22.1.1—Analyse an infrared spectrum of a simple

Infrared spectroscopy measures absorption of bond vibrations. Functional groups give characteristic absorption regions, so compare peaks with the supplied data rather than relying on one isolated feature.

Use the fingerprint region to support identity, but prioritise diagnostic absorptions such as O–H, C=O, C–H and N–H. Peak position and broadness both carry information.

A broad absorption around 3200–3600 cm⁻¹ suggests O–H; a strong peak near 1700 cm⁻¹ suggests C=O. Together they can distinguish an alcohol from a carbonyl compound.

IR identifies functional groups, not usually the complete carbon skeleton. Do not assign a group from a single noisy peak without checking the rest of the spectrum.