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Which of the following statements is correct regarding the carboxylate-type anions acetate (CH3COO-) and formate (HCOO-)?

1. The two carbon-oxygen bonds are equal in length in the acetate ion
2. The resonance (canonical) structures of the acetate ion are non-equivalent
3. The two carbon-oxygen bonds are unequal in length in the formate ion
4. The resonance structures of the formate ion are non-equivalent

Correct answer: The two carbon-oxygen bonds are equal in length in the acetate ion

Solution

Both acetate and formate ions are stabilized by two equivalent resonance structures in which the negative charge is delocalized equally over the two oxygen atoms. As a result, the two carbon-oxygen bonds in each ion are identical in length (intermediate between a single and double bond). Therefore the correct statement is that the two C-O bonds in acetate are equal in length. (The statement that formate's C-O bonds are unequal, and that the resonance structures are non-equivalent, are both false.)

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