Advanced Organic Chemistry: Infrared spectrum of butanal (butyraldehyde)  CH3CH2CH2CHO

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Interpreting the infrared spectrum of butanal (butyraldehyde)

[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of butanal [updated Mar 20th 2026 *]

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 Infrared spectroscopy - spectra index

Introductory note on the infrared spectrum of butanal

Students and teachers please note my explanation of the infrared spectrum of butanal is designed for advanced, but pre-university, chemistry courses.

Based in the infrared spectrum diagram for butanal, only some of the most prominent peaks for particular bond vibrations are discussed, particularly if butanal has a functional group with a particular characteristic wavenumber peak.

The infrared spectrum of butanal is unique and the whole, or selected wavenumbers, can be used to fingerprint its identity, sometimes analysing a mixture containing butanal or following its change of concentration in a reaction.

infrared spectrum of butanal wavenumbers cm-1 functional group detection fingerprint pattern identification of butyraldehyde doc brown's advanced organic chemistry revision notes 

Spectra obtained from a liquid film of butanal. The right-hand part of the of the infrared spectrum of butanal, wavenumbers ~1500 to 400 cm-1 is considered the fingerprint region for the identification of butanal and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of butanal.

butanal (butyraldehyde), C4H8O,  aldehydes and ketones nomenclature (c) doc b, aldehydes and ketones nomenclature (c) doc b, aldehydes and ketones nomenclature (c) doc b aldehyde

The molecular structure and naming of aldehydes and ketones

Interpretation of the infrared spectrum of butanal

The most prominent infrared absorption lines of butanal

The stretching vibrations of the C-H bond of the alkyl grouping occur at wavenumbers 2975 to 2845 cm-1.

Overlapping are the stretching vibrations of the C-H bond of the aldehyde group (CHO) occur at wavenumbers 2880 to 2650 cm-1.

The strongest and most characteristic absorption band of butanal is at wavenumbers ~1740 to 1720 cm-1 for the stretching vibrations of the carbonyl group bond C=O.

There are other multiple aliphatic aldehyde absorptions at wavenumbers ~1215 to 1435 cm-1 but not particularly characteristic.

The absence of other specific functional group bands will show that a particular functional group is absent from the butanal molecular structure.

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The chemistry of ALDEHYDES and KETONES revision notes INDEX

The mass spectrum of Butanal

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