Account for deuterated solvents and labile protons

Dissolve ethanol in CDCl₃ rather than CHCl₃ so ordinary solvent ¹H does not dominate.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Account for deuterated solvents and labile protons

Remove solvent and exchange signals

Dissolve ethanol in CDCl₃ rather than CHCl₃ so ordinary solvent ¹H does not dominate.

After D₂O exchange, ethanol's OH signal disappears while CH₃ and CH₂ signals remain.

A proton NMR solvent should dissolve the sample without contributing an overwhelming proton signal. That is why deuterated solvents such as CDCl₃ are used instead of their ordinary proton-containing analogues.

Deuterated solvent and labile proton

A deuterated solvent replaces most ¹H with 2H and greatly reduces the solvent proton signal.

A labile OH or NH proton can exchange with deuterium; 2H is not observed at the same ¹H frequency.

A residual solvent peak can remain because commercial deuterated solvent is not perfectly deuterated. It should be recognised as a known reference impurity rather than assigned to the sample.

Interpret before/after spectra

A disappearing broad signal after D₂O supports an exchangeable OH/NH environment; it does not by itself distinguish the two.

Residual solvent peaks and water peaks can remain, so use supplied reference positions and the whole spectrum.

O–H and N–H protons exchange between molecules and with traces of water, so their positions and line shapes depend strongly on concentration, temperature and solvent. Rapid exchange can also remove the simple coupling that an n + 1 prediction would suggest.

Ethanol D₂O shake

Before: 3H triplet, 2H quartet, broad ¹H OH. After: triplet and quartet remain, OH vanishes.

The carbon-bound protons do not exchange under this simple test.

For ethanol in CDCl₃, assign the stable CH₃/CH₂ pattern first. Treat the variable broad O–H signal separately, and do not force it to split the CH₂ unless the question supplies slow-exchange evidence.

Try this

Identify the disappearing signal and explain why.

Check your answer

A strong answer should include OH exchange to OD and absence from the ¹H spectrum.

Choose CDCl₃

Compare what CHCl₃ and CDCl₃ each contribute to a ¹H spectrum of a dissolved sample.

The solvent must also dissolve the sample and be chemically compatible.

In a D₂O shake, an exchangeable O–H or N–H proton is replaced by deuterium and its ¹H signal disappears. Carbon-bound C–H signals normally remain.

Try this

Explain the analytical advantage of CDCl₃.

Check your answer

CHCl₃ would add a large solvent ¹H signal that could swamp the sample's peaks. Deuterium does not resonate in ¹H NMR, so CDCl₃ leaves only a small residual CHCl₃ peak. The solvent signal is suppressed, not removed entirely.

Compare alcohol and amine

Supplied data: a broad 2H signal disappears after a D₂O shake. The proposed structure is a primary amine, RNH₂.

Decide what the disappearance proves on its own, then what other evidence you would need.

Use disappearance after D₂O as evidence for an exchangeable proton, then check whether the proposed structure actually contains O–H or N–H. Disappearance is not a general test for every broad signal.

Try this

Interpret a disappearing 2H broad signal in a primary amine candidate.

Check your answer

The disappearance shows two exchangeable protons, which is consistent with NH₂. It does not by itself rule out O–H, because OH signals also exchange. Use the molecular formula (nitrogen present), the integration and shifts of the carbon-bound signals to decide; do not call it an alcohol, or an amine, automatically.

D₂O does not erase every proton

Exchange occurs at sufficiently labile heteroatom-bound protons, not ordinary alkyl C–H under this test.

Remaining carbon-bound signals retain their integrations except for referencing/experimental details.

Deuterated solvent is not ‘invisible’ to every form of NMR. The practical point here is that deuterium does not give the ordinary ¹H solvent signal that would obscure the sample spectrum.

Try this

Better reasoning: “Adding D₂O makes the whole ¹H spectrum disappear.”

Check your answer

Limit disappearance to exchangeable OH/NH protons.

Use controlled before/after evidence

After the check questions, interpret a phenol exchange test without notes. Return later for a different amide question.

Try this next: count equivalent proton environments.

A strong spectrum note distinguishes sample peaks, residual-solvent peaks, water and exchangeable protons, and states which assignment is supported by a D₂O experiment.

Try this

Annotate fixed ethanol spectra before and after D₂O.

Check your answer

A strong answer should include unchanged CH₃/CH₂, disappearing OH and the role of deuterated solvent.