Use carbon-13 NMR environments to compare structures
Compare the expected carbon-13 environments of candidate structures to interpret a simple spectrum.
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
What does a smaller number of carbon-13 signals than carbon atoms suggest?
What you will learn
- Read the number of distinct carbon environments
- Compare possible structures by symmetry
- Combine environment count with chemical shifts
Signal count constrains the structure
A carbon-13 spectrum's number of signals gives the number of distinct carbon environments. Use this to test candidate structures: a symmetric candidate may have fewer signals than an asymmetric isomer with the same molecular formula.
The chemical-shift positions provide additional information about carbon environments, such as carbonyl, alkene/aromatic or alkyl carbons when the supplied table supports those assignments.
The syllabus data table gives typical ¹³C ranges in ppm: alkyl sp³ 0–50; sp³ next to alkene/arene 25–50, carbonyl/carboxyl 30–65, halogen 30–60, or oxygen 50–70; alkene/arene sp² 110–160; carboxyl sp² 160–185; carbonyl sp² 190–220; and nitrile sp 100–125. Use these ranges as evidence alongside the number of environments, not as unique identifiers.
Use all available evidence
A candidate should match the molecular formula, number of signals and plausible shift regions. A signal in a carbonyl range cannot be assigned to a simple alkyl carbon without explaining the discrepancy.
A simple spectrum may not uniquely identify a structure. State which candidates are consistent with the evidence and what additional data would distinguish them.
Pause and explain
Which evidence should be combined when proposing a structure from carbon-13 NMR?
Worked example
An unknown C₃H₆O has two carbon-13 signals, one in a carbonyl region and one in an alkyl region. Which common structure is consistent?
Show the worked solution
- The carbonyl signal indicates one carbonyl carbon.
- Three carbons but two environments means two carbons are equivalent.
- Test propanone's symmetry.
Answer Propanone, CH₃COCH₃, is consistent: it has a carbonyl carbon and two equivalent methyl carbons. Other evidence would be needed if multiple candidates fit.
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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.
A C₄H₈O₂ candidate has two carbon-13 signals, one in a carboxyl/ester carbonyl region. Give one structural feature that could explain the low signal count.
Give me a hint
Consider symmetry among the remaining carbons.
Compare my reasoning
- One environment is the carbonyl carbon.
- Other carbons may be equivalent by symmetry.
- Check whether the complete formula still fits.
A symmetric candidate could have equivalent alkyl carbons, reducing the number of signals; the exact structure must also match formula and shifts.
Look for these in your work
- Carbonyl assigned
- Equivalence used
- Formula retained
Two candidate structures have the same formula, but one predicts two carbon environments and the other predicts four. The spectrum has two signals. Which is more consistent?
Give me a hint
Compare each candidate's predicted environment count.
Compare my reasoning
- The two-signal candidate matches the observed count.
- The four-signal candidate predicts too many distinct environments.
- Use chemical shifts as a further check.
The candidate predicting two environments is more consistent, subject to matching the observed chemical shifts and other evidence.
Look for these in your work
- Signal count compared
- Alternative excluded provisionally
- Further check named
Why is ‘three carbon-13 signals means three carbons’ not a safe conclusion?
Give me a hint
Remember equivalent atoms share a signal.
Compare my reasoning
- Signal count is environment count.
- A molecule can have more carbon atoms than environments.
- Use the formula or another measurement for total carbon count.
Three signals indicate three distinct environments; multiple equivalent carbons can mean the molecule contains more than three carbon atoms.
Look for these in your work
- Meaning corrected
- Equivalence explained
- Total-count evidence noted
Common mistakes
- Choosing a structure using peak count alone
- Ignoring the molecular formula
- Assigning a carbonyl-range signal to an alkyl carbon
- Claiming uniqueness when evidence allows isomers
What can you explain now?
What does a smaller number of carbon-13 signals than carbon atoms suggest?
Compare with the explanation
Some carbon atoms are chemically equivalent
Equivalent environments contribute one signal.
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Make it stick
How can carbon-13 signal counts and shifts narrow down possible structures without always proving a unique answer?
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