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Considering the stability of sigma-complex, explain why -OCH_3 is o-/p-orienting -NO_2 is meta-orienting . |
Answer» Solution :Electrophilic SUBSTITUATION reaction in anisole proceeds VIA the following reaction mechanism-  There is an extraordinarily stable (every atom has its octer fulfilled) resonance structure in both ortho-and para- `SIGMA`-complex, but there is no such resonance structure in the meta-`sigma`-complex. So, ortho-and para-`sigma`-complex are more stable than meta-`simga`-complex. Consequently, electrophilic substitution proceeds easily and rapidly via ortho-and para-`sigma`-complex resulting in ortho-and para-substituted compounds as major products. Thus, `-OCH_(3)` is ortho-/para-orienting group. Electrophilic substitution reaction in nitrobenzene proceeds via the following reaction mechanism-  Both ortho-and para-`sigma`-COMPLEXES are extraordinarily unstable resonance structures (due to presence of positive charge on two adjacent atoms). However, in meta-`sigma`-complex there is no such resonance structure and so it is more stable than ortho-and para-`sigma`-complex. Thus, the reaction proceeds rapidly via the meta-`sigma`-complex and the meta-substituted compound is obtained as the major product. Thus, `-NO_(2)` is meta-orienting group. |
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