CCEA A Level Chemistry
CCEA A Level Chemistry 5.2 Nuclear Magnetic Resonance Spectroscopy Exam Questions
Free CCEA A Level Chemistry revision for 5.2 Nuclear Magnetic Resonance Spectroscopy: what you need to know, common mistakes and exam-style questions with answers. Covers proton NMR, TMS, chemical shift, equivalent hydrogens, integration, n+1 splitting and deducing structures.
Practise 5.2 Nuclear Magnetic Resonance Spectroscopy with past papers
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About CCEA A Level Chemistry 5.2 Nuclear Magnetic Resonance Spectroscopy
Section 5.2 of Unit A2 2 covers proton NMR spectroscopy, the most powerful structure determination technique on the course. You need to distinguish low and high resolution spectra, explain why tetramethylsilane (TMS) is used as the standard, identify chemically equivalent hydrogen atoms and explain why chemical shift depends on chemical environment. You must use integration curves to find relative numbers of hydrogens in each environment, and apply the n+1 rule to interpret doublets, triplets and quartets. Questions usually ask you to deduce a structure from a spectrum, often alongside IR (2.7) and mass spectrometry (5.1) data, using the chemical shift table in the data leaflet.
What you need to know
- Environments Hydrogens in the same chemical environment give one signal. The number of signals in a low resolution spectrum equals the number of environments.
- Chemical shift Measured in ppm relative to TMS at 0. Hydrogens near electronegative atoms or C=O are deshielded and appear at higher chemical shift. Use the data leaflet table.
- TMS Si(CH3)4 is used because it gives a single strong signal from 12 equivalent hydrogens, at a value away from most other signals; it is inert, non-toxic and volatile, so easily removed.
- Integration The integration trace or area under each signal is proportional to the number of hydrogens in that environment, giving a ratio such as 3:2:1.
- Splitting (n+1 rule) A signal is split into n+1 peaks by n hydrogens on adjacent carbons: 1 neighbour gives a doublet, 2 a triplet, 3 a quartet. In ethanol, CH3 is a triplet, CH2 a quartet and OH usually a singlet.
Common mistakes in 5.2 Nuclear Magnetic Resonance Spectroscopy
- Counting hydrogens on the same carbon when applying n+1. Only count hydrogens on adjacent carbon atoms.
- Saying the number of peaks equals the number of hydrogen atoms. It equals the number of environments in low resolution.
- Forgetting that OH and NH hydrogens often appear as singlets. They usually do not split or get split by neighbours.
- Treating the integration ratio as the actual numbers. It gives a ratio; use the molecular formula to scale it.
- Saying TMS is used because it is reactive. It is chosen because it is inert and gives one signal at 0 ppm.
Try three 5.2 Nuclear Magnetic Resonance Spectroscopy exam-style questions
1. Predict the high resolution NMR spectrum of ethyl ethanoate, CH3COOCH2CH3, giving the number of signals, their splitting and integration ratio. (4 marks)
Show the mark scheme answer
Three signals; CH3 next to C=O is a singlet; OCH2 is a quartet; CH3 of ethyl is a triplet; ratio 3:2:3.
2. Give two reasons why TMS is used as a standard in NMR. (2 marks)
Show the mark scheme answer
Gives a single signal (all 12 H equivalent); signal at a lower shift than almost all other compounds; inert or non-toxic or volatile (any two).
3. A compound C2H4Cl2 has a spectrum with a doublet (3H) and a quartet (1H). Suggest its structure. (2 marks)
Show the mark scheme answer
CH3 split into a doublet so it is next to a CH; CH split into a quartet so it is next to CH3; 1,1-dichloroethane, CH3CHCl2.
5.2 Nuclear Magnetic Resonance Spectroscopy FAQs
What does CCEA A Level Chemistry 5.2 NMR Spectroscopy cover?
It covers low and high resolution proton NMR, TMS, chemical shift, equivalent hydrogens, integration and n+1 splitting, used to deduce simple structures.
Which unit is 5.2 NMR Spectroscopy examined in?
It is examined in Unit A2 2: Analytical, Transition Metals, Electrochemistry and Organic Nitrogen Chemistry, a 2 hour paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit A2 3.
How do I work out a structure from an NMR spectrum?
Count environments, use integration for hydrogen ratios, use splitting to find neighbouring hydrogens, use chemical shift to place groups near O or C=O, then check against the molecular formula.
