All subjects · Chemistry · WCH12 (Unit 2)

Infrared spectroscopy

Bonds absorb infrared radiation at characteristic wavenumbers. An IR spectrum shows which functional groups a molecule contains.

How it works

Covalent bonds vibrate (stretch and bend). A bond absorbs IR radiation whose frequency matches its vibration, but only if the vibration changes the polarity (dipole) of the molecule. A spectrum plots % transmittance against wavenumber (cm-1); absorptions show as downward troughs.

Key absorptions

Approximate ranges (use the data booklet in the exam):

BondWhere foundWavenumber / cm-1Appearance
O-Halcoholsabout 3200 to 3600broad
O-Hcarboxylic acidsabout 2500 to 3300very broad
N-Haminesabout 3300 to 3500medium
C-Halkanes, alkyl groupsabout 2850 to 3000sharp
C=Oaldehydes, ketones, acidsabout 1680 to 1750strong, sharp
C=Calkenesabout 1640 to 1680medium

Using spectra

Look first for the obvious peaks: a strong C=O near 1700 and a broad O-H. Both together suggest a carboxylic acid; C=O without O-H suggests an aldehyde or ketone; broad O-H without C=O suggests an alcohol. Below about 1500 cm-1 is the fingerprint region, unique to each compound. In Core Practical 7, IR shows whether an alcohol has been oxidised (O-H disappears, C=O appears).

Worked example

A compound's IR spectrum has a strong absorption at 1715 cm-1 and no broad absorption above 2500 cm-1. Which functional group is present?

  1. 1715 cm-1 is C=O.
  2. No O-H, so it is not an alcohol or acid.

Answer: An aldehyde or ketone (a carbonyl group).

Key idea: Broad O-H about 3200 to 3600 (alcohol) or very broad 2500 to 3300 (acid); C=O about 1700. Absorption needs a change in dipole.

Check yourself

  1. A spectrum shows a very broad absorption from 2500 to 3300 cm-1 and a strong peak at 1710 cm-1. What type of compound is it?
    • A carboxylic acid
    • An alcohol
    • A ketone
    • An alkene
    Show answer

    A carboxylic acid. Very broad O-H plus C=O indicates COOH.

  2. In Core Practical 7, how does IR show that propan-1-ol has been oxidised to propanal?
    • A C=C absorption appears
    • The C-H absorptions disappear
    • The broad O-H absorption disappears and a C=O absorption appears
    • The fingerprint region becomes empty
    Show answer

    The broad O-H absorption disappears and a C=O absorption appears. Propanal has C=O but no O-H.

  3. Which absorption is typical of an alcohol?
    • A broad absorption at about 3200 to 3600 cm-1
    • A sharp absorption at 1700 cm-1
    • A sharp absorption at 1650 cm-1
    • No absorptions above 1500 cm-1
    Show answer

    A broad absorption at about 3200 to 3600 cm-1. Hydrogen bonding broadens the O-H absorption.

  4. What is the fingerprint region used for?
    • Finding the Mr
    • Detecting C=O groups
    • Measuring concentration
    • Identifying a specific compound by comparison with known spectra
    Show answer

    Identifying a specific compound by comparison with known spectra. The pattern below 1500 cm-1 is unique to each compound.

  5. Why does N2 not absorb infrared radiation?
    • It has no bonds
    • Its vibration does not change the dipole of the molecule
    • Its bond is too weak
    • It is a liquid
    Show answer

    Its vibration does not change the dipole of the molecule. Symmetrical diatomic molecules have no dipole change when they stretch.

  6. A hydrocarbon has an absorption at about 1650 cm-1. Which group is present?
    • C=O
    • O-H
    • N-H
    • C=C
    Show answer

    C=C. C=C absorbs at about 1640 to 1680 cm-1; a hydrocarbon has no oxygen.

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Last updated 7 November 2026. Spotted a mistake? Tell us.