IR spectrum identifies carbonyl functional group in C4H8O

Steps:

• Calculate degree of unsaturation: (2C + 2 - H - X + N)/2 = (10 - 8)/2 = 1, indicating one double bond or ring.
• Examine IR for key absorptions: strong band ~1710 cm⁻¹ suggests C=O stretch.
• Rule out O-H stretch ~3300 cm⁻¹ (broad), absent here, eliminating alcohols.
• Match to C4H8O isomers: absorption fits aldehyde or ketone, not alkene or ether.

Why D is correct:

• D is butanal, with C=O stretch at 1725 cm⁻¹ for aldehydes per IR correlation tables.

Why the others are wrong:

• A (2-butanone): ketone C=O at 1715 cm⁻¹, but spectrum shows 1725 cm⁻¹ specific to aldehyde.
• B (2-butanol): O-H stretch expected at 3300 cm⁻¹, absent in spectrum.
• C (cyclobutanol): cyclic alcohol with O-H absorption, not matching carbonyl band.

Final answer: D