A pharmaceutical company is developing a new painkiller. A chemist uses mass spectrometry and infrared spectroscopy to identify the structure of an unknown organic compound isolated during the synthesis process. The mass spectrum shows a molecular ion peak at m/z = 46, and the infrared spectrum shows a broad absorption in the range 2500–3300 cm⁻¹. Explain how mass spectrometry and infrared spectroscopy are used together to identify the unknown compound, and identify the functional group present in the compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The unknown compound has a molecular ion peak at m/z = 46 in its mass spectrum. Its infrared spectrum shows a broad absorption in the range 2500–3300 cm⁻¹.
Model answer (5 marks)
Mass spectrometry gives the molecular ion peak at m/z = 46, so the relative molecular mass of the unknown is 46.
Fragmentation peaks can be examined to see which parts of the molecule are lost; for a compound of this mass the most common fragment is a CH₂O⁺ ion (m/z = 30) or a C₂H₄⁺ ion (m/z = 28), indicating the presence of a carbonyl and a small hydrocarbon fragment.
Infrared spectroscopy shows a broad absorption between 2500–3300 cm⁻¹. This region is characteristic of an O–H stretch in a carboxylic acid, and the accompanying sharp peak near 1700 cm⁻¹ would confirm the C=O stretch of the same group.
Thus, the mass spectrum tells us the molecular mass and suggests a small carbonyl-containing fragment, while the IR spectrum confirms the presence of a carboxylic‑acid O–H stretch. Together they identify the unknown as a carboxylic acid with a molecular mass of 46.
The functional group present is a carboxylic acid (–COOH).
Fragmentation peaks can be examined to see which parts of the molecule are lost; for a compound of this mass the most common fragment is a CH₂O⁺ ion (m/z = 30) or a C₂H₄⁺ ion (m/z = 28), indicating the presence of a carbonyl and a small hydrocarbon fragment.
Infrared spectroscopy shows a broad absorption between 2500–3300 cm⁻¹. This region is characteristic of an O–H stretch in a carboxylic acid, and the accompanying sharp peak near 1700 cm⁻¹ would confirm the C=O stretch of the same group.
Thus, the mass spectrum tells us the molecular mass and suggests a small carbonyl-containing fragment, while the IR spectrum confirms the presence of a carboxylic‑acid O–H stretch. Together they identify the unknown as a carboxylic acid with a molecular mass of 46.
The functional group present is a carboxylic acid (–COOH).
Examiner tips
- State the m/z of the molecular ion and link it to the molecular mass. Explain how fragment peaks give clues to the structure. Identify the IR band and match it to the O–H stretch of a carboxylic acid. Show how the two techniques complement each other.
Mark scheme (5 marks)
- The molecular ion peak in the mass spectrum gives the relative molecular mass of the compound
- Fragmentation peaks in the mass spectrum can be used to identify fragments / parts of the molecule
- Infrared spectroscopy identifies functional groups by matching absorptions to a reference / data table of known absorptions
- The broad absorption at 2500–3300 cm⁻¹ indicates the presence of an O–H bond in a carboxylic acid / the functional group is a carboxylic acid (–COOH)
- Using both techniques together provides more complete / reliable identification than either technique alone
Key terms in this question
mass spectrometry · molecular ion peak · infrared spectroscopy · functional group
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →