Last updated: June 2026
Use this IR absorption and functional group identifier to interpret common infrared peaks and chemical-test clues. It is designed for O-Level, IGCSE, A-Level and Singapore H2 Chemistry students identifying alcohols, carboxylic acids, aldehydes, ketones, esters, amines, nitriles, alkenes and aromatic compounds.
- Enter each important IR absorption on a separate line, such as 1710 strong or 2500–3300 very broad.
- Add descriptive words such as broad, very broad, strong or sharp when the peak shape or intensity is known.
- Open the Chemical tests tab and select any positive test results provided in the question.
- Select Identify functional groups to obtain a ranked shortlist of possible groups.
- Use the explanation beneath the result to see which IR regions and chemical tests produced each suggestion.
IR Absorption Identifier
An IR spectrum shows which bonds absorb infrared radiation. Different bonds absorb at different approximate wavenumber ranges, measured in cm−1.
A strong absorption near 1700 cm−1 often suggests a C=O bond. A broad absorption around 3200–3600 cm−1 may suggest an O–H bond in an alcohol or phenol. A very broad absorption extending from about 2500 to 3300 cm−1, together with a C=O absorption, strongly suggests a carboxylic acid.
Functional Group Identifier
A functional group identifier uses several clues together. IR peaks are important, but chemical tests can make the conclusion much more specific.
A compound with a strong C=O absorption and a positive 2,4-DNPH test may be an aldehyde or ketone. If the same compound also gives a positive Tollens’ test, the evidence points towards an aldehyde.
Common IR Absorption Table
| Absorption range / cm−1 | Possible bond or group | Comment |
|---|---|---|
| 3200–3600, broad | O–H in an alcohol or phenol | Usually broad, but not as extensive as the O–H absorption of a carboxylic acid. |
| 2500–3300, very broad | O–H in a carboxylic acid | Often overlaps with the C–H absorption region. |
| 3300–3500 | N–H in an amine or amide | Often sharper than O–H and may appear as one or two absorptions. |
| 2850–2960 | C–H in an alkane | Common in compounds containing alkyl groups. |
| 3000–3100 | =C–H in an alkene or aromatic compound | Appears just above 3000 cm−1. |
| 1680–1750, strong | C=O carbonyl group | Present in aldehydes, ketones, carboxylic acids, esters and amides. |
| 1620–1680 | C=C in an alkene | Often medium or variable in intensity. |
| 1450–1600 | Aromatic C=C | Several absorptions may appear in this region. |
| 2210–2260, sharp | C≡N in a nitrile | Usually a distinctive sharp absorption. |
| 1000–1300 | C–O | May occur in alcohols, esters, ethers and carboxylic acids. |
Carbonyl Region Guide
The carbonyl region is one of the most useful parts of an IR spectrum. A strong peak around 1680–1750 cm−1 suggests C=O, but this absorption alone does not identify the exact type of carbonyl compound.
| Evidence | Possible conclusion |
|---|---|
| C=O and very broad O–H from about 2500–3300 cm−1 | Carboxylic acid |
| C=O and C–O in the 1000–1300 cm−1 region | Ester or carboxylic acid may be present |
| C=O and absorptions near 2720 and 2820 cm−1 | Aldehyde may be present |
| C=O and positive 2,4-DNPH test | Aldehyde or ketone |
| C=O and positive Tollens’ test | Aldehyde |
| C=O and N–H absorption | Amide may be present |
O–H, N–H and C–H Region Guide
Absorptions around 3000–3600 cm−1 can be confusing because O–H, N–H and C–H absorptions occur in nearby regions. The shape and exact position of the absorption are therefore important.
Alcohol O–H is usually broad around 3200–3600 cm−1. Carboxylic acid O–H is usually very broad and extends from roughly 2500 to 3300 cm−1. Alkane C–H appears below 3000 cm−1, whereas alkene or aromatic C–H appears just above 3000 cm−1.
Chemical Test Clues
Chemical tests can support or eliminate possible functional groups suggested by the IR spectrum.
| Chemical test | Positive result | What it suggests |
|---|---|---|
| 2,4-DNPH | Orange precipitate | Aldehyde or ketone |
| Tollens’ reagent | Silver mirror | Aldehyde |
| Fehling’s solution | Brick-red precipitate | Usually an aliphatic aldehyde |
| Carbonate | Effervescence | Carboxylic acid |
| Bromine water | Decolourised | Alkene or phenol, depending on the conditions |
| Acidified KMnO4 | Purple solution becomes colourless | An oxidisable group such as an alkene or aldehyde |
| Sodium metal | Effervescence | Alcohol, phenol or carboxylic acid |
Worked examples
Example 1: Strong C=O and very broad O–H
A strong absorption near 1710 cm−1 together with a very broad absorption from about 2500–3300 cm−1 strongly suggests a carboxylic acid.
Example 2: Strong C=O and positive Tollens’ test
A strong C=O absorption suggests a carbonyl compound. A positive Tollens’ test then distinguishes an aldehyde from a ketone.
Example 3: Broad O–H but no C=O
A broad O–H absorption around 3200–3600 cm−1 without a carbonyl absorption may suggest an alcohol or phenol.
Example 4: Sharp absorption around 2250 cm−1
A sharp absorption around 2210–2260 cm−1 suggests a nitrile group containing C≡N.
Common exam traps
Frequently Asked Questions
What does an IR Absorption Identifier do?
It matches common IR absorption ranges with possible bonds and functional groups such as O–H, C=O, C≡N and C–O.
What does a Functional Group Identifier do?
It combines IR absorptions and chemical-test clues to produce a ranked shortlist of possible functional groups.
Can IR identify the whole molecule?
Usually not. IR is most useful for identifying functional groups. The complete structure normally requires additional evidence from molecular formulae, mass spectrometry and NMR.
Why are the two identifiers on the same page?
Students use IR absorptions to infer functional groups, so interpreting the peaks and identifying the group are naturally connected tasks.
What is the most important IR absorption for carbonyl compounds?
A strong absorption around 1680–1750 cm−1 is important because it suggests the presence of a C=O bond.