IR spectrum identifies carboxylic acid functional group

Steps:

• Examine the spectrum for a broad absorption band between 2500–3300 cm⁻¹, characteristic of O-H stretching in carboxylic acids due to hydrogen bonding.
• Confirm presence of a sharp C=O stretch around 1710 cm⁻¹, typical for carbonyl in carboxylic acids.
• Absence of sharp O-H peak rules out alcohols; no C=O rules out ketones.
• Lack of functional group absorptions beyond C-H stretches eliminates alkanes.

Why C is correct:

• CH₃COOH exhibits a broad, intense O-H stretch (2500–3300 cm⁻¹) from dimeric hydrogen bonding and C=O stretch at ~1710 cm⁻¹, matching carboxylic acid IR signature.

Why the others are wrong:

• A: CH₃COCH₃ shows C=O at ~1715 cm⁻¹ but no broad O-H, indicating ketone only.
• B: CH₃CH₂OH displays broad O-H at ~3300 cm⁻¹ but lacks C=O stretch, typical of alcohol.
• D: CH₃CH₂CH₃ has only C-H stretches (~2900–3000 cm⁻¹), no O-H or C=O for alkane.

Final answer: C