Where does the O–H bond of a carboxylic acid absorb?

2500–3300 cm⁻¹, very broad.

Which two absorptions identify a carboxylic acid?

C=O at 1680–1750 cm⁻¹ and a very broad O–H at 2500–3300 cm⁻¹.

How do alcohols and ketones differ in infrared spectra?

Alcohols show O–H at 3200–3600 cm⁻¹ and no C=O; ketones show C=O at 1680–1750 cm⁻¹ and no O–H.

Why can infrared not tell an aldehyde from a ketone?

Both contain C=O absorbing in the same range.

How would oxidising propan-1-ol to propanal change the spectrum?

The O–H absorption disappears and a C=O absorption appears.

Why may an amine and an alcohol be confused in infrared spectra?

N–H (3300–3500) and O–H (3200–3600) absorptions overlap.

Infrared spectroscopyEdexcel A-Level Chemistry: Flashcards

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What causes a bond to absorb infrared radiation?

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What causes a bond to absorb infrared radiation?: The radiation frequency matches the frequency at which the bond vibrates (stretches or bends).
What unit is used for infrared absorptions?: Wavenumber in cm⁻¹.
What is the C=O stretching range?: 1680–1750 cm⁻¹.
What is the C=C stretching range?: 1620–1669 cm⁻¹.
What is the O–H stretching range for alcohols?: 3200–3600 cm⁻¹.
Where does the O–H bond of a carboxylic acid absorb?: 2500–3300 cm⁻¹, very broad.
What is the N–H stretching range in amines?: 3300–3500 cm⁻¹.
What is the C–H stretching range?: 2850–3100 cm⁻¹.
Which two absorptions identify a carboxylic acid?: C=O at 1680–1750 cm⁻¹ and a very broad O–H at 2500–3300 cm⁻¹.
How do alcohols and ketones differ in infrared spectra?: Alcohols show O–H at 3200–3600 cm⁻¹ and no C=O; ketones show C=O at 1680–1750 cm⁻¹ and no O–H.
Why can infrared not tell an aldehyde from a ketone?: Both contain C=O absorbing in the same range.
How would oxidising propan-1-ol to propanal change the spectrum?: The O–H absorption disappears and a C=O absorption appears.
Why may an amine and an alcohol be confused in infrared spectra?: N–H (3300–3500) and O–H (3200–3600) absorptions overlap.

Exam questions on Infrared spectroscopy

An organic compound P has the molecular formula C₃H₆O. A student records its infrared spectrum and finds a strong absorption at 1715 cm⁻¹ and no absorption between 3200 and 3600 cm⁻¹.
Explain why the infrared spectrum cannot distinguish between propanal and propanone, and describe a chemical test that does.2 marks
A chemist is given two colourless liquids, ethanol and ethanoic acid, whose labels have fallen off. She records an infrared spectrum of each.
Describe how the two spectra differ, so that the chemist can identify each liquid.2 marks
A student heats propan-1-ol with acidified potassium dichromate(VI) and distils off the product as soon as it forms. The infrared spectrum of the distillate shows absorptions at 2980 cm⁻¹ and 1725 cm⁻¹ only, apart from the fingerprint region.
Identify the bonds responsible for the two absorptions, and state what the absence of any absorption at 3200–3600 cm⁻¹ shows about the distillate.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).