9701 · 22.1

Infrared spectroscopy — practice questions

Practice and worked examples for 9701 Infrared spectroscopy. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

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 $C=O$ 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 $O-H$ group within a carboxylic acid. The broadness is due to extensive hydrogen bonding.
- The peaks around 2900 cm⁻¹ are due to $C-H$ bonds.
Combine the evidence: The compound contains both a $C=O$ group and an $O-H$ group characteristic of an acid.
Conclusion: The presence of both functional groups on the same molecule means that compound Y is a carboxylic acid.

Worked example 2

Two isomers, A and B, have the molecular formula $C_3H_8O$ . 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 $O-H$ group in an alcohol.
- The absence of a strong peak at 1600-1800 cm⁻¹ means there is no $C=O$ group.
- Therefore, Isomer A is an alcohol. Possible structures for $C_3H_8O$ are propan-1-ol ( $CH_3CH_2CH_2OH$ ) or propan-2-ol ( $CH_3CH(OH)CH_3$ ).
Analyse Isomer B:
- The absence of a broad absorption above 3100 cm⁻¹ indicates there is no $O-H$ group.
- The molecular formula $C_3H_8O$ suggests the oxygen must be in an ether linkage.
- Therefore, Isomer B is an ether. The only possible structure is methoxyethane ( $CH_3OCH_2CH_3$ ).
Conclusion: Isomer A is an alcohol (propan-1-ol or propan-2-ol) and Isomer B is an ether (methoxyethane).

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