Suggest structures from IR spectra
Unknown U has formula C₃H₆O, a strong 1715 cm⁻¹ band and no broad O–H absorption.
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The core idea
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Learning objectives
- Suggest structures from IR spectra
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.
Formula first, spectrum second
Use the formula to restrict atoms and unsaturation, then assign understanding check 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.
Check your answer
Choose propanone only after using C=O presence and aldehydic-feature absence; acknowledge IR resolution limits.
Choose ethanol or methoxymethane
C2H6O shows broad 3340 cm⁻¹ and no 1700–1750 cm⁻¹ band.
The two candidates are ethanol, CH₃CH₂OH, and methoxymethane, CH₃OCH₃.
Select the consistent structure and cite one present and one absent feature.
Check your answer
Ethanol. The broad 3340 cm⁻¹ O–H band is present, and methoxymethane has no O–H bond. The absent 1700–1750 cm⁻¹ band is consistent with both candidates, because neither contains C=O, so it cannot be the deciding feature.
Test an ester candidate
C4H8O2 shows strong 1740 cm⁻¹, strong 1240 cm⁻¹ and no broad O–H.
Consider more than one functional-group class that fits C4H8O2, and use the absent band as well as the present ones.
Propose one structure and predict two bands it should contain.
Check your answer
An ester fits: for example ethyl ethanoate, with C=O near 1740 and C–O near 1240 cm⁻¹. Butanoic acid also has formula C4H8O2 but would show a very broad 2500–3300 cm⁻¹ O–H band, which is absent. IR alone does not pick out which ester isomer it is.
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.
Better reasoning: “The deepest 1715 cm⁻¹ trough proves propanone.”
Check your answer
Replace proof with carbonyl support, then add formula and absence evidence before proposing propanone.
Deduce, predict, falsify
After the check questions, deduce the C₃H₆O₂ structure without notes. Return later for a different C₄H₈O question.
Try this next: predict bands from a known structure.
Write a three-column evidence table for U: observation, inference, candidates rejected.
Check your answer
A strong answer should include formula, 1715 cm⁻¹ C=O, absent O–H and an explicit test of propanone against propanal.
Suggest structures from IR spectra scientific representation
Two separate transmittance traces run from 4000 to 500 cm⁻¹. Unknown U, C3H6O, has a weak band near 2970 and a strong trough at 1715, with no broad trough from 2500 to 3600. Unknown V, C4H8O2, has a weak band near 2985 and strong troughs at 1740 and 1240, with no broad trough from 2500 to 3600.
About 5 minutes
| 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: Two separate transmittance traces run from 4000 to 500 cm⁻¹. Unknown U, C3H6O, has a weak band near 2970 and a strong trough at 1715, with no broad trough from 2500 to 3600. Unknown V, C4H8O2, has a weak band near 2985 and strong troughs at 1740 and 1240, with no broad trough from 2500 to 3600.