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Compare the carbon-nitrogen (C-N) bond connecting the nitro group to the ring in two compounds: structure (I) is nitrobenzene, and structure (II) is nitrobenzene bearing two bulky alkyl substituents at both ortho positions (relative to the -NO2 group). Choose the correct statement about the C-N bond length.

1. carbon-nitrogen bond length structure I is greater than that in structure II
2. carbon-nitrogen bond length in structure I is less than in structure II
3. carbon-nitrogen bond length in both structure is same
4. It can not be compared

Correct answer: carbon-nitrogen bond length in structure I is less than in structure II

Solution

In nitrobenzene (I) the nitro group is coplanar with the ring, so the ring pi-system conjugates with the -NO2 group. This conjugation gives the C-N bond partial double-bond character, shortening it. In (II) the two bulky ortho substituents sterically force the -NO2 group to rotate out of the aromatic plane; conjugation is lost, the C-N bond becomes essentially a pure single bond, and is therefore longer than in (I). Thus the C-N bond in I is shorter (less) than in II.

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