IR spectrum identifies a saturated ketone

Steps:

• Calculate degree of unsaturation for C4H8O: (2×4 + 2 - 8)/2 = 1, matching a carbonyl group.
• Strong peak at 1720 cm⁻¹ indicates C=O stretch typical of ketones or aldehydes.
• No strong peak above 3000 cm⁻¹ rules out O-H stretch from alcohols or carboxylic acids.
• Strong peak at 1200 cm⁻¹ aligns with fingerprint vibrations in saturated ketones, excluding conjugated systems.

Why B is correct:

• B is butan-2-one (CH₃COCH₂CH₃), whose ketone C=O absorbs near 1715 cm⁻¹ per IR correlation tables, fitting all data with one unsaturation and no O-H.

Why the others are wrong:

• A (alcohol like butenol) shows broad O-H above 3000 cm⁻¹.
• C (aldehyde like butanal) exhibits characteristic aldehydic C-H stretches at 2700-2800 cm⁻¹.
• D (ether like methoxypropene) lacks C=O absorption at 1720 cm⁻¹.

Final answer: B