H3 Chemistry 9813 · Study focus: H3 Chemistry: Explain hydrogen-bonding effects on chemical shift
H3 Chemistry: Explain hydrogen-bonding effects on chemical shift
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Explain hydrogen-bonding effects on chemical shift
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Control OH conditions
Ethanol OH can move from δ1.6 dilute to δ4.8 neat while CH3 stays near δ1.2.
The selective movement identifies a condition-sensitive exchangeable proton.
Hydrogen-bond deshielding
Hydrogen bonding withdraws electron density from O–H and commonly raises its δ.
A distribution of associated environments and exchange can broaden the OH line.
Concentration, temperature, solvent hydrogen-bond basicity and impurities can change OH shift and width.
Compare spectra only with the other variables controlled.
| Sample | Condition | OH δ / ppm | Non-exchanging signal | Inference |
|---|---|---|---|---|
| ethanol | 0.02 mol dm⁻³, 298 K | 1.6 | CH₃ 1.2 ppm | less hydrogen bonding |
| ethanol | neat, 298 K | 4.8 | CH₃ 1.2 ppm | more hydrogen bonding |
| phenol | 298 K | 7.1 | — | broader, more associated |
| phenol | 338 K | 6.2 | — | narrower, less associated |
| phenol + D₂O | after exchange | absent | other peaks retained | labile proton confirmed |
Text alternative: Text gives compound, condition, shift, width and exchange result.
Separate variables
Concentration, temperature, solvent hydrogen-bond basicity and impurities can change OH shift and width.
Compare spectra only with the other variables controlled.
Read concentration data
At 298 K ethanol OH moves δ1.6→4.8 as concentration rises; CH3 remains δ1.2.
The trend supports stronger average intermolecular association.
- Explain the fixed series.
Open the feedback checkpoint after attempting
- Credit selective OH deshielding and the CH3 control.
Read temperature data
A supplied phenol OH moves δ7.1→6.2 and narrows on heating 298→338 K.
This supports weaker/shorter-lived hydrogen bonding and faster exchange in that sample.
- Interpret both changes.
Open the feedback checkpoint after attempting
- Credit the bounded shift and width conclusions.
Use solvent and D2O controls
A hydrogen-bond-accepting solvent may move phenol OH downfield; D2O replacement removes its 1H signal.
Match concentration and temperature before comparing solvents.
- Design a controlled exchange check.
Open the feedback checkpoint after attempting
- Credit matched conditions and selective OH disappearance.
Repair universal lookup
No single δ identifies every alcohol OH because association and exchange vary.
Use D2O plus non-labile signals instead of forcing a variable peak.
- Repair: ‘all alcohol OH is δ2.0.’
Open the feedback checkpoint after attempting
- Reject the invariant value and name two controls.
Cross-check structure
A broad δ11.8 signal lost with D2O plus a C=O IR band supports CO2H; δ11.8 alone does not.
Next: ionisation/fragmentation at /learning/h3-mass-spectrometry-ionisation-fragmentation-lesson.html.
- Evaluate the acid evidence.
Open the feedback checkpoint after attempting
- Credit labile OH, hydrogen-bond shift and carbonyl corroboration.