Compare the C-N bond lengths a and b marked in the two compounds.
(a) (bond a = ring C-NO₂) (b) (bond b = ring C-NO₂)

Question

Compare the C-N bond lengths a and b marked in the two compounds. (a) (bond a = ring C-NO₂) (b) (bond b = ring C-NO₂)

TestHub · Accepted Answer

In (a) the C-N bond has partial double-bond character because the nitro group is coplanar with the ring and conjugates with it, shortening the bond. In (b) the two ortho methoxy groups force the NO₂ group out of the ring plane (steric inhibition of resonance), the C-N bond loses its double-bond character and becomes longer. Hence b > a.

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