compounds in increasing order of length of their C-N bond.

The bond order of C-N bond is 2 in III, shortest, I in II, longest bond. There is greater extent of delocalization of nitrogen lone pair in IV because it brings negative charge on oxygen than in I where negative charge moves on carbon. Hence, C-N bond acquire greater partial double bondcharacter in IV than in I. This results in greater shortening of C-N bond length in IV than in I, although I has more resonating structures.