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Formic acid (HCOOH) can be represented as a resonance hybrid of four contributing canonical structures, labelled I, II, III and IV, that differ in the placement of the formal charges and the C=O / C-O bonds. Arrange these four contributing structures in the correct order of decreasing stability (most stable first).

1. I > II > III > IV
2. I > II > IV > III
3. I > III > II > IV
4. I > IV > III > II

Correct answer: I > II > III > IV

Solution

For a resonance hybrid, the relative weight (stability) of a canonical form follows standard rules: (1) structures with more covalent bonds and no charge separation are most stable; (2) structures with negative charge on the more electronegative atom (O) are favoured; (3) structures with complete octets are favoured; (4) structures with like-charge proximity or charge on electropositive atoms are least stable. The neutral, fully bonded Kekule form of formic acid (I) is the largest contributor, followed by the singly charge-separated forms in order of how favourably the charges are distributed, giving the order I > II > III > IV.

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