IR Spectrum Identifies Carboxylic Acid Functional Group

Steps:

• Identify key IR absorption bands: broad O-H stretch (2500–3300 cm⁻¹) and C=O stretch (1710 cm⁻¹).
• Match broad O-H to hydrogen-bonded carboxylic acid, not sharp alcohol O-H.
• Confirm C=O at 1710 cm⁻¹ typical for carboxylic acids, distinguishing from esters or alcohols.
• Rule out compounds lacking both features.

Why C is correct:

• HOCH₂CH₂COOH is a carboxylic acid, exhibiting broad O-H stretch due to dimer hydrogen bonding and C=O stretch per Hooke's law for carbonyl vibration.

Why the others are wrong:

• A: Ester shows C=O (1735 cm⁻¹) but lacks broad O-H; any O-H is sharp from alcohol.
• B: Ether-alcohol shows broad O-H but no C=O absorption.
• D: Identical to C, but question specifies C as correct; possible notation error.

Final answer: C