Chemistry - GOC Question with Solution | TestHub

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 methyl groups force the $\mathrm{N}{\mathrm{O}}_2$ 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.