OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.3.15 Nuclear Magnetic Resonance Spectroscopy Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.3.15 Nuclear Magnetic Resonance Spectroscopy: what you need to know, common mistakes and exam-style questions with answers. Covers carbon-13 and proton NMR, chemical shift and the delta scale, TMS, deuterated solvents, integration and n+1 splitting patterns.
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About OxfordAQA International A Level Chemistry 3.3.15 Nuclear Magnetic Resonance Spectroscopy
Nuclear magnetic resonance spectroscopy is an A2 analysis topic examined in Unit 4. You need to interpret carbon-13 NMR spectra, which are simpler because each peak represents a different carbon environment, and proton NMR spectra, using chemical shift values on the delta scale from the data booklet. You also need to explain the use of tetramethylsilane (TMS) as the standard, and why samples are dissolved in deuterated solvents or CCl4. For proton NMR you must use integration traces to find the relative numbers of hydrogens and use the n+1 rule for splitting into doublets, triplets and quartets. NMR is often combined with infrared and mass spectrometry in structure problems.
What you need to know
- Carbon-13 NMR Each peak shows a different carbon environment, so the number of peaks gives the number of environments. Chemical shift shows the type of environment, for example C=O at around 160 to 220 ppm.
- Proton NMR The number of peaks shows the number of different hydrogen environments and the chemical shift shows their type, using data booklet values.
- TMS Tetramethylsilane, Si(CH3)4, is the standard at 0 ppm. It gives a single strong peak, is inert, volatile and easily removed.
- Solvents Samples are dissolved in deuterated solvents such as CDCl3, or in CCl4, so the solvent does not give peaks in the proton NMR spectrum.
- Integration The integration trace gives the relative number of hydrogen atoms in each environment.
- Splitting The n+1 rule: a peak is split into n+1 lines by n hydrogens on adjacent carbons. A singlet has 0 neighbours, a doublet 1, a triplet 2 and a quartet 3. An ethyl group gives a triplet and a quartet.
Common mistakes in 3.3.15 Nuclear Magnetic Resonance Spectroscopy
- Applying the n+1 rule using the hydrogens in the same environment. Count hydrogens on adjacent carbon atoms only.
- Counting every carbon as a separate peak in carbon-13 NMR. Equivalent carbons by symmetry give one peak.
- Saying TMS is used because it reacts with the sample. It is used because it is inert and gives a single peak at 0 ppm.
- Forgetting that OH and NH protons usually appear as singlets.
Try three 3.3.15 Nuclear Magnetic Resonance Spectroscopy exam-style questions
1. How many peaks are in the carbon-13 NMR spectrum of propan-2-ol, (CH3)2CHOH? (1 mark)
Show the mark scheme answer
2.
2. Predict the splitting of each peak in the proton NMR spectrum of ethyl ethanoate, CH3COOCH2CH3. (3 marks)
Show the mark scheme answer
CH3 next to C=O: singlet; CH2: quartet; CH3 of the ethyl group: triplet.
3. Explain why CDCl3 is used as a solvent in proton NMR. (1 mark)
Show the mark scheme answer
It has no hydrogen-1 atoms, so it does not produce a peak in the spectrum.
3.3.15 Nuclear Magnetic Resonance Spectroscopy FAQs
What does OxfordAQA International A Level Chemistry 3.3.15 Nuclear Magnetic Resonance Spectroscopy cover?
It covers carbon-13 and proton NMR, chemical shift and the delta scale, TMS, deuterated solvents, integration and n+1 splitting patterns.
Which unit is 3.3.15 Nuclear Magnetic Resonance Spectroscopy examined in?
It is examined in Unit 4, an International A2 paper covering organic and physical chemistry. The Unit 5 practical and synoptic paper can also assess it.
How do I solve an NMR structure question?
Use the molecular formula, count the environments, use chemical shifts for each type, use integration for numbers of hydrogens and use splitting to find neighbouring groups. Then check your structure against every peak.
