In simple terms
A friendly intro before the formal notes — no formulas yet.
The Molecular Vibe Check
Infrared spectroscopy is like playing different notes to make molecular bonds 'dance'. By seeing which 'notes' (frequencies) the bonds respond to, we can figure out what types of bonds are in the molecule.
Imagine you have a collection of different tuning forks. If you play a specific sound frequency, only the tuning fork designed for that exact frequency will start to vibrate. In a molecule, the covalent bonds are like different tuning forks. When we shine a range of infrared frequencies on them, each type of bond (like C=O or O-H) absorbs the specific frequency it needs to vibrate, telling us it's there.
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A sample of the organic compound is placed in an infrared spectrometer.
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A beam of infrared radiation, containing a wide range of frequencies, is passed through the sample.
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Different covalent bonds within the molecules absorb radiation at their own specific resonant frequencies, causing them to stretch or bend.
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A detector measures the amount of radiation transmitted at each frequency, producing a spectrum that shows which frequencies were absorbed.
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The Principles of IR Spectroscopy
Covalent bonds are not rigid sticks; they are more like springs that are constantly vibrating. These vibrations can be a 'stretch' (along the bond axis) or a 'bend' (changing the bond angle). Each type of bond, such as C-H, O-H, or C=O, has a natural frequency at which it vibrates. When infrared radiation with that exact same frequency is shone on the molecule, the bond absorbs the energy, causing it to vibrate with a greater amplitude. The specific frequencies absorbed depend on the strength of the bond (stronger bonds like double bonds vibrate at higher frequencies) and the masses of the atoms involved (bonds with lighter atoms vibrate at higher frequencies).
Interpreting an IR Spectrum
An IR spectrum is a graph that plots the amount of infrared radiation that passes through a sample against the wavenumber of the radiation. The vertical axis is Percentage Transmittance (%T), which is the percentage of radiation that gets through the sample without being absorbed. The horizontal axis is Wavenumber in cm⁻¹, which is proportional to frequency. By convention, the wavenumber scale runs from high to low, from left to right (e.g., 4000 cm⁻¹ to 400 cm⁻¹).
A dip or trough in the graph is called an 'absorption' or a 'peak'.
A low transmittance value (e.g., 10%) means a high amount of absorption has occurred at that wavenumber.
The position of the peak (wavenumber) tells you what type of bond is present.
The intensity (how far down the peak goes) can be described as strong (s), medium (m), or weak (w).
The shape of the peak can be described as sharp or broad.
Characteristic Absorption Frequencies
The region of the spectrum above 1500 cm⁻¹ is known as the functional group region. Absorptions here are typically due to simple stretching vibrations and can be clearly correlated with specific functional groups. You must be able to recognise the following key absorptions, using the values provided in your Data Booklet.
O-H bond in alcohols: A broad peak at 3200–3600 cm⁻¹. The broadness is due to hydrogen bonding.
O-H bond in carboxylic acids: A very broad peak at 2500–3300 cm⁻¹. This is even broader than in alcohols and often overlaps the C-H region.
C=O bond in carbonyl compounds: A strong, sharp peak at 1680–1750 cm⁻¹. This is often the most obvious peak in the spectrum.
C-H bond: Peaks are found around 2850–3100 cm⁻¹. While almost all organic molecules have these, they confirm the presence of an alkyl skeleton. The C-H stretch in an aldehyde (~2720-2820 cm⁻¹) is a useful diagnostic peak.
Examiners often test your ability to distinguish between an alcohol and a carboxylic acid. The key is the C=O peak. An alcohol has a broad O-H peak but NO C=O peak. A carboxylic acid has BOTH a very broad O-H peak AND a strong, sharp C=O peak.
The Fingerprint Region
The region of the spectrum below 1500 cm⁻¹ is called the fingerprint region. It contains a large number of complex absorptions caused by bending vibrations and other motions involving the entire molecule. While it is difficult to assign individual peaks in this region, the overall pattern is unique to a specific compound. Therefore, by comparing the fingerprint region of an unknown compound to a database of spectra from known compounds, a positive identification can be made.
Pay as much attention to the absence of peaks as their presence. If you are trying to decide between a ketone and an alcohol, the lack of a broad O-H peak is definitive evidence against the alcohol structure.
Worked examples
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A liquid compound, Y, is analysed by IR spectroscopy. The spectrum shows a very broad absorption centred at approximately 3000 cm⁻¹ and a strong, sharp absorption at 1710 cm⁻¹. There are also peaks around 2900 cm⁻¹. Deduce the functional groups present and identify the class of organic compound to which Y belongs.
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Analyse the peaks:
- A strong, sharp absorption at 1710 cm⁻¹ is characteristic of a carbonyl group. (Data booklet range: 1680–1750 cm⁻¹).
- A very broad absorption from 2500-3300 cm⁻¹ (centred at 3000 cm⁻¹) is characteristic of the group within a carboxylic acid. The broadness is due to extensive hydrogen bonding.
- The peaks around 2900 cm⁻¹ are due to bonds.
Two isomers, A and B, have the molecular formula . The IR spectrum of isomer A shows a broad absorption at 3350 cm⁻¹ but no absorption between 1600-1800 cm⁻¹. The IR spectrum of isomer B shows no broad absorption above 3100 cm⁻¹. Identify the functional groups in A and B and suggest their structures.
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Analyse Isomer A:
- A broad absorption at 3350 cm⁻¹ is characteristic of an group in an alcohol.
- The absence of a strong peak at 1600-1800 cm⁻¹ means there is no group.
- Therefore, Isomer A is an alcohol. Possible structures for are propan-1-ol () or propan-2-ol ().
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Glossary
Key terms for this topic — skim now; the Check step will test them.
- O-H bond in alcohols
A broad peak at 3200–3600 cm⁻¹. The broadness is due to hydrogen bonding.
- O-H bond in carboxylic acids
A very broad peak at 2500–3300 cm⁻¹. This is even broader than in alcohols and often overlaps the C-H region.
- C-H bond
Peaks are found around 2850–3100 cm⁻¹. While almost all organic molecules have these, they confirm the presence of an alkyl skeleton.
- Fingerprint region
The complex region of the spectrum below 1500 cm⁻¹. It is unique to a particular molecule and is used for identification by comparison with known spectra.
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Key takeaways
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A dip or trough in the graph is called an 'absorption' or a 'peak'.
A low transmittance value (e.g., 10%) means a high amount of absorption has occurred at that wavenumber.
The position of the peak (wavenumber) tells you what type of bond is present.
The intensity (how far down the peak goes) can be described as strong (s), medium (m), or weak (w).
The shape of the peak can be described as sharp or broad.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
The infrared spectrum of R is shown in Fig. 5.3. Use the absorptions in the region above 1500cm¯¹ in Table 5.1 when answering this question. Add F to Fig. 5.3 to identify the peak that is present in an infrared spectrum of both Q and R. Identify the bond that corresponds to the absorption for F. Add G to Fig. 5.3 to identify the peak that is not present in an infrared spectrum of Q. Identify the bond that corresponds to the absorption for G.
Fig. 5.3 shows the structure of Z. Fig. 5.4 is the infrared spectrum of Z. Identify the bond and functional group responsible for each of the absorptions labelled A, B, C and D in Fig. 5.4. A B C D
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