IR spectrum identifies functional groups via characteristic absorption bands

Steps:

• Examine the IR spectrum for key peaks: broad O-H stretch around 3000 cm⁻¹ and carbonyl C=O at 1710 cm⁻¹ indicate carboxylic acid.
• Rule out alcohols (no sharp O-H, only broad with C=O suggests acid, not ester).
• Check for absence of C=C or other unsaturations that would show 1600-1700 cm⁻¹ without O-H.
• Match to options: D has both -COOH features.

Why D is correct:

• Carboxylic acids show broad, dimeric O-H stretch (2500-3300 cm⁻¹) and C=O at 1710 cm⁻¹ due to hydrogen bonding.

Why the others are wrong:

• A: Ketone lacks O-H stretch, only C=O at 1715 cm⁻¹.
• B: Alcohol has broad O-H but no C=O absorption.
• C: Ester shows C=O at 1735 cm⁻¹ and C-O at 1100 cm⁻¹, but no broad O-H.

Final answer: D