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Arrange the C-N bond length (involving the amino nitrogen) in the following species in increasing order: (I) H2N-CH=CH-C(=O)-CH3, (II) H2N-CH=CH-CH=CH2, (III) H2N-CH=CH-CH2-CH3.

1. I < II < III
2. III < II < I
3. II < I < III
4. I < III < II

Correct answer: I < II < III

Solution

The nitrogen lone pair conjugates into the adjacent pi system. In (I) the lone pair is delocalized all the way onto the electron-withdrawing carbonyl, giving the C-N bond the most double-bond character and the shortest length. In (II) it delocalizes into an extended diene but with no strong acceptor, giving intermediate shortening. In (III) beyond the single C=C there is only a saturated chain, so the least delocalization and the longest C-N bond. Increasing bond length: I < II < III.

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