The acetate ion, CH3COO−, is a common carboxylate that delocalizes its negative charge over two oxygen atoms through resonance. Omitting the hydrogen atoms bonded to carbon for clarity, the canonical Lewis structures show one C=O double bond and one C–O single bond with a negative charge on the singly bonded oxygen.
(a) Draw the two equivalent resonance structures for the acetate ion, indicating all lone pairs and formal charges.
(b) Calculate the average bond order for each C–O bond in the resonance hybrid.
(c) Identify which C–O bonds in acetate are expected to have identical bond lengths. Justify your answer using resonance and bond-order arguments.
(d) The C=O stretching frequency in molecular acetic acid (CH3COOH) is about 1760 cm−1, whereas the symmetric C–O stretch in acetate is observed near 1415 cm−1. Explain how the difference in vibrational frequency is consistent with the bond orders you calculated in part (b).