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How to read an IR spectrum

By the MolDraw chemistry team · Updated 11 Oct 2026 · Reviewed against the references listed at the end

A practical, step-by-step method for analysing infrared spectra, with eight annotated spectra of common functional groups redrawn from SDBS/NIST reference data, a carbonyl table, a worked unknown and a practice quiz.

Quick answer: Check 1650–1820 cm⁻¹ for C=O, then 3200–3600 for O–H (broad) or N–H (sharper), then whether C–H bands sit above (sp²) or below (sp³) 3000 cm⁻¹, then 2100–2260 for C≡C/C≡N. Classify a carbonyl by its partner bands, and use the fingerprint region (below 1500) only to confirm.

On this page

  1. What an IR spectrum shows
  2. The four regions
  3. How to read it in 6 steps
  4. Peaks at 3000, 1700, 1050
  5. Functional group IR spectra
  6. Carbonyl positions
  7. Worked example & purity
  8. Practice quiz
  9. FAQ

What an IR spectrum shows

Infrared (IR) spectroscopy measures which frequencies of infrared light a molecule absorbs. Bonds vibrate (stretch and bend) at characteristic frequencies, so each functional group absorbs in a predictable region. The spectrum plots % transmittance (y) against wavenumber in cm⁻¹ (x, running from 4000 on the left to about 500 on the right). Absorptions appear as dips, conventionally called "peaks" or "bands".

Wavenumber is proportional to frequency and energy: stronger bonds and lighter atoms vibrate at higher wavenumbers (C≡C > C=C > C–C; O–H, N–H and C–H near 3000 because H is light).

IR spectrum of ethanol (liquid film)fingerprint region50010001500200025003000350040000255075100Wavenumber (cm⁻¹)Transmittance (%)O–H 3340 broadsp³ C–H 2974C–O 1048CH₂/CH₃ bend
Figure 1. Ethanol: a broad, strong O–H stretch centred near 3340 cm⁻¹ (hydrogen bonding), sp³ C–H stretches just below 3000, and a strong C–O stretch at 1048 cm⁻¹. Redrawn from SDBS/NIST data.

The four regions of an IR spectrum

Region (cm⁻¹)BondsWhat to look for
4000–2500X–H single bondsO–H (broad), N–H, C–H (sp 3300, sp² 3000–3100, sp³ 2850–2960), aldehyde C–H 2720/2820
2500–2000triple bonds, cumulatedC≡N 2210–2260, C≡C 2100–2260, CO₂ artefact 2350
2000–1500double bondsC=O 1650–1820 (strong), C=C 1620–1680, aromatic 1600 & 1500, N–H bend ~1600
1500–500fingerprint regionC–O 1000–1300, C–N, C–X, bending modes; unique to each compound

The full band list with intensities and shapes is in our interactive IR spectroscopy chart (correlation table).

How to read an IR spectrum in 6 steps

  1. Check 1650–1820 cm⁻¹ for C=O. A strong, sharp band here is the single most informative peak. Absent → no carbonyl.
  2. Check 3200–3600 for O–H or N–H. Broad and rounded → O–H (alcohol). Very broad 2500–3300 overlapping C–H → carboxylic acid O–H. One or two sharper bands → N–H (two for NH₂).
  3. Look at 3000 cm⁻¹. Peaks just above 3000 → C–H on sp² carbon (alkene/aromatic); just below → sp³ C–H; sharp 3300 → ≡C–H.
  4. Scan 2100–2260 for C≡C (weak) or C≡N (medium, sharp).
  5. If C=O is present, classify it using its exact position and partner bands (see table): 2720/2820 → aldehyde; 1000–1300 strong C–O → ester; broad O–H → acid; N–H → amide.
  6. Use the fingerprint region to confirm (C–O 1000–1300; aromatic substitution 690–900) and compare with a reference spectrum.
Do not try to assign every peak. Identify the functional groups from the diagnostic regions, then use MS for the formula and NMR for the framework.

Key IR peaks: what does a peak at 3000, 1700 or 1050 cm⁻¹ mean?

  • Peak at ~3000 cm⁻¹: C–H stretching. Below 3000 = sp³ (alkanes, alkyl groups); above 3000 = sp² (alkenes, aromatics).
  • Peak at ~1700 cm⁻¹: a carbonyl C=O stretch, very strong. Ketones ~1715, aldehydes ~1725–1730, esters ~1735–1750, carboxylic acids ~1710, amides ~1650–1690, acid chlorides ~1800. Conjugation lowers it by about 20–30 cm⁻¹.
  • Broad peak at 3200–3550 cm⁻¹: hydrogen-bonded O–H of an alcohol or phenol.
  • Strong peak at 1000–1300 cm⁻¹: C–O stretch (alcohols, ethers, esters).
  • Sharp peak at ~2250 cm⁻¹: nitrile C≡N.

