Identify O–H and N–H protons by D₂O exchange
Compare a proton NMR spectrum before and after adding D₂O to identify exchangeable O–H or N–H signals.
Section 37 · Analytical techniques
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
Which proton types can be identified using D₂O exchange in the syllabus?
What you will learn
- Recognise exchangeable O–H and N–H protons
- Interpret a signal change after D₂O
- Distinguish exchangeable H from carbon-bound H
D₂O exchanges with some O–H and N–H protons
When D₂O is added to a sample containing O–H or N–H groups, exchange can replace those hydrogen-1 atoms with deuterium. The corresponding proton signal may disappear or change in the spectrum.
This provides evidence for an exchangeable O–H or N–H proton. It does not identify which group by itself if both are possible; use the molecular formula, chemical shifts and other evidence.
Carbon-bound protons behave differently
Protons bonded to carbon do not normally undergo this simple D₂O exchange test under the syllabus conditions, so their signals remain.
Compare spectra before and after treatment. A disappearing signal supports an exchangeable proton, but avoid overclaiming the exact functional group without the rest of the structure.
Pause and explain
What observation supports the presence of an exchangeable proton after D₂O is added?
Worked example
A spectrum has a broad signal that disappears after D₂O is added. What type of proton is indicated?
Show the worked solution
- D₂O exchanges with some O–H and N–H protons.
- The proton signal is lost or changed.
- Use other evidence to distinguish O–H from N–H.
Answer An exchangeable O–H or N–H proton is indicated; the change alone may not distinguish which one.
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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 proton signal vanishes after adding D₂O. Give the supported conclusion and one limitation.
Give me a hint
State the exchangeable proton types and the ambiguity.
Compare my reasoning
- O–H or N–H can exchange.
- The signal change supports an exchangeable proton.
- Other evidence is needed to choose between O–H and N–H.
The result supports an O–H or N–H proton, but the D₂O test alone does not necessarily distinguish them.
Look for these in your work
- Exchangeable types named
- Evidence interpreted
- Limitation stated
Which signals should generally remain unchanged in a simple D₂O exchange comparison: O–H, N–H or carbon-bound C–H?
Give me a hint
Distinguish heteroatom-bound exchangeable hydrogen from carbon-bound hydrogen.
Compare my reasoning
- O–H and N–H may exchange.
- Carbon-bound C–H normally remains.
- Compare the before/after spectra.
Carbon-bound C–H signals generally remain; O–H and N–H signals may change or disappear.
Look for these in your work
- Classes distinguished
- Expected change stated
- Comparison invoked
A candidate contains an amide N–H and an alcohol O–H, and one signal disappears after D₂O. Can the test alone identify which one?
Give me a hint
Both listed groups have exchangeable protons.
Compare my reasoning
- Either O–H or N–H can exchange.
- The observation alone identifies exchangeability, not its exact source.
- Use chemical shift or other structural evidence.
No. The disappearance indicates an exchangeable proton but does not alone distinguish the alcohol O–H from the amide N–H.
Look for these in your work
- Both candidates recognized
- Inference limited
- Further evidence named
Common mistakes
- Calling the disappearing peak a carbon-bound proton
- Claiming D₂O distinguishes O–H from N–H by itself
- Ignoring the before/after comparison
- Assuming every O–H or N–H signal has a fixed chemical shift
What can you explain now?
Which proton types can be identified using D₂O exchange in the syllabus?
Compare with the explanation
O–H and N–H protons
These exchangeable protons can be replaced by deuterium.
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Make it stick
What does D₂O exchange show, and which proton assignments still need other evidence?
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.