Identify characteristic functional-group IR absorptions
A fixed ethanoic-acid trace has a very broad 2500–3300 cm⁻¹ envelope and a strong 1712 cm⁻¹ band.
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The core idea
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Learning objectives
- Identify characteristic functional-group IR absorptions
Read a spectrum as combined evidence
A fixed ethanoic-acid trace has a very broad 2500–3300 cm⁻¹ envelope and a strong 1712 cm⁻¹ band.
Together these support carboxylic-acid O–H and C=O; neither alone identifies the whole molecule.
Characteristic region and fingerprint
A characteristic absorption is a supplied range associated with a bond vibration, such as C=O near 1680–1750 cm⁻¹.
The fingerprint region contains many coupled vibrations and is used by comparison; it should not be treated as one functional-group label.
Use presence, absence, shape and intensity
Alcohol O–H is broad around 3200–3600 cm⁻¹; carboxylic-acid O–H is typically very broad around 2500–3300 cm⁻¹; C=O is strong near 1700 cm⁻¹.
A sharp band near 2250 cm⁻¹ may support C≡N, while absence of a broad O–H band can discriminate carbonyl candidates.
Distinguish acid from ester
Spectrum A: broad 2500–3300 plus strong 1712 cm⁻¹. Spectrum B: strong 1740 cm⁻¹ but no broad O–H.
A supports a carboxylic acid; B supports an ester or another non-O–H carbonyl and needs further evidence.
Classify the decisive functional groups in spectra A and B.
Check your answer
A strong answer should include acid O–H+C=O for A and carbonyl without O–H for B; do not claim a unique ester from one band.
Annotate a three-band trace
Mark 3350 cm⁻¹ broad, 1718 cm⁻¹ strong and 1050 cm⁻¹ strong on a hydroxy-carbonyl spectrum.
Use the supplied Data Booklet ranges and label uncertainty where bands overlap.
State which groups are supported by the three fixed bands.
Check your answer
Support O–H, C=O and C–O; a whole structure still requires formula or other evidence.
Use an important absence
A molecular formula C₃H₆O and a strong 1725 cm⁻¹ band support a carbonyl.
No aldehydic C–H features and no O–H narrow the candidates but do not replace full-spectrum reasoning.
Compare propanone and propanal using the supplied positive and negative evidence.
Check your answer
Propanone is better supported only if the aldehydic C–H evidence is absent; state the limitations of IR alone.
One peak is not one structure
Functional-group assignment is probabilistic evidence constrained by ranges, band shape and important absences.
Combine at least two independent features before naming a functional-group class where possible.
Better reasoning: “A 1715 cm⁻¹ band proves the sample is propanone.”
Check your answer
State only that a carbonyl is supported, then request formula and other bands before choosing propanone.
From bands to candidate structures
After the check questions, try the nitrile example without notes. Return later for a different amide question.
Try this next: deduce structures.
Annotate the fixed ethanoic-acid trace with range, shape and group.
Check your answer
A strong answer should include broad acid O–H at 2500–3300 and strong C=O near 1712 cm⁻¹, with a combined rather than single-peak conclusion.
Identify characteristic functional-group IR absorptions scientific representation
Two separate transmittance traces run from 4000 to 500 cm⁻¹. Ethanoic acid has a very broad trough from 2500 to 3300, a strong trough at 1712 and a band around 1260. Ethyl ethanoate has a weak band just below 3000, a strong trough at 1740 and a band around 1160, with no broad trough from 2500 to 3600.
About 5 minutes
| Sample | Wavenumber / cm⁻¹ | Strength / width | Assignment | Explicit absence |
|---|---|---|---|---|
| ethanoic acid | 2500–3300 | very broad, strong | O–H | — |
| ethanoic acid | 1712 | strong | C=O | — |
| ethanoic acid | 1210–1320 | strong cluster | C–O | — |
| ethyl ethanoate | 1740; 1050–1300 | strong; strong cluster | C=O; C–O | no broad 2500–3600 O–H |
Text alternative: Two separate transmittance traces run from 4000 to 500 cm⁻¹. Ethanoic acid has a very broad trough from 2500 to 3300, a strong trough at 1712 and a band around 1260. Ethyl ethanoate has a weak band just below 3000, a strong trough at 1740 and a band around 1160, with no broad trough from 2500 to 3600.