Identify functional groups from infrared spectra
Use the supplied characteristic absorption ranges and more than one spectral feature to identify functional groups.
AS Level; Cambridge International Chemistry 9701, syllabus for 2028–2030. Use the functional-group data supplied by Cambridge; knowledge of how a mass spectrometer works is not required. Spectra and values in these questions are illustrative. Written practice is self-checked
Before you begin
- Recognise the functional groups from Topics 13–20 and calculate relative mass and weighted means.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which bond commonly gives an IR absorption in the 2200–2250 cm⁻¹ range?
What you will learn
- Read an IR absorption in wavenumbers
- Match characteristic ranges to functional-group bonds
- Combine overlapping evidence to identify a simple molecule's groups
Match absorption ranges to bonds
Use the Data section provided for the examination. Characteristic ranges include C–O 1040–1300 cm⁻¹; C=C 1500–1680; C=O in amides 1640–1690, carbonyl/carboxyl groups 1670–1740 and esters 1710–1750; C≡N 2200–2250; alkane C–H 2850–2950; and N–H 3300–3500.
O–H absorptions can help distinguish groups: carboxyl O–H is 2500–3000 cm⁻¹ and hydroxy O–H is 3200–3650. Use the supplied table rather than relying on memorised values that may differ from the examination data.
Use a pattern, not one isolated peak
Some ranges overlap, so a single absorption may not identify a unique group. Combine the carbonyl region with C–O, O–H, N–H or C≡N evidence and the molecular formula or other information supplied.
For example, a broad carboxyl O–H absorption together with a C=O absorption supports a carboxylic acid. An ester has C=O and C–O evidence but no carboxyl O–H group. The task is to identify groups in a simple molecule, not to explain instrument operation.
Pause and explain
Which pair best supports an ester rather than a carboxylic acid?
Worked example
An IR spectrum shows absorptions at 1718 cm⁻¹ and 1150 cm⁻¹, with no broad O–H absorption. Which group is most consistent with these features?
Show the worked solution
- 1718 cm⁻¹ lies in the ester C=O range.
- 1150 cm⁻¹ lies in the C–O range.
- Absence of carboxyl O–H supports an ester rather than a carboxylic acid.
Answer An ester group is consistent with the C=O and C–O absorptions and no carboxyl O–H feature.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
An unknown has a strong absorption at 1730 cm⁻¹ and a C–O absorption at 1180 cm⁻¹, but no O–H feature. Which functional group is most consistent?
Give me a hint
Use both carbonyl and single-bond evidence.
Compare my reasoning
- 1730 cm⁻¹ lies in the ester C=O range.
- 1180 cm⁻¹ lies in the C–O range.
- No O–H feature argues against a carboxylic acid or alcohol.
An ester is most consistent with the C=O and C–O absorptions and no O–H absorption.
Look for these in your work
- Two characteristic ranges used
- O–H evidence considered
- Conclusion identifies ester
A spectrum has a C=O absorption and a broad feature at 2600 cm⁻¹. Explain why this supports a carboxylic acid rather than an ester.
Give me a hint
Compare the supplied carboxyl O–H range with the ester features.
Compare my reasoning
- 2600 cm⁻¹ is in the carboxyl O–H range.
- Both acids and esters can show a carbonyl absorption.
- The broad O–H feature distinguishes the acid.
The broad feature at 2600 cm⁻¹ matches carboxyl O–H. Together with C=O, it supports a carboxylic acid; an ester does not have that O–H group.
Look for these in your work
- Correct range applied
- Carbonyl overlap recognised
- Group distinguished by combined evidence
A simple molecule has absorptions in the 3300–3500 and 1710–1740 cm⁻¹ ranges. Give a plausible pair of functional groups and explain why the spectrum alone may not name one unique molecule.
Give me a hint
Match N–H and carbonyl ranges, then consider other structures that can contain them.
Compare my reasoning
- 3300–3500 cm⁻¹ indicates N–H.
- 1710–1740 cm⁻¹ can indicate a carbonyl/carboxyl group.
- Different structures can share the same functional groups.
An amine or amide N–H and a carbonyl-containing group are plausible. The data identify features, not a unique whole structure; additional evidence such as molecular formula is needed.
Look for these in your work
- Both ranges matched
- Group inference qualified
- Need for further evidence explained
Common mistakes
- Treating one overlapping peak as conclusive
- Mixing up carboxyl and hydroxy O–H ranges
- Ignoring the supplied data table
- Explaining the mass spectrometer instead of interpreting the spectrum
What can you explain now?
Which bond commonly gives an IR absorption in the 2200–2250 cm⁻¹ range?
Compare with the explanation
C≡N in a nitrile
The supplied data list this range for nitrile C≡N.
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Make it stick
How do you combine overlapping IR ranges with positive and negative evidence to identify a functional group?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.