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WAChemistrySyllabus dot point

How does infrared spectroscopy identify the functional groups present in a molecule?

Interpret infrared spectra to identify functional groups from characteristic absorption bands

A focused answer to the WACE Year 12 Chemistry dot point on infrared spectroscopy, how molecular bonds absorb infrared radiation at characteristic wavenumbers, how to identify functional groups such as O-H, C=O and N-H from absorption bands, with a worked example and common exam mistakes.

Generated by Claude Opus 4.76 min answer

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What this dot point is asking

Infrared (IR) spectroscopy identifies the functional groups in a molecule. Different bonds absorb infrared radiation at characteristic energies, so the pattern of absorptions acts as a fingerprint of the bonds present.

How it works

When a molecule absorbs infrared radiation, its bonds vibrate (stretch and bend) more vigorously. A bond absorbs only at the frequency that matches its natural vibration, which depends on the masses of the atoms and the stiffness of the bond. The spectrum plots absorption (or transmittance) against wavenumber (in cm−1^{-1}), and each absorption is recorded as a dip or peak.

Diagnostic absorption bands

The bands you must recognise (values are approximate ranges from the data table) include:

  • O-H (alcohol): broad, around 3200 to 3550 cm−1^{-1}.
  • O-H (carboxylic acid): very broad, around 2500 to 3300 cm−1^{-1}.
  • N-H (amine, amide): around 3300 to 3500 cm−1^{-1}.
  • C=O (carbonyl, in aldehydes, ketones, acids, esters): strong and sharp, around 1670 to 1750 cm−1^{-1}.
  • C-H: around 2850 to 3100 cm−1^{-1}.

The fingerprint region

The region below about 1500 cm−1^{-1} is the fingerprint region, a complex pattern unique to each compound. It is too complex to assign band by band, but it can confirm identity by matching against a reference spectrum of a known compound.

Why this matters

Infrared spectroscopy complements mass spectrometry and NMR: mass spectrometry gives the molar mass, IR identifies the functional groups present, and NMR reveals the carbon-hydrogen framework. Used together they let chemists determine a complete structure, which is the core skill of the instrumental analysis dot points.

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