22.1 Infrared spectroscopy
- Syllabus
- 9701–2028–2029
- Topic
- 22.1
- Level
- AS
Learning objectives
Match IR absorptions in combination to identify functional groups
| Bond / functional-group context | Official absorption range / cm⁻¹ |
|---|---|
| C–O in hydroxy or ester | 1040–1300 |
| C=C in aromatic compound or alkene | 1500–1680 |
| C=O in amide | 1640–1690 |
| C=O in carbonyl or carboxyl | 1670–1740 |
| C=O in ester | 1710–1750 |
| C≡N in nitrile | 2200–2250 |
| O–H in carboxyl | 2500–3000 |
| C–H in alkane | 2850–2950 |
| O–H in hydroxy | 3200–3650 |
| N–H in amine or amide | 3300–3500 |
- Read the wavenumber axis in cm⁻¹. 2. Match clear absorptions to the supplied ranges. 3. Combine bonds required by one functional group. 4. Check that expected companion absorptions are present and that a proposed group does not conflict with major absent features.
| Spectrum evidence | Supported deduction |
|---|---|
| O–H 2500–3000 plus C=O 1670–1740 | carboxylic acid |
| C=O 1710–1750 plus C–O 1040–1300, without O–H | ester |
| C≡N 2200–2250 | nitrile |
| O–H 3200–3650 plus C–O 1040–1300 | alcohol / hydroxy group |
Several ranges overlap: a peak near 1680 cm⁻¹ cannot by itself distinguish every C=C and C=O context. Use a set of present and absent absorptions; IR normally identifies functional groups rather than a unique complete carbon skeleton.