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For each of the following pairs of resonance (canonical) structures, identify which member of the pair is the LESS stable contributor: (a) CH2(+)-O-CH3 vs CH2=O(+)-CH3; (b) two canonical forms of a fused bicyclic aromatic system; (c) H-C(=O)-CH2(-) vs H-C(-O(-))=CH2; (d) an amide-type delocalized system vs its charge-separated form; (e) a five-membered ring with exocyclic CH2(+) vs the same ring with exocyclic CH2(-).

1. The first structure in each pair
2. The second structure in each pair
3. Both structures are equally stable
4. Cannot be determined

Correct answer: The first structure in each pair

Solution

Stability of resonance contributors is judged by: maximum number of covalent bonds, complete octets, and placement of negative charge on electronegative atoms (and positive charge on electropositive atoms). In each pair as written, the first structure places the positive charge on carbon or leaves carbon electron-deficient, while the second has the charge on the more electronegative oxygen/nitrogen with better octet satisfaction, making the first the less stable contributor.

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