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Which of the following groups act as electron-donating groups through resonance (+M effect) when attached to a benzene ring? (a) -CONH2 (b) -NO2 (c) -OCOCH3 (d) -COOCH3 (e) -CHO (f) -NHCOCH3

1. (c) -OCOCH3 and (f) -NHCOCH3
2. (a) -CONH2 and (e) -CHO
3. (b) -NO2 and (d) -COOCH3
4. (c) -OCOCH3 and (b) -NO2

Correct answer: (c) -OCOCH3 and (f) -NHCOCH3

Solution

+M (electron releasing by resonance) requires the atom attached to the ring to have an available lone pair. In -OCOCH3 the ring is bonded to O (lone pairs), and in -NHCOCH3 the ring is bonded to N (lone pair); both donate into the ring. By contrast -CONH2, -COOCH3, -CHO are bonded to a carbonyl carbon and -NO2 to N=O, all of which are -M (electron withdrawing).

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