H3 Chemistry 9813 · Study focus: H3 Chemistry: Account for deuterated solvents and labile protons
H3 Chemistry: Account for deuterated solvents and labile protons
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Account for deuterated solvents and labile protons
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
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.
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 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.
| Signal | Before D2O | After D2O | Meaning |
|---|---|---|---|
| OH | δ2.4 broad, 1H | absent | labile proton exchanged |
| CH2 | δ3.6 quartet, 2H | unchanged | non-labile analyte signal |
| CH3 | δ1.2 triplet, 3H | unchanged | non-labile analyte signal |
| CDCl3 residual | δ7.26 small | remains | solvent suppression/reference, not exchange |
Text alternative: Every before/after signal is listed with δ, multiplicity, integral and persistence/disappearance status.
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.
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.
- Identify the disappearing signal and explain why.
Open the feedback checkpoint after attempting
- Credit OH exchange to OD and absence from the 1H spectrum.
Choose CDCl3
CHCl3 would contribute a large proton resonance; CDCl3 contributes mainly deuterium plus a small residual proton peak.
The solvent must also dissolve the sample and be chemically compatible.
- Explain the analytical advantage of CDCl3.
Open the feedback checkpoint after attempting
- Credit suppression of the solvent 1H signal, not removal of all solvent features.
Compare alcohol and amine
Both OH and NH signals may broaden and disappear after D2O.
Use integration, chemical shift and other molecular signals to distinguish candidates.
- Interpret a disappearing 2H broad signal in a primary amine candidate.
Open the feedback checkpoint after attempting
- Support NH2 exchange but require other evidence; do not call it an alcohol 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.
- Repair: “Adding D2O makes the whole 1H spectrum disappear.”
Open the feedback checkpoint after attempting
- Limit disappearance to exchangeable OH/NH protons.
Use controlled before/after evidence
Complete twelve solvent/exchange checks, then unseen phenol assessment and different amide re-test.
Next objective: count equivalent proton environments at /learning/h3-nmr-proton-equivalence-lesson.html.
- Annotate fixed ethanol spectra before and after D2O.
Open the feedback checkpoint after attempting
- Credit unchanged CH3/CH2, disappearing OH and the role of deuterated solvent.