A-Level Chemistry 9701 / Topic 22 / AS

Bonds that vibrate leave a signature.

Shine infrared radiation through an organic compound and some frequencies go missing. Each missing band matches a bond vibrating in the molecule, so the dips in a spectrum tell you which functional groups are present.

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The key idea

Covalent bonds are always vibrating (stretching and bending). A bond absorbs infrared radiation whose frequency matches its natural vibration frequency, provided the vibration changes the dipole of the molecule. Positions are quoted as wavenumber in cm⁻¹, and each bond absorbs in a characteristic range listed in the Data Booklet, for example O–H in alcohols 3200–3600 cm⁻¹ (broad), O–H in carboxylic acids 2500–3000 cm⁻¹ (very broad) and C=O 1670–1740 cm⁻¹. A spectrum plots transmittance against wavenumber, so every absorption is a dip. The fingerprint region below 1500 cm⁻¹ is unique to each compound and is matched against a database. The same absorption by C=O and O–H bonds in CO₂ and H₂O is what makes them greenhouse gases.

Fig. 1: simplified infrared spectrum of ethanol, with the broad alcohol O–H, the C–H and the C–O absorptions

Section 01

Why bonds absorb.

Stretch each bond in the simulator and watch the dipole: a vibration is only IR-active if it changes the dipole. Then read the notes on vibration and the greenhouse effect.

Vibrating bonds: a bond behaves like a spring between two atoms. It stretches and bends at a natural frequency set by the bond strength and the masses of the atoms: stronger bonds and lighter atoms vibrate faster and absorb at higher wavenumber.
Wavenumber: 1 / wavelength, in cm⁻¹. Organic spectra run from 4000 cm⁻¹ on the left to about 500 cm⁻¹ on the right.
Transmittance: 100% means all the radiation passes through. An absorption shows as a dip towards 0%; a broad dip from O–H is caused by hydrogen bonding, which gives the O–H bonds a spread of strengths.
Greenhouse effect: the Earth's surface emits infrared. CO₂, H₂O and CH₄ have bonds whose vibrations change the dipole, so they absorb this radiation and re-emit it, warming the lower atmosphere. N₂ and O₂ are non-polar and do not absorb.

Section 02

The Data Booklet ranges.

In the exam you are given these ranges. The skill is matching each strong dip above 1500 cm⁻¹ to a bond, then deciding which functional group it belongs to.

Bond	Functional group	Wavenumber / cm⁻¹	Appearance
O–H	alcohols	3200–3600	broad
N–H	amines, amides	3300–3500	medium, sharper than O–H
C–H	alkanes and most organics	2850–2950	sharp; almost always present
O–H	carboxylic acids	2500–3000	very broad, overlaps C–H
C≡N	nitriles	2200–2250	sharp, in an otherwise empty region
C=O	aldehydes, ketones, acids, esters	1670–1740	strong and sharp
C=C	alkenes	1640–1680	medium
C–O	alcohols, acids, esters	1040–1300	strong, in the fingerprint region

Fingerprint region (below 1500 cm⁻¹): many overlapping bending and skeletal vibrations. No two compounds match exactly, so comparing it with a database spectrum confirms an identity.
Method: look first at 3600–2500 (O–H, N–H), then near 1700 (C=O), then 1040–1300 (C–O). Ignore C–H near 2900: nearly every organic compound has it.

Stage 1 · Learn

Check what the sim just showed you

Four quick checks. Each correct answer earns XP and records this skill on your profile.

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What causes a molecule to absorb infrared radiation?

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Which range in cm⁻¹ shows the O–H of an alcohol?

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What is the fingerprint region of an infrared spectrum?

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Why is nitrogen, N₂, not a greenhouse gas?

Section 03

Telling compounds apart.

Four compounds with two carbon atoms give four different patterns. Learn the combination, not just one peak.

Compound	Broad O–H?	C=O near 1700?	C–O 1040–1300?
ethanol, CH₃CH₂OH	yes, 3200–3600	no	yes
ethanal, CH₃CHO	no	yes	no
ethanoic acid, CH₃COOH	yes, very broad 2500–3000	yes	yes
methyl methanoate, HCOOCH₃	no	yes	yes

Following a reaction: when ethanol is oxidised under reflux to ethanoic acid, the O–H dip moves from 3200–3600 down to a very broad band at 2500–3000, and a strong C=O appears near 1710 cm⁻¹.
Distillation to the aldehyde: a pure ethanal sample shows C=O but no broad O–H, which proves no unreacted ethanol remains.

Examiner trap

Spectra are drawn with wavenumber decreasing from left to right, and absorptions point down. Do not call every peak near 3000 an O–H: C–H at 2850–2950 is in almost every compound. The acid O–H is very broad and lower (2500–3000) than the alcohol O–H (3200–3600). An ester has C=O and C–O but no O–H. You cannot identify a compound from the fingerprint region by eye: it needs a database match.

Stage 2 · Exam

Exam-style questions

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A compound shows a very broad absorption from 2500 to 3000 cm⁻¹ and a strong absorption at 1710 cm⁻¹. What type of compound is it?

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Propan-1-ol is heated under reflux with excess acidified K₂Cr₂O₇. Which change in the IR spectrum confirms the reaction is complete?

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Which compound would show a strong absorption at 1670–1740 cm⁻¹ and at 1040–1300 cm⁻¹, but NO absorption from 2500 to 3600 cm⁻¹ other than C–H?

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Why is the O–H absorption of an alcohol broad rather than sharp?

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Six original Cambridge-style questions

Why bonds absorb infrared, Data Booklet wavenumber ranges, the fingerprint region, identifying functional groups from spectra and the greenhouse effect.