A level Chemistry · Edexcel 9CH0 · Topic 7B
Infrared spectroscopy
Key points
- Bonds vibrate (stretch and bend); a bond absorbs infrared radiation when the frequency matches its natural vibration, and only if the vibration changes the dipole.
- Spectra plot % transmittance against wavenumber (cm⁻¹); absorptions appear as downward troughs.
- Key absorptions: O–H in alcohols broad, about 3200–3600; O–H in carboxylic acids very broad, 2500–3300; C=O sharp, about 1650–1750; C–H about 2850–3000; N–H about 3300–3500.
- The region below about 1500 cm⁻¹ is the fingerprint region, unique to each compound; compare with a database to confirm identity.
- Use IR to follow reactions, e.g. oxidation of an alcohol to a ketone: the O–H peak disappears and a C=O peak appears.
- Always use the data booklet values and link each peak to a named bond.
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Covalent bonds are not rigid: they stretch and bend like springs. Each type of bond vibrates at its own natural frequency and absorbs infrared radiation of that frequency, so an infrared spectrum tells you which bonds, and therefore which…
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