H3 Chemistry 9813 · Study focus: H3 Chemistry: Suggest structures from IR spectra
H3 Chemistry: Suggest structures from IR spectra
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Suggest structures from IR spectra
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Constrain an unknown
Unknown U has formula C3H6O, a strong 1715 cm⁻¹ band and no broad O–H absorption.
The formula permits one degree of unsaturation; the carbonyl accounts for it and propanone is consistent, but the whole evidence set must be checked.
A deduction is a tested candidate
Structure deduction combines molecular formula, characteristic bands and important absences to propose a structure.
A consistent candidate predicts every supplied feature; IR alone may leave constitutional isomers unresolved.
Use the formula to restrict atoms and unsaturation, then assign diagnostic bands. Reject candidates that require an absent band.
For C2H6O, broad 3200–3600 cm⁻¹ and no 1700 cm⁻¹ supports ethanol over methoxymethane only when C–O/fingerprint evidence is also considered.
| Unknown / formula | Exact band / cm⁻¹ | Strength / width | Candidate comparison | Explicit absence |
|---|---|---|---|---|
| U / C3H6O | 1715 | strong, narrow | propanone supported; propanal would require aldehydic C–H | no broad O–H; no aldehydic C–H |
| V / C4H8O2 | 1740; 1240 | strong; strong | ester supported; acid would require broad O–H | no broad O–H |
Text alternative: Every plotted trough is duplicated in a text table with exact wavenumber, width, strength and explicit absence; formulas are read in linear notation.
Formula first, spectrum second
Use the formula to restrict atoms and unsaturation, then assign diagnostic bands. Reject candidates that require an absent band.
For C2H6O, broad 3200–3600 cm⁻¹ and no 1700 cm⁻¹ supports ethanol over methoxymethane only when C–O/fingerprint evidence is also considered.
Deduce C3H6O
The 1715 cm⁻¹ band supports C=O and no broad O–H excludes an alcohol. Absence of aldehydic C–H evidence favours propanone over propanal.
State propanone as the best supplied candidate, not as a conclusion from 1715 cm⁻¹ alone.
- Use formula C3H6O and the fixed spectrum to choose propanone or propanal.
Open the feedback checkpoint after attempting
- Choose propanone only after using C=O presence and aldehydic-feature absence; acknowledge IR resolution limits.
Choose ethanol or ethanal
C2H6O shows broad 3340 cm⁻¹ and no 1700–1750 cm⁻¹ band.
Ethanol contains O–H and no C=O; ethanal requires C=O.
- Select the consistent structure and cite one present and one absent feature.
Open the feedback checkpoint after attempting
- Ethanol; present broad O–H and absent carbonyl. Both observations must be cited.
Test an ester candidate
C4H8O2 shows strong 1740 cm⁻¹, strong 1240 cm⁻¹ and no broad O–H.
An ester such as ethyl ethanoate is consistent; an acid candidate conflicts with O–H absence.
- Propose one structure and predict two bands it should contain.
Open the feedback checkpoint after attempting
- Ethyl ethanoate is acceptable with C=O near 1740 and C–O near 1240; formula and absent O–H must also fit.
Do not name from the tallest peak
The base peak concept belongs to mass spectrometry; an IR trough's intensity does not uniquely name a molecule.
Deduction requires formula plus multiple positive/negative constraints.
- Repair: “The deepest 1715 cm⁻¹ trough proves propanone.”
Open the feedback checkpoint after attempting
- Replace proof with carbonyl support, then add formula and absence evidence before proposing propanone.
Deduce, predict, falsify
Complete twelve fixed unknowns, then the unseen C3H6O2 assessment and different C4H8O re-test.
Next objective: predict bands from a known structure at /learning/h3-infrared-predict-bands-lesson.html.
- Write a three-column evidence table for U: observation, inference, candidates rejected.
Open the feedback checkpoint after attempting
- Credit formula, 1715 cm⁻¹ C=O, absent O–H and an explicit test of propanone against propanal.