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Which of the following pairs of structures does NOT represent resonance?

1. H2C=CH-C(=O)-H; CH2(+)-CH=C(-O-)-H
2. CH2=CH-CHCl(+); CH2(+)-CH=CH-Cl
3. (CH3)2CH-C(=O)-O(-); (CH3)2CH-C(-O-)=O
4. CH3-CH2-C(=O)-CH3; CH3-CH=C(-O-)-CH3

Correct answer: CH3-CH2-C(=O)-CH3; CH3-CH=C(-O-)-CH3

Solution

The keto and enol forms of butan-2-one (CH3-CH2-CO-CH3 and CH3-CH=C(OH)-CH3 shown as the enolate) differ by the position of a hydrogen atom, so they are tautomers, not resonance structures. In the other pairs, only electrons move while every atom stays in place, which is genuine resonance.

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