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Consider the delocalization of the negative charge in the following carbanions by drawing their resonance (canonical) structures: (a) CH3-C(=O)-CH2⁻ and (b) H-C(=O)-CH=CH-CH2⁻. Counting the original structure plus every additional contributing structure, what is the total number of resonance structures for (a) and (b) respectively?

1. 2 and 3
2. 2 and 2
3. 3 and 4
4. 1 and 2

Correct answer: 2 and 3

Solution

(a) CH3-C(=O)-CH2⁻ is an enolate. The carbanion lone pair conjugates with the C=O: one structure has the charge on carbon (C=O intact), the other has the charge on oxygen with a C=C formed. That is 2 contributing structures. (b) H-C(=O)-CH=CH-CH2⁻ is an extended/conjugated carbanion. The negative charge can stay on the terminal CH2, shift onto the internal carbon as the C=C migrates, and finally reach the carbonyl oxygen giving an alkoxide with the full conjugated chain. That gives 3 contributing structures. So the answers are 2 and 3.

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