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IR spectroscopy table and how to read a spectrum

An IR spectrum tells you which functional groups are present. You only need a handful of absorptions, and the region above 1500 cm⁻¹ is where to look first.

Spectroscopy · Updated 3 November 2026

The key absorptions

BondWhere it's foundWavenumber / cm⁻¹What the peak looks like
O–Halcohols, phenols3200–3550strong, broad (hydrogen bonding)
O–Hcarboxylic acids2500–3300very broad, often overlapping C–H
N–Hamines, amides3300–3500medium; primary amines often show two peaks
C–Halkanes (sp³)2850–3000strong, sharp
C–Halkenes, arenes (sp²)3000–3100medium, just above 3000
C–Hterminal alkynes (sp)about 3300sharp
C–Haldehydesabout 2720 and 2820two weak peaks
C≡Nnitriles2210–2260medium, sharp
C≡Calkynes2100–2260weak
C=Oketones, aldehydes, acids, esters, amides1650–1820strong, sharp: the easiest peak to spot
C=Calkenes1620–1680weak to medium
C=Caromatic ringsabout 1450–1600several medium peaks
C–Oalcohols, ethers, esters, acids1000–1300strong

Telling carbonyl compounds apart

CompoundC=O / cm⁻¹ (approx.)Other clues
Acyl chlorideabout 1800highest C=O
Ester1735–1750strong C–O at 1000–1300
Aldehydeabout 1725C–H at about 2720 and 2820
Ketoneabout 1715no O–H, no aldehyde C–H
Carboxylic acidabout 1710very broad O–H, 2500–3300
Amide1650–1690N–H near 3300 (unless N is fully substituted)

How to read a spectrum in four checks

  1. Is there a strong peak at 1650–1820? Yes → there's a C=O.
  2. Is there a broad peak above 2500? Very broad from 2500–3300 with a C=O → carboxylic acid. Broad at 3200–3550 without C=O → alcohol.
  3. Anything at 2100–2260? → C≡N or C≡C.
  4. C–H just above or below 3000? Above → alkene or arene C–H. Only below → saturated.

The region below about 1500 cm⁻¹ is the fingerprint region: it's unique to each molecule, so it's used to match a compound against a known spectrum rather than to identify groups.

Example

OOH
ethanoic acid: very broad O–H at 2500–3300 and C=O near 1710
O
butan-2-one (butanone): C=O near 1715, no O–H
OH
propan-1-ol: broad O–H near 3330, C–O near 1060

Use IR with NMR to pin down the full structure: see ¹H NMR splitting.

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