4.9.4Unit 4. Rates, Equilibria and Further Organic Chemistry
High Resolution Proton NMR — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 54 of 56 Unit quiz
Section 1 of 4Splitting
In high resolution spectra each peak is split into a multiplet by neighbouring hydrogens.
n + 1 rule
- A hydrogen with n equivalent hydrogens on the adjacent carbon atoms appears as a peak split into n + 1 lines.
Singlet
- No hydrogens on adjacent carbons gives a single line, as for CH₃ in CH₃COO–.
Doublet and triplet
- One neighbouring H gives a doublet with ratio 1:1, and two neighbouring H atoms give a triplet with ratio 1:2:1.
Quartet
- Three neighbouring H atoms give a quartet with the line intensities in the ratio 1:3:3:1.
Section 2 of 4Worked Pattern: Ethyl Ethanoate
The ester CH₃COOCH₂CH₃ has three different hydrogen environments.
Worked Pattern: Ethyl Ethanoate
CH₃CO–
- This group has no adjacent hydrogens, so it appears as a singlet with area 3, at about 2.0 ppm.
–OCH₂–
- The two hydrogens have three neighbours on the CH₃ group, so they appear as a quartet with area 2 at about 4.1 ppm.
–CH₃ of ethyl
- These three hydrogens have two neighbours on the CH₂ group, so they appear as a triplet with area 3 at about 1.2 ppm.
Link
- The quartet and triplet pair together indicate the presence of an ethyl group in the molecule.
Section 3 of 4Deducing a Structure
1
Count the peaks and use integration to find how many hydrogens are in each environment.
2
Use the shifts to assign each peak to the group it is attached to.
3
Use the splitting pattern of each peak to find how many neighbouring hydrogens it has.
4
Piece together the fragments so that every peak is explained by the neighbours.
5
Hydrogens on oxygen or nitrogen usually appear as singlets because they exchange rapidly and are not coupled to neighbours.
Section 4 of 4Exam Checklist & Tips
4.9.4 – Key Points for the Exam
- Spin–spin splitting Splitting of a peak caused by hydrogens on adjacent carbon atoms.
- Multiplet Group of lines produced by splitting.
- Singlet Peak with no splitting.
- Coupling Interaction between neighbouring hydrogen nuclei.
Common Mistakes
- Using n instead of n + 1 lines for the splitting pattern.
- Counting hydrogens on the same carbon as neighbours.
- Forgetting that equivalent hydrogens do not split each other.
- Expecting –OH hydrogens to show splitting in most spectra.
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