Advanced Organic Chemistry: Infrared spectrum of but-1-ene CH3CH2CH=CH2

Interpreting and explaining the infrared spectrum of but-1-ene (1-butene)

[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 spectrometry - analysing the infrared spectra of butan-1ene [spectra page updated April 3rd 2026 *]

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 Infrared spectroscopy - spectra index

Introductory note on the infrared spectrum of but-1-ene (1-butene)

Students and teachers please note my explanation of the infrared spectrum of but-1-ene (1-butene) is designed for advanced, but pre-university, chemistry courses.

Based in the infrared spectrum diagram for but-1-ene (1-butene), only some of the most prominent peaks for particular bond vibrations are discussed, particularly if but-1-ene (1-butene) has a functional group with a particular characteristic wavenumber peak.

The infrared spectrum of but-1-ene (1-butene) is unique and the whole, or selected wavenumbers, can be used to fingerprint its identity, sometimes analysing a mixture containing but-1-ene (1-butene) or following its change of concentration in a reaction.

infrared spectrum of but-1-ene C4H8 CH3CH2CH=CH3 wavenumbers cm-1 functional group detection fingerprint pattern identification of 1-butene doc brown's advanced organic chemistry revision notes 

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

But-1-ene, alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b , alkene

The molecular structure and naming of alkenes

Interpretation of the infrared spectrum of but-1-ene

The most prominent infrared absorption lines of but-1-ene

There are prominent bands of C-H stretching vibration absorptions peaking at wavenumbers ~3095 to 3010 cm-1, and also 995 to 905 cm-1, associated with the C-H bond in a RCH=CH2 grouping e.g. as in but-1-ene.

Overlapping with the above, are prominent bands due to C-H stretching vibration absorptions peak at wavenumbers ~2975 to 2860 cm-1, typical of molecules with a methyl alkyl group.

There is a very strong characteristic absorption for C=C stretching vibrations, peaking at wavenumbers 1645 to 1640 cm-1, typical of an alkene like but-1-ene.

Around wavenumbers ~1470 to 1370 cm-1 are absorptions dues to C-H deformation vibrations of the C-CH3 grouping in but-1-ene.

The absence of other specific functional group bands will show that a particular functional group is absent from the but-1-ene molecular structure.

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Links associated with but-1-ene

The chemistry of ALKENES revision notes INDEX

The mass spectrum of but-1-ene

The H-1 NMR spectrum of but-1-ene  (Please read 8 points at the top of the 1H NMR index page)

The C-13 NMR spectrum of but-1-ene

Infrared spectroscopy index

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