3.6.2Organic Chemistry
Infrared (IR) Spectroscopy — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 32 of 42
Section 1 of 5Why Bonds Absorb Infrared
The technique relies on the energy of infrared radiation matching bond vibration energies.
1
Infrared radiation is passed through a sample, and the instrument measures how much of each wavenumber emerges at the other side.
2
Covalent bonds are constantly vibrating by stretching, bending and twisting, each with its own natural frequency.
3
When the frequency of the radiation matches the vibration frequency of a bond, the bond absorbs energy and vibrates more strongly.
4
The absorbed wavenumbers show as downward troughs in the spectrum, where transmittance drops towards the bottom.
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Because the frequency depends on the atoms joined and their surroundings, each functional group absorbs in a characteristic range.
Section 2 of 5Characteristic Absorptions
Learn the main ranges (in cm⁻¹) and what each trough looks like.
O–H alcohol
- A strong, broad trough at about 3750–3200 cm⁻¹, widened by hydrogen bonding between molecules.
O–H acid
- A very broad trough at about 3300–2500 cm⁻¹ that overlaps the C–H region.
C=O
- A strong, sharp trough at roughly 1680–1750 cm⁻¹ for aldehydes, ketones, acids and esters.
C–H alkane
- Absorption at about 2850–2960 cm⁻¹ from stretching of C–H bonds in saturated chains.
C≡N and C≡C
- Triple bonds absorb at about 2100–2260 cm⁻¹, a region that few other bonds use.
| Bond | Wavenumber / cm⁻¹ | Appearance |
|---|---|---|
| O–H (alcohol) | 3750–3200 | Strong, broad |
| C–H (alkane) | 2850–2960 | Medium to strong |
| C=O | 1680–1750 | Strong, sharp |
| C≡N | 2220–2260 | Medium |
Section 3 of 5Interpreting a Spectrum
Interpreting a Spectrum
Check the C=O region
- A strong sharp trough near 1700 cm⁻¹ tells you a carbonyl group is present.
Check the O–H region
- A broad trough above 3000 cm⁻¹ points to an alcohol or, if much broader, a carboxylic acid.
Use both together
- C=O plus a very broad O–H suggests a carboxylic acid, while C=O alone suggests an aldehyde, ketone or ester.
Fingerprint region
- The complex pattern below 1500 cm⁻¹ is unique to a molecule and is used to confirm identity against a database.
Section 4 of 5Uses and Limits
Identifying groups
- IR quickly shows which functional groups are present but does not give the full structure on its own.
Distinguishing isomers
- Propanone and propan-2-ol have the same carbon skeleton but give different key troughs, C=O versus O–H.
Monitoring reactions
- Appearance of a C=O trough and loss of an O–H trough shows that an alcohol has been oxidised.
Breathalysers
- Instruments detect ethanol in breath because its C–H and O–H bonds absorb infrared radiation.
Section 5 of 5Exam Checklist & Tips
3.6.2 – Key Points for the Exam
- Wavenumber The reciprocal of wavelength, measured in cm⁻¹, used to label the horizontal axis of an IR spectrum.
- Transmittance The percentage of infrared radiation that passes through the sample without being absorbed.
- Fingerprint region The complex region below about 1500 cm⁻¹ that is unique to each compound.
- Functional group An atom or group of atoms that gives an organic molecule its characteristic reactions.
Common Mistakes
- Reading troughs as peaks pointing upwards; in IR spectra absorption appears as downward troughs.
- Stating that propanone has an O–H trough; it only shows a strong C=O absorption near 1700 cm⁻¹.
- Giving a wavenumber range without units or quoting it in nm instead of cm⁻¹.
- Describing the O–H of an alcohol as sharp; hydrogen bonding makes it broad.
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