Use the n + 1 rule to interpret proton splitting
Relate a proton signal's splitting pattern to the number of equivalent neighbouring protons on adjacent carbon atoms.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 37. Use the supplied spectra, shift table and chromatography conditions. Interpret peaks as evidence and state when a short trace cannot uniquely identify a substance. Written practice is self-checked
Before you begin
- Recognise functional groups, molecular structures, polarity, intermolecular interactions and the distinction between equivalent and different chemical environments.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A signal is split by two equivalent adjacent protons. What pattern is expected?
What you will learn
- Count equivalent adjacent protons
- Apply the n + 1 rule
- Recognise the syllabus-limited splitting patterns
Count neighbours on adjacent carbons
For the syllabus n + 1 rule, a set of equivalent protons is split by n equivalent protons on adjacent carbon atoms into n + 1 lines. One neighbour gives a doublet, two give a triplet and three give a quartet.
The rule concerns adjacent carbon-bound protons in the simple structures covered. Do not count the protons in the signal itself as neighbours, and do not count atoms beyond the adjacent carbon.
Translate lines back into neighbours
A singlet corresponds to no equivalent neighbouring protons in this simple rule; a multiplet can arise when the pattern is more complex. The syllabus limits named patterns to singlet, doublet, triplet, quartet and multiplet.
Splitting and integration answer different questions: the number of lines indicates neighbouring protons, while the total area indicates the relative number in the signal environment.
Pause and explain
A quartet indicates how many equivalent adjacent protons under the n + 1 rule?
Worked example
A CH₃ group is next to CH₂ in an alkane. What splitting is expected for its signal under the n + 1 rule?
Show the worked solution
- The methyl protons have two equivalent neighbouring protons on the adjacent CH₂ carbon.
- Set n = 2.
- Calculate n + 1 lines.
Answer Three lines: a triplet, because n + 1 = 2 + 1 = 3.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
What splitting is expected for a CH₂ group next to a CH₃ group?
Give me a hint
Count the three equivalent protons on the adjacent carbon.
Compare my reasoning
- n = 3 neighbouring protons.
- Use n + 1.
- Name the four-line pattern.
Quartet, because 3 + 1 = 4 lines.
Look for these in your work
- Neighbours counted
- Rule applied
- Pattern named
A signal is a doublet. How many equivalent neighbouring protons are indicated by the simple n + 1 rule?
Give me a hint
Subtract one from the number of lines.
Compare my reasoning
- A doublet has two lines.
- n + 1 = 2.
- Therefore n = 1.
One equivalent neighbouring proton.
Look for these in your work
- Line count read
- Equation rearranged
- Neighbour count correct
A learner sees a triplet and says the signal represents three protons. What distinction corrects the statement?
Give me a hint
Splitting lines and integrated area measure different properties.
Compare my reasoning
- A triplet has three lines because there are two neighbouring equivalent protons.
- The integral gives the number of protons in the signal environment.
- A triplet could integrate to a different proton count.
A triplet indicates two equivalent neighbouring protons under n + 1; the signal's integrated area, not its line count, gives the relative number of protons in that environment.
Look for these in your work
- Splitting interpreted
- Integral distinguished
- Overclaim corrected
Common mistakes
- Counting protons on the same carbon as neighbours
- Using integration to predict splitting
- Forgetting to add one
- Applying the rule beyond the simple syllabus patterns without evidence
What can you explain now?
A signal is split by two equivalent adjacent protons. What pattern is expected?
Compare with the explanation
Triplet
n + 1 = 2 + 1 = 3 lines.
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Make it stick
What does splitting reveal about neighbouring protons, and how is it different from integration?
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