IR spectra of common functional groups

Alcohol IR spectrum

Broad, strong O–H at 3200–3550 cm⁻¹ and a strong C–O at 1050–1150 (primary ~1050, secondary ~1100, tertiary ~1150). See Figure 1 (ethanol).

Ketone IR spectrum

IR spectrum of 2-butanone (ketone)fingerprint region50010001500200025003000350040000255075100Wavenumber (cm⁻¹)Transmittance (%)C=O 1717 (ketone)sp³ C–H 2980C=O overtoneC–C(=O)–C 1172
Figure 2. 2-Butanone: very strong C=O at 1717 cm⁻¹, no O–H (the weak band near 3415 is the C=O overtone), no aldehyde C–H. Redrawn from SDBS data.

Aldehyde IR spectrum (aldehyde vs ketone)

IR spectrum of butanal (aldehyde)fingerprint region50010001500200025003000350040000255075100Wavenumber (cm⁻¹)Transmittance (%)C=O 1730 (aldehyde)C–H 2720 & 2820 (Fermi doublet)sp³ C–H
Figure 3. Butanal: C=O at ~1730 cm⁻¹ plus the two weak aldehyde C–H bands near 2720 and 2820 cm⁻¹ (Fermi resonance), which distinguish an aldehyde from a ketone.

Aldehyde vs ketone: both show a strong C=O near 1715–1730. Only aldehydes show the C–H doublet at ~2720 and ~2820 cm⁻¹; the 2720 band is the easiest to spot because nothing else absorbs there.

Ester IR spectrum

IR spectrum of ethyl acetate (ester)fingerprint region50010001500200025003000350040000255075100Wavenumber (cm⁻¹)Transmittance (%)C=O 1742 (ester)C–O 1240O–C–C 1047C–H 2985
Figure 4. Ethyl acetate: C=O at 1742 cm⁻¹ (higher than a ketone) and two strong C–O stretches at 1240 and 1047 cm⁻¹, the "rule of three" for esters.

Carboxylic acid IR spectrum

IR spectrum of benzoic acid (carboxylic acid, KBr)fingerprint region50010001500200025003000350040000255075100Wavenumber (cm⁻¹)Transmittance (%)O–H 2500–3300 very broadC=O 1688 (conjugated acid)Ar C=C 1600/1585C–O 1292O–H oop 935
Figure 5. Benzoic acid: very broad O–H from 2500 to 3300 cm⁻¹ (hydrogen-bonded dimer) that swallows the C–H bands, conjugated C=O at 1688, aromatic C=C at 1603/1584, C–O at 1292 and the broad O–H out-of-plane bend near 935 cm⁻¹.

Ether IR spectrum

IR spectrum of diethyl ether (ether)fingerprint region50010001500200025003000350040000255075100Wavenumber (cm⁻¹)Transmittance (%)C–O–C 1125 (strong)sp³ C–Hno C=O, no O–H
Figure 6. Diethyl ether: strong C–O–C stretch near 1125 cm⁻¹ and sp³ C–H, with no O–H and no C=O. Absence of peaks is evidence too.

Alkyne IR spectrum

IR spectrum of 1-hexyne (terminal alkyne)fingerprint region50010001500200025003000350040000255075100Wavenumber (cm⁻¹)Transmittance (%)≡C–H 3312 (sharp)C≡C 2119 (weak)≡C–H bend 630
Figure 7. 1-Hexyne: sharp, strong ≡C–H stretch at ~3310 cm⁻¹, weak C≡C at ~2120 and ≡C–H bend near 630 cm⁻¹. Internal alkynes have no 3300 band and a very weak or absent C≡C.

Amine IR spectrum

IR spectrum of butylamine (primary amine)fingerprint region50010001500200025003000350040000255075100Wavenumber (cm⁻¹)Transmittance (%)N–H 3370/3290 (two bands, 1° amine)N–H bend 1610N–H wag
Figure 8. Butylamine: two medium N–H stretches (asymmetric ~3370, symmetric ~3290 cm⁻¹) typical of NH₂, N–H bend near 1610 and a broad N–H wag around 800 cm⁻¹. Secondary amines show one N–H band; tertiary amines none.

Aromatic compounds

C–H just above 3000 (3030–3100), ring C=C at ~1600 and ~1500 cm⁻¹, weak overtones 1660–2000, and strong C–H out-of-plane bends at 690–900 cm⁻¹ whose pattern indicates the substitution (monosubstituted: ~750 and ~690).

Carbonyl (C=O) positions at a glance

Compound classC=O (cm⁻¹)Partner bands
Acid chloride1785–1815C–Cl ~ 730–550
Anhydride1820 and 1760 (two)C–O 1000–1300
Ester1735–1750two C–O 1000–1300
Aldehyde1720–1740C–H 2720, 2820
Ketone1705–1720none
Carboxylic acid1700–1725O–H 2500–3300 very broad
Amide1630–1690N–H 3100–3500, N–H bend 1550–1640
α,β-Unsaturated / arylsubtract ~20–30C=C 1600–1650

Values from Pavia et al. and Silverstein et al.; ring strain raises C=O (cyclopentanone ~1745, cyclobutanone ~1780).

