IR spectrum identifies functional groups via characteristic absorption peaks

Steps:

• Count carbons: All options have three carbons, so eliminate based on IR features.
• Recall IR ranges: Alkenes show C=C stretch at 1620–1680 cm⁻¹; alcohols show broad O–H at 3200–3600 cm⁻¹; carbonyls (ketones/esters) show C=O at 1700–1750 cm⁻¹.
• Assume spectrum shows only C=C peak, no O–H or C=O, matching an alkene.
• Match to options: Only ethene fits pure alkene structure.

Why C is correct:

• CH₂=CH₂ has C=C bond, absorbing at 1640 cm⁻¹ per Hooke's law for vibrational frequencies.

Why the others are wrong:

• A: Shows broad O–H stretch from alcohol group.
• B: Shows strong C=O stretch from ketone.
• D: Shows C=O stretch from ester.

Final answer: C