4.7.1Core Organic Chemistry
Infrared Spectroscopy
5 min read 5 parts Lesson 27 of 29
Section 1 of 5How IR Spectroscopy Works
Different bonds absorb at different wavenumbers because they have different strengths and atom masses.
Bond vibrations
- Covalent bonds behave like springs and stretch and bend at characteristic frequencies.
Absorption
- A bond absorbs infrared radiation when the radiation frequency matches the natural vibration frequency of that bond.
Spectrum
- The spectrum plots percentage transmittance against wavenumber in cm⁻¹, so absorptions appear as downward peaks.
Characteristic peaks
- Each type of bond absorbs in a typical range of wavenumber, allowing functional groups to be identified.
Fingerprint region
- The complex pattern below about 1500 cm⁻¹ is unique to each molecule and can be matched against a database.
Section 2 of 5Characteristic Absorptions
Characteristic Absorptions
O–H in alcohols
- A broad absorption between about 3200 and 3640 cm⁻¹ indicates an alcohol.
O–H in acids
- A very broad absorption between about 2500 and 3300 cm⁻¹ indicates a carboxylic acid.
C=O
- A strong, sharp peak between 1640 and 1750 cm⁻¹ shows a carbonyl group in aldehydes, ketones, acids and esters.
C–H
- Absorptions around 2850–3100 cm⁻¹ come from C–H bonds in almost all organic molecules.
N–H, C≡N, C–O
- N–H absorbs near 3300–3500 cm⁻¹, C≡N near 2220–2260 cm⁻¹ and C–O between 1000 and 1300 cm⁻¹.
| Bond | Wavenumber / cm⁻¹ | Appearance |
|---|---|---|
| O–H (alcohol) | 3200–3640 | Broad |
| O–H (acid) | 2500–3300 | Very broad |
| N–H | 3300–3500 | Medium |
| C=O | 1640–1750 | Strong, sharp |
| C–O | 1000–1300 | Strong |
Section 3 of 5Interpreting a Spectrum
1
Look first for a strong sharp peak near 1700 cm⁻¹, which indicates a carbonyl group.
2
Then look at the region above 2500 cm⁻¹ for O–H or N–H absorptions.
3
A broad O–H peak together with C=O suggests a carboxylic acid, whereas C=O alone suggests an aldehyde, ketone or ester.
4
Ignore the fingerprint region unless a database comparison is required.
5
Match the peaks with the data table and state the bond and functional group for each absorption.
Section 4 of 5Applications
Monitoring
- IR can show when a reaction has finished by checking that the reactant peak, such as O–H, has disappeared.
Breathalysers
- Some breath analysers use IR absorption by C–H and O–H bonds of ethanol to measure alcohol in breath.
Greenhouse gases
- CO₂, CH₄ and H₂O absorb infrared radiation from the Earth through their bond vibrations, warming the atmosphere.
Identification
- Unknown samples can be compared with reference spectra to identify them quickly.
Section 5 of 5Exam Checklist & Tips
4.7.1 – Key Points for the Exam
- Wavenumber The number of waves per centimetre, measured in cm⁻¹ and used to label IR absorptions.
- Fingerprint region The part of an IR spectrum below about 1500 cm⁻¹ that is unique to a molecule.
- Infrared spectroscopy A technique using absorption of IR radiation by bond vibrations to identify functional groups.
- Transmittance The percentage of radiation passing through a sample.
Common Mistakes
- Reading peak height as the number of bonds without considering the type of bond.
- Confusing the sharp C=O peak with the broad O–H peak.
- Quoting wavenumbers in nm or Hz rather than cm⁻¹.
- Using the fingerprint region to identify a functional group instead of the diagnostic region.