Infrared spectroscopy

AS Level Chemistry · Analytical techniques. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Covalent bonds vibrate, and each type of bond absorbs infrared radiation in its own range of frequencies. An infrared spectrum shows these absorptions against wavenumber, measured in cm⁻¹. Each absorption appears as a dip in the trace, called a peak.

You use the spectrum to find which functional groups are present. Match each strong absorption to a bond using the table in the Data section. You do not need to explain every peak.

What is missing matters as much as what is there. No strong peak near 1700 cm⁻¹ means no C=O group. No broad peak above 2500 cm⁻¹ means no O–H group. Use both kinds of evidence to choose between possible compounds.

Rules to remember

O–H in an alcohol: broad absorption at 3200–3600 cm⁻¹
O–H in a carboxylic acid: very broad absorption at 2500–3000 cm⁻¹
C=O (aldehyde, ketone, carboxylic acid, ester): strong, sharp absorption at 1680–1750 cm⁻¹
C≡N in a nitrile: sharp absorption near 2250 cm⁻¹
N–H in an amine: absorption at 3300–3500 cm⁻¹
C=C in an alkene: absorption near 1650 cm⁻¹
C–O (alcohol, ester): 1040–1300 cm⁻¹; C–H: 2850–2950 cm⁻¹, present in almost every organic compound

Common mistake

Students see a C=O peak near 1700 cm⁻¹ and at once choose a carboxylic acid. An aldehyde, ketone or ester has this peak too: it is an acid only if the very broad O–H absorption at 2500–3000 cm⁻¹ is also there.

Worked example

A compound C₃H₆O has a broad absorption at 3200–3600 cm⁻¹, an absorption near 1650 cm⁻¹ and no strong absorption at 1680–1750 cm⁻¹. Is it propanal, propanone or prop-2-en-1-ol, CH₂=CHCH₂OH?

The broad absorption at 3200–3600 cm⁻¹ shows the O–H group of an alcohol.
The absorption near 1650 cm⁻¹ shows a C=C bond.
No absorption at 1680–1750 cm⁻¹ means there is no C=O group, so it is not propanal or propanone.

Answer: Prop-2-en-1-ol, CH₂=CHCH₂OH.

Practice questions

Try each one, then open the answer.

1. How could infrared spectroscopy distinguish propanal from propan-1-ol?

A
propanal shows a strong absorption near 1720 cm⁻¹ and no broad O–H band
B
propan-1-ol shows a strong absorption near 1720 cm⁻¹
C
both show a broad band at 3200–3600 cm⁻¹
D
the two spectra are identical

Show answer

Answer: A. Propanal has a C=O group (strong band at about 1720 cm⁻¹) and no O–H, while propan-1-ol has the broad O–H band at 3200–3600 cm⁻¹ and no C=O. Both show C–H bands, which do not help.

2. An infrared spectrum shows strong absorptions at 1740 cm⁻¹ and 1200 cm⁻¹, and no broad absorption between 2500 and 3600 cm⁻¹. Which compound is it?

A
ethanoic acid
B
butan-1-ol
C
but-1-ene
D
ethyl ethanoate

Show answer

Answer: D. 1740 cm⁻¹ is a C=O stretch and 1200 cm⁻¹ a C–O stretch, with no O–H band, which fits an ester. Ethanoic acid would also show a broad O–H band at 2500–3000 cm⁻¹.

3. Which compound shows an absorption at 3300–3500 cm⁻¹ due to an N–H bond?

A
ethanenitrile
B
chloroethane
C
ethylamine
D
propanone

Show answer

Answer: C. Primary amines contain N–H bonds, which absorb at 3300–3500 cm⁻¹. Ethanenitrile contains nitrogen but only in a C≡N bond, with no N–H.

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Infrared spectroscopy

The idea

Rules to remember

Common mistake

Worked example

A compound C3H6O has a broad absorption at 3200–3600 cm−1, an absorption near 1650 cm−1 and no strong absorption at 1680–1750 cm−1. Is it propanal, propanone or prop-2-en-1-ol, CH2=CHCH2OH?

Practice questions

1. How could infrared spectroscopy distinguish propanal from propan-1-ol?

2. An infrared spectrum shows strong absorptions at 1740 cm−1 and 1200 cm−1, and no broad absorption between 2500 and 3600 cm−1. Which compound is it?

3. Which compound shows an absorption at 3300–3500 cm−1 due to an N–H bond?

Read the full notes

Keep going

A compound C₃H₆O has a broad absorption at 3200–3600 cm⁻¹, an absorption near 1650 cm⁻¹ and no strong absorption at 1680–1750 cm⁻¹. Is it propanal, propanone or prop-2-en-1-ol, CH₂=CHCH₂OH?

2. An infrared spectrum shows strong absorptions at 1740 cm⁻¹ and 1200 cm⁻¹, and no broad absorption between 2500 and 3600 cm⁻¹. Which compound is it?

3. Which compound shows an absorption at 3300–3500 cm⁻¹ due to an N–H bond?