The C-O bond in carboxylic acid is:  

CONCEPT:

The C-O Bond in Carboxylic Acid

• Carboxylic acids contain a carboxyl group (-COOH), which includes a C=O (carbonyl) bond and a C-O (single bond).
• The C-O bond in carboxylic acids is a result of sp²-sp³ orbital overlap.
• The oxygen of the hydroxyl group (-OH) is sp³ hybridized, while the carbon of the carboxyl group is sp² hybridized due to the presence of the double bond with the oxygen atom in the carbonyl group.
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EXPLANATION:

• The bond length of the C-O bond in carboxylic acid is influenced by resonance.
• In carboxylic acids, resonance occurs between the carbonyl group (C=O) and the hydroxyl group (C-OH):
  O=C-OH ↔ O-C=O^-
• This resonance gives partial double-bond character to the C-O bond, making it shorter than a typical sp²-sp³ single bond.
• Therefore, the C-O bond in carboxylic acid is sp²-sp³ and shorter than expected due to resonance.

Therefore, the correct answer is (sp²-sp³ shorter).