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

Steps:

• Locate broad absorption band around 2500–3300 cm⁻¹, indicating hydrogen-bonded O-H stretch.
• Identify sharp C=O stretch at 1700–1725 cm⁻¹, typical for carbonyl compounds.
• Check for absence of bands like sharp O-H (alcohols) or ester C-O at 1100–1300 cm⁻¹.
• Match combination to carboxylic acid functional group.

Why A is correct:

• Ethanoic acid (CH₃COOH) exhibits broad, dimeric O-H stretch from hydrogen bonding and C=O at 1710 cm⁻¹ per Hooke's law for carbonyl vibration.

Why the others are wrong:

• B. Ethanol: Shows broad O-H but lacks C=O band.
• C. Ethyl ethanoate: Displays ester C=O at 1735 cm⁻¹ without broad O-H.
• D. Propanone: Has only C=O at 1715 cm⁻¹, no O-H absorption.

Final answer: A