Distinguishing functional groups by IR absorption bands

Steps:

• Identify key IR features: alcohols show broad O-H at 3200-3600 cm⁻¹; carbonyls at 1700-1750 cm⁻¹; aldehydes have unique C-H at 2700-2850 cm⁻¹.
• Match spectra: one with O-H only (alcohol), one with C=O and no O-H (ketone), one with C=O, no O-H, plus aldehyde C-H (aldehyde).
• Eliminate options lacking aldehyde (unique C-H band) or mismatching alcohol type.
• Confirm B fits: propanone (ketone C=O), propanal (aldehyde C=O + C-H), propan-2-ol (secondary alcohol O-H).

Why B is correct:

• Aldehyde's characteristic C-H stretch at 2700-2850 cm⁻¹ (from sp² C-H in CHO group) uniquely identifies propanal, distinguishing it from ketone and alcohol.

Why the others are wrong:

• A lacks aldehyde; includes primary alcohol and acid, both with O-H but no aldehyde C-H.
• C includes acid (broad O-H + C=O) and primary alcohol, missing secondary alcohol distinction.
• D lacks aldehyde; has secondary alcohol and acid, but no propanal's C-H band.

Final answer: B