Infrared (IR) spectroscopy

Specification Reference Topic 7, point 2 (Edexcel A-Level Chemistry)

Quick Notes

IR spectroscopy identifies functional groups by detecting bond vibrations.
Bonds absorb IR radiation at specific wavenumbers (cm⁻¹).
An absorption peak in an IR spectra indicates the presence of a bond.
Key absorptions:
- C–H (alkanes, alkenes, aldehydes): 2850–3100 cm⁻¹
- C=C (alkenes): 1620–1680 cm⁻¹
- O–H (alcohols): 3200–3600 cm⁻¹ (broad)
- C=O (aldehydes and ketones): 1680–1750 cm⁻¹
- O–H and C=O (carboxylic acids): very broad 2500–3300 cm⁻¹ for O–H and 1680–1750 cm⁻¹ for C=O
- N–H (amines): 3300–3500 cm⁻¹ (usually sharp)

Full Notes

IR Spectroscopy and how it works is covered in more detail here.
This page is just what you need to know for Edexcel A-level :)

Infrared spectroscopy is used to detect bond vibrations in organic compounds.

When a compound is exposed to infrared radiation, some bonds absorb energy and vibrate more strongly. This absorption happens at characteristic wavenumbers, depending on the type of bond and its environment.

Edexcel A-Level Chemistry diagram of an IR spectrometer showing source, sample, interferometer and detector path.

IR radiation is passed through a sample.
Bonds in the sample can vibrate in specific ways, absorbing IR radiation at characteristic wavenumbers (measured in cm⁻¹).
An infrared spectrum is produced, showing absorption peaks.

Edexcel A-Level Chemistry IR spectrum with labelled axes showing percentage transmittance versus wavenumber and typical absorption peaks.

Identifying Functional Groups Using IR Spectroscopy

Bonds from different functional groups absorb IR radiation at specific wavenumbers (you are given a data table in exams, you don’t need to remember them).

Edexcel A-Level Chemistry data table of IR absorption ranges for common bonds such as O–H, C=O, C–H, N–H and C=C.

We can use absorbances in spectra to identify bond types present.

Example Ethanoic acid (CH₃COOH)

Ethanoic acid has two absorbances in its IR spectra that helps identify it. One for the O–H bond (2500 to 3000) and one for the C=O bond (1680 to 1750).

Edexcel A-Level Chemistry IR spectrum for ethanoic acid showing broad O–H absorption 2500–3000 cm−1 and sharp C=O peak 1680–1750 cm−1.

The ‘Fingerprint Region’ (Below 1500 cm⁻¹)

The region on a spectra below 1500cm^-1 is called the fingerprint region.

Edexcel A-Level Chemistry IR spectrum highlighting the fingerprint region below 1500 cm−1.

It is unique to a given molecule can be used for exact identification by comparing spectra to known compounds from a database.

This region is very complex and isn’t analysed in detail at this level of study.

Summary

IR spectroscopy detects bond vibrations and shows characteristic absorption peaks for functional groups.
Use key ranges such as O–H, C=O, C–H, N–H and C=C to identify bonds in a compound.
The molecular fingerprint region below 1500 cm⁻¹ is unique and useful for exact identification by comparison to databases.