Predict characteristic IR absorptions from structure

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.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Predict characteristic IR absorptions from structure

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 understanding check 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.

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 understanding check.

Try this

Predict the two most understanding check ethanoic-acid features and one useful additional band.

Check your answer

A strong answer should include 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.

For each bond type present, give an expected range; then state one band that must be absent and why.

Try this

Write a three-row propanone prediction table.

Check your answer

Rows: strong C=O near 1715 cm⁻¹ present; sp3 C–H just below 3000 cm⁻¹ present; broad O–H at 3200–3600 cm⁻¹ absent because propanone has no O–H bond.

Forecast a nitrile

Propanenitrile is CH₃CH₂C≡N.

Build the same kind of table: present bands with expected ranges, then the regions that must be empty.

Try this

Predict a minimal understanding check spectrum for CH₃CH₂CN.

Check your answer

Present: sharp C≡N near 2250 cm⁻¹ and alkyl C–H just below 3000 cm⁻¹. Absent: C=O near 1700 cm⁻¹ and broad O–H, because the molecule has neither bond.

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 understanding check absorptions from functional groups and symmetry, not one peak per drawn bond.

Try this

Better reasoning: “Ethanol has six bonds, so its IR spectrum must have six peaks.”

Check your answer

Reject bond counting; predict broad O–H, C–O and C–H regions while acknowledging many coupled fingerprint modes.

Forecast before seeing data

After the check questions, predict bands for 2-hydroxypropanone without notes. Return later for a different chloropropanenitrile question.

Try this next: connect IR-active greenhouse gases.

Try this

Draw an annotated predicted trace for propanone and ethanoic acid.

Check your answer

The two traces must differ in O–H envelope and carbonyl position/context; axes run 4000→500 cm⁻¹.