H3 Chemistry 9813 · Study focus: H3 Chemistry: Predict characteristic IR absorptions from structure
H3 Chemistry: Predict characteristic IR absorptions from structure
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Predict characteristic IR absorptions from structure
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Turn structure into a spectral forecast
Ethanoic acid contains C=O, O–H and C–O bonds; predict strong C=O near 1710 cm⁻¹ and very broad acid O–H at 2500–3300 cm⁻¹.
Predictions should prioritise diagnostic regions rather than promise one resolved peak for every bond.
Characteristic prediction
A characteristic-band prediction names the bond/group, approximate supplied range, shape or strength where useful, and important absent features.
The fingerprint region can be forecast qualitatively but complex coupled vibrations are not assigned one-for-one.
Propanone predicts strong C=O near 1700 and no broad O–H. Ethanol predicts broad alcohol O–H 3200–3600 and C–O around 1000–1300 but no C=O.
Methyl ethanoate predicts ester C=O near 1740 and C–O bands, with no acid O–H envelope.
| Compound | Present ranges / shape | Structural bond | Absent ranges |
|---|---|---|---|
| ethanoic acid | 2500–3300 very broad; 1710 strong; 1210–1320 | O–H; C=O; C–O | no nitrile 2250 |
| propanone | 1715 strong; alkyl C–H below 3000 | C=O; C–H | no broad O–H |
| propanenitrile | 2250 sharp; alkyl C–H below 3000 | C≡N; C–H | no C=O; no O–H |
Text alternative: Each forecast has an adjacent text list of present and absent ranges, strength/shape and the exact structural bond responsible.
Compare structures before drawing traces
Propanone predicts strong C=O near 1700 and no broad O–H. Ethanol predicts broad alcohol O–H 3200–3600 and C–O around 1000–1300 but no C=O.
Methyl ethanoate predicts ester C=O near 1740 and C–O bands, with no acid O–H envelope.
Forecast ethanoic acid
List acid O–H as very broad 2500–3300 cm⁻¹ and C=O as strong near 1710 cm⁻¹.
Add C–O in the fingerprint region but do not call every CH3 vibration diagnostic.
- Predict the two most diagnostic ethanoic-acid features and one useful additional band.
Open the feedback checkpoint after attempting
- Credit acid O–H, carbonyl and C–O with sensible ranges/shapes.
Forecast propanone
The only oxygen is carbonyl oxygen; there is no O–H bond.
Predict strong C=O around 1715 cm⁻¹, alkyl C–H below 3000 cm⁻¹ and explicit absence of broad O–H.
- Write a three-row propanone prediction table.
Open the feedback checkpoint after attempting
- Rows must include C=O present, sp3 C–H present and O–H absent.
Forecast a nitrile
Propanenitrile contains C≡N and alkyl C–H but no carbonyl or O–H.
Predict a sharp C≡N band near 2250 cm⁻¹ and state the two absent regions.
- Predict a minimal diagnostic spectrum for CH3CH2CN.
Open the feedback checkpoint after attempting
- Credit C≡N near 2250, C–H below 3000, absent C=O and absent broad O–H.
Bonds do not map one-to-one onto peaks
A molecule's normal modes combine atomic motions; equivalent or coupled bonds need not yield separately resolved bands.
Predict diagnostic absorptions from functional groups and symmetry, not one peak per drawn bond.
- Repair: “Ethanol has six bonds, so its IR spectrum must have six peaks.”
Open the feedback checkpoint after attempting
- Reject bond counting; predict broad O–H, C–O and C–H regions while acknowledging many coupled fingerprint modes.
Forecast before seeing data
Complete twelve structure-to-band checks, then unseen 2-hydroxypropanone and different chloropropanenitrile re-test.
Next objective: connect IR-active greenhouse gases at /learning/h3-infrared-greenhouse-gases-lesson.html.
- Draw an annotated predicted trace for propanone and ethanoic acid.
Open the feedback checkpoint after attempting
- The two traces must differ in O–H envelope and carbonyl position/context; axes run 4000→500 cm⁻¹.