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Which of the following groups behave as electron-withdrawing groups by resonance (-M / -R effect)? (a) -COOH (b) -CONHCH3 (c) -COCl (d) -CN (e) -O-CH=CH2 (f) an amine N with its lone pair on a ring

1. (a), (b), (c) and (d) only
2. (a), (c), (d) and (e) only
3. (b), (c), (e) and (f) only
4. all of (a) to (f)

Correct answer: (a), (b), (c) and (d) only

Solution

Groups -COOH, -CONHCH3, -COCl and -CN all contain a multiply-bonded electronegative atom (carbonyl or nitrile) that pulls pi electron density toward itself by resonance, so they are -M (electron-withdrawing). The ether-vinyl group -O-CH=CH2 donates the oxygen lone pair (+M, electron-donating), and the amine nitrogen lone pair on a ring also donates electrons (+M). Hence only (a), (b), (c), (d) are electron withdrawing by resonance.

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