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Which one of the following groups is unable to take part in resonance with an adjacent suitable group?

1. (a) -COOH
2. (b) -COOCH3
3. (c) -COCl
4. (d) -NH3

Correct answer: (d) -NH3

Solution

Groups like -COOH, -COOCH3 and -COCl all contain a C=O pi bond (and adjacent lone pairs) that can delocalize, so they participate in resonance. In -NH3 the nitrogen has used all four orbitals for sigma bonds (three N-H plus one to the ring) and bears a positive charge with no lone pair and no pi system, so it cannot donate or accept electrons through resonance; it acts only by an inductive (-I) effect.

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