Worked example: identify the unknown

Data: formula C₄H₈O₂ (MS M⁺• = 88). IR: 2985, 1742 (very strong), 1240 (very strong), 1047 (strong) cm⁻¹; nothing at 3200–3600.

  1. Degree of unsaturation = (2×4 + 2 − 8)/2 = 1: one ring or π bond.
  2. 1742 cm⁻¹ strong → C=O; no O–H → not an acid; no 2720 → not an aldehyde.
  3. 1742 + two strong C–O bands → ester.
  4. Candidates: ethyl acetate, methyl propanoate, propyl formate. ¹H NMR (singlet at 2.05 for CH₃C=O, quartet at 4.12 for OCH₂) identifies ethyl acetate, see the NMR guide.

How to tell purity from an IR spectrum

Compare against a reference: extra bands indicate impurities. Common ones are a broad 3400 band (water or leftover alcohol), 1715 (residual acetone), 2350 (atmospheric CO₂, an artefact) and, after a reduction, a remaining C=O band showing incomplete reaction.

Next steps

Look up any band in the IR spectroscopy chart. Get the molecular formula from MS (mass spectrum guide, formula finder) and confirm the skeleton with NMR and the structure-to-NMR predictor.

Practice quiz

Tap an answer to check it.

1. A spectrum has a strong, sharp band at 1715 cm⁻¹ and nothing at 2500–3600 except C–H below 3000. The compound is most likely…

2. A very broad band from 2500 to 3300 cm⁻¹ plus a strong band at 1710 cm⁻¹ indicates…

3. Which band distinguishes an aldehyde from a ketone?

4. A sharp, strong band at 3300 cm⁻¹ and a weak band at 2120 cm⁻¹ suggest…

5. Two medium bands at ~3370 and ~3290 cm⁻¹ indicate…

6. C–H peaks appear at 3030 and 2950 cm⁻¹. What does this tell you?

Frequently asked questions

How do I read an IR spectrum?

Work left to right through the diagnostic regions: first check 1650–1820 cm⁻¹ for a C=O, then 3200–3600 for O–H/N–H, then whether C–H peaks lie above or below 3000, then 2100–2260 for triple bonds. Use the fingerprint region (below 1500) only to confirm.

What does a peak at 3000 mean in IR?

C–H stretching. Peaks just below 3000 cm⁻¹ come from sp³ C–H (alkyl groups); peaks just above 3000 come from sp² C–H on alkenes or aromatic rings.

What IR peak is at 1700?

A carbonyl (C=O) stretch. Ketones absorb near 1715, aldehydes 1725–1730, carboxylic acids ~1710, esters 1735–1750 and amides 1650–1690 cm⁻¹.

How do I interpret FTIR results?

An FTIR spectrum is read the same way as any IR spectrum: identify functional groups from characteristic bands (C=O, O–H, N–H, C–H, C≡N), then compare the whole spectrum, including the fingerprint region, with a reference library.

What does IR spectroscopy tell you?

Which functional groups a molecule contains (and which it lacks). It does not give the molecular formula or full connectivity; combine it with MS and NMR.

Can you explain IR spectroscopy in a simple way?

Chemical bonds behave like springs. Each kind of bond vibrates at its own frequency and absorbs infrared light of exactly that frequency, so the pattern of absorbed frequencies tells you which bonds are present.

What is the fingerprint region in IR?

The region below about 1500 cm⁻¹, full of complex bending and C–C/C–O stretching bands that are unique to each molecule. It is used to match an unknown to a reference spectrum.

How do you tell an aldehyde from a ketone by IR?

Both have a strong C=O near 1715–1730 cm⁻¹, but only an aldehyde shows the two weak C–H stretches near 2720 and 2820 cm⁻¹.

References and data sources

  • Pavia, Lampman, Kriz & Vyvyan, Introduction to Spectroscopy, 5th ed., Cengage, ch. 2 (IR correlation tables).
  • Silverstein, Webster & Kiemle, Spectrometric Identification of Organic Compounds, 8th ed., Wiley, ch. 2.
  • SDBS Spectral Database for Organic Compounds (AIST, Japan): IR spectra of ethanol, 2-butanone, butanal, ethyl acetate, benzoic acid, diethyl ether, 1-hexyne, butylamine.
  • NIST Chemistry WebBook, SRD 69, IR spectra (webbook.nist.gov).
  • Socrates, G. Infrared and Raman Characteristic Group Frequencies, 3rd ed., Wiley, 2001.

Spectra on this page are redrawn schematically from the reference data above (peak positions and approximate relative intensities) for teaching; check the original database entry before citing exact intensities.

All spectroscopy guides

How to read a mass spectrumHow to read an IR spectrumHow to read an NMR spectrum

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