IR Spectroscopy and the Functional Group Chart

Infrared (IR) spectroscopy is an analytical technique that identifies functional groups within an organic molecule. A sample is exposed to infrared radiation, and different covalent bonds absorb specific frequencies of that radiation depending on the atoms involved and the strength of the bond. The resulting pattern of absorption, called an IR spectrum, acts like a fingerprint for the bonds present in a compound, making it a key tool for confirming or ruling out particular functional groups.

Covalent bonds are not rigid, they constantly stretch and bend, behaving a little like tiny springs. Each type of bond has a natural vibrational frequency that depends on the masses of the atoms and the bond strength. When the frequency of incoming infrared radiation matches this natural vibration, the bond absorbs energy and vibrates with greater amplitude. Because different bonds (such as \(O\)–\(H\), \(N\)–\(H\), \(C\)–\(H\), and \(C=O\)) vibrate at different frequencies, each produces an absorption peak at a characteristic position on the spectrum.

An IR spectrum plots percentage transmittance (how much radiation passes through the sample) on the vertical axis against wavenumber, measured in cm\(^{-1}\), on the horizontal axis. Wavenumber decreases from left to right, typically from 4000 cm\(^{-1}\) down to around 400 cm\(^{-1}\). A dip, or peak, in the trace shows a wavenumber at which the sample strongly absorbs radiation, and it is this pattern of dips that reveals which functional groups are present.

The spectrum splits into two useful zones. Above roughly 1500 cm\(^{-1}\) lies the functional group region, where individual bonds such as \(O\)–\(H\), \(N\)–\(H\), \(C\)–\(H\), and \(C=O\) give clear, identifiable peaks. Below 1500 cm\(^{-1}\) lies the fingerprint region, a complex tangle of peaks caused by interacting vibrations across the whole molecule. No two compounds produce the same fingerprint region, so it is used to confirm a compound's identity by comparison with a known reference spectrum, rather than to pick out individual bonds.

The table below summarises the key absorptions expected at A Level. Wavenumber ranges are approximate, since the exact position of a peak can shift slightly depending on the rest of the molecule.

A three carbon compound gives an IR spectrum with a very broad absorption between 2500 and 3300 cm\(^{-1}\) and a strong sharp peak at around 1710 cm\(^{-1}\). What functional group is present?

The peak at 1710 cm\(^{-1}\) is consistent with a \(C=O\) bond. The very broad absorption spanning 2500 to 3300 cm\(^{-1}\), rather than a narrower peak nearer 3200 to 3550 cm\(^{-1}\), is the signature of a hydrogen bonded \(O\)–\(H\) bond in a carboxylic acid rather than a simple alcohol. Together, the two peaks identify propanoic acid rather than propan-1-ol (which would show only the O–H peak, with no C=O) or propanal (which shows the C=O peak but not the broad acid O–H).

IR spectroscopy is powerful for spotting the presence or absence of a functional group, but it rarely gives the full structure of a molecule on its own. Chemists usually combine it with other data, such as the fragmentation pattern from mass spectrometry, the hydrogen environments revealed by proton NMR, and the separation and purity checks provided by chromatography, to build a complete and confident picture of an unknown compound.

Related lessons

• Uk A Level Chemistry