Infrared spectroscopy — AQA A-Level Chemistry
Test yourself on Infrared spectroscopy with AQA A-Level practice questions.
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Infrared spectroscopy explained
A covalent bond behaves like two masses joined by a spring, vibrating at a frequency fixed by the strength of the bond and the masses of the atoms it joins.
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When infrared radiation of exactly that frequency passes through the sample it is absorbed, the vibration becomes larger, and a trough appears on the spectrum. Because each type of bond has its own natural frequency, absorptions appear at characteristic wavenumbers, measured in cm⁻¹, which you look up on the data sheet rather than recalling. AQA credits any value inside the tabulated range, so the broad alcohol O–H is accepted anywhere from about 3230 to 3550 cm⁻¹ and the carbonyl C=O from about 1680 to 1750 cm⁻¹.
‘Fingerprinting’ allows identification of a molecule by comparison of spectra.
Below about 1500 cm⁻¹ an infrared spectrum becomes a dense pattern of overlapping absorptions produced by bending and by vibrations of the whole carbon skeleton. Individual peaks there cannot reliably be assigned to single bonds, but the pattern as a whole is unique to one compound, which is why it is called the fingerprint region. Identification therefore works by matching: the spectrum of the unknown is compared with spectra of known compounds held in a database, and a match across the fingerprint region identifies the compound. This technique is highly effective for separating chain or positional isomers, which contain the same functional groups and so look alike above 1500 cm⁻¹ but differ in skeletal vibrations below it. NMR spectroscopy can also distinguish isomers.
Students should be able to: use infrared spectra and the Chemistry Data Sheet or Booklet to identify particular bonds, and therefore functional groups, and also to identify impurities.
Read a spectrum in a fixed order. Look first for a strong sharp absorption near 1700 cm⁻¹: if C=O is present the compound could be an aldehyde, ketone, carboxylic acid, ester, amide, or acyl chloride. Then look for O–H. A broad absorption between about 2500 and 3000 cm⁻¹ alongside the C=O means a carboxylic acid; a broad absorption between about 3230 and 3550 cm⁻¹ with no C=O means an alcohol. Ignore the C–H absorptions near 2900 cm⁻¹, which nearly every organic compound shows. Impurities appear as absorptions that should not be there: an alcohol O–H still visible in a spectrum of a ketone product shows unreacted alcohol, and a C=O in a spectrum of an alcohol shows unwanted oxidation or incomplete reduction.
Your focus
- Explain why different bonds absorb infrared radiation at different wavenumbers.
- Use the data sheet to quote the wavenumber range for a named bond, with the correct unit.
- Identify the bond responsible for a labelled absorption on a given infrared spectrum.
Show all 9 objectives
- State where the fingerprint region lies and explain why its pattern is unique to a compound.
- Explain how a compound is identified from its fingerprint region, naming what the spectrum is compared with.
- Justify choosing infrared fingerprinting over mass spectrometry to distinguish two positional isomers.
- Deduce the functional group present in a compound from the peaks present and absent in its infrared spectrum.
- Distinguish an alcohol from a carboxylic acid using the position and shape of the O–H absorption.
- Identify an impurity in a product from an unexpected absorption and name the likely substance.
Infrared spectroscopy exam tips
Marking Points
- one mark for quoting a wavenumber lying inside the range given on the data sheet, such as 1720 cm⁻¹ for C=O
- one mark for naming the bond responsible for that absorption
- one mark for describing the shape of the peak where it distinguishes, such as the broad alcohol O–H
- one mark for linking the absorption to vibration of the bond at a characteristic frequency
- one mark for identifying the fingerprint region as below about 1500 cm⁻¹
- one mark for saying the pattern there is unique to a particular compound
- one mark for saying identification requires comparison with the spectrum of a known compound or a database
- one mark for explaining that chain or positional isomers share functional groups, so the fingerprint region is used to separate them
- one mark for quoting the wavenumber of the peak used and the range for it on the data sheet
- one mark for assigning that peak to the correct bond
- one mark for deducing the functional group from the combination of peaks present and absent
- one mark for identifying an impurity from a peak that should not be present and naming the substance
Examiner Tips
- 💡Always open the data sheet before quoting a value; examiners accept the tabulated range, not your memory.
- 💡Write the unit cm⁻¹ every time, including inside explanations.
- 💡Describe the peak as broad or sharp where the data sheet distinguishes them; the shape can carry a mark.
- 💡Say "compared with a database or known spectrum"; identification by matching is the point of the region.
- 💡If two compounds are chain or positional isomers, the fingerprint region is a standard intended answer for distinguishing them.
- 💡Quote the boundary as about 1500 cm⁻¹ rather than an exact figure.
- 💡Say which peaks are absent as well as which are present; absence is often the deciding evidence.
- 💡Quote your value and then the data sheet range beside it, so the examiner can see the match.
- 💡For a purity question, ask what the starting material would show and check whether that peak is still there.
Common Mistakes
- quoting a remembered wavenumber that falls outside the range on the data sheet
- reading the horizontal axis the wrong way, since wavenumber decreases from left to right
- giving the unit as cm or nm rather than cm⁻¹
- naming a functional group when the question asked for the bond
- trying to assign individual bonds within the fingerprint region, which is the job of the region above 1500 cm⁻¹
- claiming a compound can be named from its fingerprint region with no reference spectrum
- choosing high resolution mass spectrometry to distinguish isomers, which AQA sets as a wrong option
- assuming all isomers have identical IR spectra above 1500 cm⁻¹ (functional group isomers will differ)
- naming a functional group from one peak when the absence of a second peak is what settles it
- treating the C–H absorption near 2900 cm⁻¹ as diagnostic, when almost every organic compound shows it
- reading any broad O–H as an alcohol without checking whether it stretches down towards 2500 cm⁻¹
- quoting a peak position without the unit cm⁻¹