IR Identification via Functional Group Absorptions

Steps:

• Examine spectrum for broad O-H stretch (3200-3600 cm⁻¹) indicating alcohol.
• Check for C=C stretch (1620-1680 cm⁻¹) suggesting alkene, without C=O (1700-1750 cm⁻¹).
• Rule out carbonyl compounds lacking O-H or showing extra bands like aldehyde C-H (2700-2800 cm⁻¹).
• Match to unsaturated alcohol structure with no other functional groups.

Why B is correct:

• CH₃CH=CHCH₂OH features allylic alcohol, showing broad O-H and C=C stretches per Hooke's law for vibrational frequencies, without carbonyl absorption.

Why the others are wrong:

• A: CH₃COCH₃ is a ketone with strong C=O stretch but no O-H or C=C.
• C: CH₃CH₂COCH₃ is a ketone with C=O stretch but lacks O-H and C=C.
• D: CH₃CH(OH)CH₂CHO has O-H but also aldehyde C=O and C-H stretches absent in spectrum.

Final answer: B