Assign proton environments using chemical shifts
Use the Cambridge shift ranges to link proton signals to their chemical environments.
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 reference is used for the chemical-shift scale?
What you will learn
- Use chemical shift as evidence for proton environments
- Apply the supplied Cambridge shift ranges
- Account for O–H and N–H variability
Read shifts from the reference table
Chemical shift δ is measured in ppm relative to TMS at 0. The Cambridge table gives typical ranges: alkyl protons are about 0.9–1.7; alkyl next to C=O about 2.2–3.0; next to O or halogen about 3.2–4.0; alkene about 4.5–6.0; aromatic about 6.0–9.0; aldehyde about 9.3–10.5; and carboxylic-acid O–H about 9–13 ppm.
Use the full table provided for alcohol, phenol, amine and amide O–H/N–H regions. Those exchangeable proton shifts can vary with solvent and concentration, so do not over-interpret an exact value.
Assign by environment, not just a number
Identify the atoms adjacent to the proton-bearing atom. Nearby electronegative atoms and carbonyl or aromatic groups alter electron shielding and shift signals downfield relative to ordinary alkyl protons.
Typical ranges can overlap. Combine shift with integration, splitting and the molecular formula rather than claiming a unique group from one value when the ranges overlap.
Pause and explain
A proton signal at 3.6 ppm is consistent with which common environment in the Cambridge table?
Worked example
A proton signal lies at 9.8 ppm. Which common environments from the Cambridge table are plausible?
Show the worked solution
- Compare 9.8 with the aldehyde range.
- Compare it with the carboxylic-acid O–H range.
- Use other evidence to distinguish them.
Answer An aldehyde proton (9.3–10.5 ppm) or a carboxylic-acid O–H proton (9.0–13.0 ppm) is plausible; the shift alone does not distinguish them.
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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.
Assign plausible environments for signals at 1.2, 4.8 and 10.0 ppm using typical Cambridge ranges.
Give me a hint
Compare each value with the supplied reference ranges.
Compare my reasoning
- 1.2 ppm falls in the alkane range.
- 4.8 ppm can be alkene or some exchangeable environments.
- 10.0 ppm can be aldehyde or carboxylic-acid O–H.
1.2 ppm: alkyl; 4.8 ppm: alkene or overlapping O–H/N–H range depending on structure; 10.0 ppm: aldehyde or carboxylic acid O–H. More evidence is needed for the overlapping cases.
Look for these in your work
- Table ranges applied
- Overlap recognized
- Ambiguity stated
Why can an O–H proton shift vary more than a carbon-bound alkyl proton?
Give me a hint
Consider hydrogen bonding, solvent and concentration.
Compare my reasoning
- The Cambridge note warns that O–H/N–H shifts vary.
- Solvent and concentration can affect exchangeable protons.
- Use the range as typical evidence, not a fixed value.
O–H shifts can vary with solvent and concentration because these protons are exchangeable and affected by their surroundings; treat the listed range as typical.
Look for these in your work
- Exchangeable group recognized
- Conditions named
- Range used cautiously
A spectrum has one signal at 9.7 ppm. Can you identify an aldehyde uniquely from this shift?
Give me a hint
Check whether another listed environment overlaps.
Compare my reasoning
- Aldehyde protons are around 9.3–10.5 ppm.
- Carboxylic-acid O–H can also be around 9.0–13.0 ppm.
- Use other signals and formula to distinguish.
No. The signal is consistent with an aldehyde but can overlap with a carboxylic-acid O–H; use integration, splitting and other data.
Look for these in your work
- Aldehyde range applied
- Overlap identified
- Further evidence requested
Common mistakes
- Treating ranges as exact single values
- Ignoring overlap
- Using a shift table as if solvent and concentration never matter
- Assigning a signal without checking integration or structure
What can you explain now?
What reference is used for the chemical-shift scale?
Compare with the explanation
TMS at δ = 0 ppm
Sample shifts are measured relative to tetramethylsilane.
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Make it stick
How should you use typical chemical-shift ranges while accounting for overlaps and variable O–H/N–H signals?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.