IR spectrum identifies carboxylic acid via broad O-H and C=O peaks

Steps:

• Examine O-H region: broad absorption 2500-3300 cm⁻¹ indicates hydrogen-bonded O-H typical of carboxylic acids.
• Check C=O region: sharp peak near 1710 cm⁻¹ confirms carbonyl group.
• Note absence of aldehyde C-H stretches around 2700-2800 cm⁻¹, ruling out aldehydes.
• Verify no sharp O-H like alcohols, confirming acidic dimer formation.

Why C is correct:

• Propanoic acid (CH₃CH₂COOH) shows broad, intense O-H stretch from dimer hydrogen bonding and C=O at 1710 cm⁻¹ per Hooke's law for carbonyl vibration.

Why the others are wrong:

• A: Propan-1-ol has sharp, narrow O-H stretch 3200-3600 cm⁻¹ from free alcohols, not broad acidic band.
• B: Propanal lacks broad O-H; shows twin C-H peaks at 2720 and 2820 cm⁻¹ for aldehydes.
• D: Propanone has only C=O at 1715 cm⁻¹, no O-H absorption at all.

Final answer: C