Infrared Spectroscopy: Reading a Spectrum for Functional Groups
Four regions, checked in order, identify most functional groups in under a minute.
The four regions
| Region (cm−¹) | Contains | Check for |
|---|---|---|
| 3600 – 3200 | O–H, N–H | Alcohols, acids, amines, amides |
| 3300 – 2700 | C–H | Whether the carbon is sp, sp² or sp³; aldehyde C–H |
| 2300 – 2100 | Triple bonds | Nitriles, alkynes |
| 1800 – 1600 | Double bonds | Carbonyls, alkenes, aromatics |
| Below 1500 | Fingerprint | Whole-molecule identity, not individual groups |
The carbonyl region in detail
Almost every organic structure question involves deciding whether a carbonyl is present and, if so, what kind. The position within the carbonyl range distinguishes them.
| Approximate position | Type | Reason for the shift |
|---|---|---|
| Higher end | Anhydride, ester | Adjacent oxygen withdraws inductively, strengthening the bond |
| Middle | Aldehyde, ketone | Reference position |
| Lower end | Amide | Nitrogen donates by resonance, weakening the bond |
| Lowered by conjugation | Any conjugated carbonyl | Delocalisation reduces double bond character |
The pattern is consistent: anything donating electron density into the carbonyl lowers the frequency, and anything withdrawing raises it. Reasoning from that principle is more reliable than recalling numbers.
Ring strain raises the frequency, because a smaller ring forces a bond angle that increases the carbonyl bond's s character. A strained cyclic ketone therefore absorbs noticeably higher than an unstrained one, which is a standard identification.
Distinguishing similar compounds
| Pair | Distinguishing feature |
|---|---|
| Alcohol and carboxylic acid | The acid O–H is far broader and a carbonyl band is present |
| Aldehyde and ketone | The aldehyde shows two weak C–H bands just below 2900 |
| Primary and secondary amine | Two N–H bands versus one |
| Ester and ketone | The ester shows strong C–O bands as well as the carbonyl |
| Alkyne and nitrile | A terminal alkyne also shows a sharp C–H band near 3300 |
Why some bands are strong and others weak
Infrared intensity depends on how much the dipole moment changes during the vibration. A carbonyl stretch changes a large dipole substantially, so it is intense; a carbon–carbon stretch in a symmetrical alkene changes almost nothing, so it may be very weak or absent entirely.
That explains an observation that otherwise looks contradictory: a symmetrically substituted alkene can show no detectable band for its double bond even though the bond is certainly present. Concluding from a missing band that a group is absent is therefore unsafe for symmetrical vibrations, and a question presenting such a case is testing exactly that caution.
The fingerprint region
Below about 1500 cm−¹ the spectrum contains many overlapping bands from whole-molecule vibrations. Individual bands are rarely assignable, but the pattern as a whole is characteristic of the specific compound.
Its use is therefore comparison rather than identification: two samples with identical fingerprint regions are the same compound. It answers "is this the same substance" rather than "what functional groups are present".
What absence tells you
Negative evidence is often the most efficient. No band near 1700 rules out every carbonyl compound at once. No broad band above 3200 rules out alcohols, acids and amines. Each such observation eliminates a large family of structures in one step.
In a combined-spectra problem, listing what is absent first often narrows the possibilities faster than identifying what is present.
Frequently asked questions
Why is a carboxylic acid O–H so much broader than an alcohol O–H?
Because acids form strongly hydrogen-bonded dimers, and the range of hydrogen bond strengths present broadens the absorption considerably.
Why does conjugation lower the carbonyl frequency?
Because delocalisation reduces the double bond character of the carbonyl, weakening the bond and lowering its force constant.
Why does ring strain raise it?
Because the constrained angle increases the s character of the carbonyl bond, strengthening it.
Can infrared alone determine a structure?
Rarely. It identifies functional groups reliably but says little about the carbon skeleton, which is why it is used alongside NMR and mass spectrometry.
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