22.1 IR Spectroscopy
Candidates should be able to:
- Analyse an infrared spectrum of a simple molecule to identify functional groups.
1 What Is Infrared Spectroscopy?
In infrared (IR) spectroscopy, a sample is irradiated — exposed to radiation — with electromagnetic waves from the infrared region of the spectrum. The instrument used is a spectrophotometer, which detects the intensity of infrared wavelengths that pass through the sample.
This technique is especially useful to organic chemists because every organic molecule absorbs infrared radiation. The energy absorbed corresponds to changes in the vibration of the bonds between atoms. Covalent bonds are never static — they constantly vibrate, and they can do so in three main ways:
Every bond has a natural frequency at which it vibrates. If a molecule is irradiated with energy that exactly matches this frequency, the vibration is stimulated and energy is absorbed — this is called the resonance frequency of that vibration.
Chemists express absorbed frequencies as their reciprocal, in a unit called the wavenumber (symbol cm⁻¹) — literally the number of wave cycles per centimetre. Every IR spectrum is plotted against wavenumber, always running from high (4000 cm⁻¹) on the left to low (500 cm⁻¹) on the right.
2 Reading an IR Spectrum
On an IR spectrum, the y-axis shows absorbance. Every dip (trough) pointing downward marks a wavenumber at which the sample strongly absorbed infrared radiation — this is called an absorption band. Each functional group produces bands in a characteristic, predictable range, which is what makes IR spectra so useful for identification.
Hydrogen bonding broadens absorption bands. The O–H bond in alcohols and carboxylic acids experiences extensive hydrogen bonding, so it produces a wide, rounded trough rather than a narrow spike. By contrast, the C=O bond in a carbonyl group is not hydrogen-bonded and gives a strong, sharp, well-defined peak — one of the easiest bands to spot on any spectrum.
Characteristic infrared absorption frequencies
| Bond | Functional groups containing the bond | Absorption range / cm⁻¹ |
|---|---|---|
| C–O | hydroxy, ester | 1040 – 1300 |
| C=C | aromatic compound, alkene | 1500 – 1680 |
| C=O | amide | 1640 – 1690 |
| C=O | carbonyl, carboxyl | 1670 – 1740 |
| C=O | ester | 1710 – 1750 |
| C≡N | nitrile | 2200 – 2250 |
| C–H | alkane | 2850 – 2950 |
| N–H | amine, amide | 3300 – 3500 |
| O–H | carboxyl | 2500 – 3000 (broad) |
| O–H | hydroxy | 3200 – 3600 (broad) |
The two regions of a spectrum
Most bonds above give recognisable, isolated bands — this upper part of the spectrum is often called the functional group region. Below about 1500 cm⁻¹, bands overlap heavily and are unique to each molecule, forming a fingerprint region used to confirm an exact compound rather than identify individual bonds.
3 Case Studies: Identifying Functional Groups
The three spectra below all belong to small, related molecules — an alcohol, a carboxylic acid, and an ester — so comparing them side by side shows clearly how each functional group leaves its own signature.
Ethyl ethanoate, CH₃COOCH₂CH₃
This ester is built from the same C=O group as ethanoic acid, but with no acidic O–H left in the molecule. Using the table in Section 2, work out what you’d expect to see — then check your reasoning below.
- The x-axis always runs high → low wavenumber (4000 → 500 cm⁻¹).
- A trough = an absorption band; you are matching its position and shape to a bond.
- Hydrogen bonding = broad peak. No hydrogen bonding = sharp peak.
- An O–H band and a C=O band appearing together is the classic signature of a carboxylic acid.
- Alcohol O–H (3200–3600, broad) with carboxylic acid O–H (2500–3000, very broad).
- N–H (3300–3500) with O–H — both sit in the same region; check the shape and the rest of the molecule.
- The three overlapping C=O ranges — amide, carbonyl/carboxyl, and ester all sit close together.
IR spectroscopy irradiates a sample and measures which infrared wavenumbers it absorbs. Each bond has its own characteristic range, so matching the troughs on a spectrum to a data table identifies the functional groups present.