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Short answer
An ester shows up in an infrared (IR) spectrum as a pattern of three intense peaks: a carbonyl (C=O) stretch at about 1700 cm-1 (1755-1735 for saturated esters) and two C-O stretches near 1200 and 1100 cm-1. For a saturated ester the C=O band falls at about 1755-1735 cm-1. Reading all three bands together, rather than the carbonyl alone, is what separates an ester from a ketone, aldehyde or carboxylic acid.
What is IR spectroscopy?
IR spectroscopy records which infrared wavenumbers a sample absorbs. Each absorption band corresponds to a vibration of a bond or group of atoms, and positions are reported in reciprocal centimetres (cm-1). Because functional groups absorb in characteristic regions, an IR spectrum works as a quick check for which groups are present. For the instrument side, see the CASRAI guide to the Fourier transform infrared spectrophotometer.
How does IR spectroscopy work?
The instrument passes infrared light through or over the sample and measures how much is absorbed at each wavenumber. The result is a plot of absorbance or transmittance against wavenumber, with each peak marking a vibration. How the sample is presented to the beam changes the look of the spectrum; the guide on ATR-FTIR versus transmission FTIR covers that choice, and the Raman versus FTIR comparison places IR next to its complementary technique.
How to read an ester spectrum: the rule of three
Spectroscopy Online’s carbonyl series describes esters as having a memorable pattern of three intense peaks at about 1700, 1200 and 1100 cm-1, which it calls the rule of three. The three peaks are the C=O stretch and two C-O stretches. For saturated esters the ranges given are:
- C=O stretch: 1755-1735 cm-1
- C-C-O stretch: 1210-1160 cm-1
- O-C-C stretch: 1100-1030 cm-1
Start with the carbonyl: an intense band in the 1735-1755 region is the first signal. Then look for the strong band in the 1160-1210 region and the further band between 1030 and 1100. A spectrum with only the carbonyl peak has not yet shown an ester.
Worked example: ethyl acetate
The same source gives ethyl acetate as its example, with peaks at 1742 cm-1 (C=O), 1241 cm-1 (C-C-O) and 1047 cm-1 (O-C-C). The carbonyl and third band sit inside the saturated ranges above. The middle band does not: the author notes that the 1210-1160 range does not hold for acetates, whose C-C-O stretch typically falls around 1240 cm-1. If you are reading an acetate and the middle band sits above the usual range, that is expected, not a sign of a mixed or contaminated sample.
Saturated versus aromatic esters
Conjugation with an aromatic ring moves the bands. A second Spectroscopy Online article, on polyesters, tabulates the two cases, and notes that aromatic carbonyl stretches tend to be about 30 cm-1 lower:
| Band | Saturated ester (cm-1) | Aromatic ester (cm-1) |
|---|---|---|
| C=O stretch | 1755-1735 | 1730-1715 |
| C-C-O stretch | 1210-1160 | 1310-1250 |
| O-C-C stretch | 1100-1030 | 1130-1100 |
Two practical points follow from the same article. A C=O near 1750 cm-1 points to a saturated ester. A sample containing both ester types should show six ester peaks rather than three. The article also warns that the aromatic ester C=O range overlaps other groups, including saturated ketones, so the other two bands should carry the identification.
Telling esters from ketones, aldehydes and carboxylic acids
The carbonyl bands of these groups sit close together, so the surrounding bands decide the assignment. The Cengage group-frequency table gives these ranges:
| Group | Carbonyl C=O (cm-1) | Other diagnostic band (cm-1) |
|---|---|---|
| Ester | 1750-1735, very strong | C-O-C 1290-1180, very strong |
| Ketone | 1725-1705 | – |
| Aldehyde | 1735-1720 | C-H 2740-2720, medium |
| Carboxylic acid | 1710-1680 | O-H 3300-2400, strong, very broad |
As a reading checklist: a carbonyl near the top of the range plus strong C-O bands below 1300 suggests an ester; a weak aldehyde C-H pair near 2740-2720 suggests an aldehyde; a very broad O-H envelope across 3300-2400 suggests a carboxylic acid. Because the ester and aldehyde carbonyl ranges overlap at 1735, the carbonyl position alone cannot decide between them. The CASRAI IR functional-group frequency table lists these and other groups side by side.
Where it matters for labs
For lab staff, the practical step after reading a spectrum is to compare it with a reference. Reference spectra can be looked up in the NIST Chemistry WebBook, which lists IR collections including the Coblentz Society evaluated infrared spectra, PNNL data and a quantitative infrared database. Published ranges are guides, and observed positions vary with the compound and its environment.
Common misunderstandings
- “One peak at 1740 means ester.” Aldehydes absorb in the same region (1735-1720 in the Cengage table), and aromatic esters overlap saturated ketones. Confirm with the C-O bands.
- “The middle band is always 1210-1160.” Not for acetates, which sit near 1240 cm-1, nor for aromatic esters (1310-1250).
- “Conjugation does not change the carbonyl.” It lowers it by about 30 cm-1 for aromatic esters.
FAQ
What is IR spectroscopy?
A technique that measures which infrared wavenumbers a sample absorbs, so that bands can be matched to functional groups such as the ester carbonyl.
How does IR spectroscopy work?
Infrared light interacts with the sample and the absorption at each wavenumber is recorded as a spectrum; each band reflects a bond vibration. See the FTIR spectrophotometer guide linked above for the instrument.
How to read IR spectroscopy for an ester?
Find the intense C=O band (1755-1735 cm-1 for saturated esters), then the C-C-O band (1210-1160, or near 1240 for acetates) and the O-C-C band (1100-1030). All three together fit an ester.
Sources
- Spectroscopy Online, “The C=O Bond, Part VI: Esters and the Rule of Three”: spectroscopyonline.com (checked 2026-10-29)
- Spectroscopy Online, “Infrared Spectroscopy of Polymers VIII: Polyesters and the Rule of Three”: spectroscopyonline.com (checked 2026-10-29)
- Cengage, IR group frequencies table: cxp.cengage.com (checked 2026-10-29)
- NIST Chemistry WebBook: webbook.nist.gov (checked 2026-10-29)
