IR spectroscopy distinguishes functional groups by characteristic absorption bands

Steps:

• Examine peaks for O-H stretch (broad 3200-3600 cm⁻¹ in alcohols/acids), C=O stretch (1700-1750 cm⁻¹ in carbonyls), and aldehydic C-H (2700-2800 cm⁻¹ doublets in aldehydes).
• Spectrum with C=O at ~1715 cm⁻¹ and no O-H or aldehydic C-H identifies a ketone (propanone).
• Spectrum with C=O at ~1725 cm⁻¹ plus aldehydic C-H doublets identifies an aldehyde (propanal).
• Spectrum with broad O-H at ~3300 cm⁻¹, C-O at ~1100 cm⁻¹, and no C=O identifies a secondary alcohol (propan-2-ol).

Why B is correct:

• Option B matches distinct IR signatures: ketone (C=O only), aldehyde (C=O + aldehydic C-H per IR correlation tables), alcohol (O-H only).

Why the others are wrong:

• A includes carboxylic acid (broad O-H overlapping C=O), indistinguishable from alcohol's O-H.
• C has two alcohols (propan-1-ol and propan-2-ol) with nearly identical broad O-H peaks.
• D includes acid (broad O-H + C=O) and alcohol (O-H), lacking a unique third spectrum.

Final answer: B