Broad O-H stretch in carboxylic acids

Steps:

• Identify the molecule: but-2-enoic acid (CH3CH=CHCOOH) contains a carboxylic acid group (-COOH).
• Note the peak: broad absorption at 2500-3000 cm⁻¹ indicates hydrogen-bonded stretching vibration.
• Consult the table: O-H for hydroxy groups absorbs at 3200-3600 cm⁻¹, but carboxylic O-H extends lower due to dimerization.
• Match to functional group: this range fits the broad O-H of -COOH, not other bonds.

Why D is correct:

• Carboxylic acids show a broad O-H stretch from 2500-3300 cm⁻¹ due to strong intermolecular hydrogen bonding in dimers.

Why the others are wrong:

• A: C-O stretches at 1040-1080 cm⁻¹, far below the observed range.
• B: C=O stretches at 1660-1680 cm⁻¹ (carbonyl/amide), not broad or in 2500-3000 cm⁻¹.
• C: O-H (hydroxy) typically 3200-3600 cm⁻¹, narrower and higher than the given broad peak.

Final answer: D