Infrared (IR) spectroscopy identifies functional groups by the way bonds absorb IR light and vibrate (stretch and bend). Every bond has a characteristic vibration frequency determined by the masses of the atoms and the bond's stiffness, so specific absorptions act as fingerprints for specific groups. IR does not usually give the whole structure, but it rapidly tells you which functional groups are present.

Focus your attention on the diagnostic region (roughly 4000–1500 cm⁻¹), where the most useful signals appear. The handful of absorptions to know cold:

O–H (alcohol): ~3200–3550 cm⁻¹, broad and rounded — the most recognizable peak in all of IR.
O–H (carboxylic acid): ~2500–3300 cm⁻¹, very broad, overlapping the C–H region.
N–H (amine/amide): ~3300–3500 cm⁻¹, sharper than O–H; primary amines show two prongs.
C=O (carbonyl): ~1700–1750 cm⁻¹, a strong, sharp peak — arguably the single most important IR signal, present in aldehydes, ketones, acids, esters, and amides.
C≡N and C≡C: ~2100–2260 cm⁻¹, a narrow window where few other things absorb.

The region below ~1500 cm⁻¹ is the fingerprint region — a complex pattern unique to each molecule that is generally too intricate to interpret peak-by-peak but can confirm identity by matching to a known spectrum. The practical strategy is a checklist: Is there a broad O–H? A strong C=O? If both, think carboxylic acid. C=O but no O–H points toward a ketone, aldehyde, or ester. No C=O and no O–H narrows things toward hydrocarbons, ethers, or amines. IR pairs beautifully with MS — MS gives the formula and weight, IR tells you the functional groups within it.

"What It Absorbs"