Interpreting the infrared spectrum of E-but-2-ene and Z-but-2-ene (infrared spectra of the cis/trans stereoisomers of but-2-ene, 2-butene)

[Author ©  Dr WP Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses, IB chemistry & US K12 grade 11, grade 12 & AP honors chemistry courses: Molecular spectroscopy of E/Z isomers of but-2-ene [updated Mar 20th 2026 *]

(other names: E-2-butene & Z-2-butene, trans-but-2-ene & cis-but-2-ene, trans-2-butene & cis-2-butene)

Spectra obtained from a liquid film of E-but-2-ene and Z-but-2-ene (E-2-butene and Z-2-butene). The right-hand part of the of the infrared spectrum of E-but-2-ene and Z-but-2-ene, wavenumbers ~1500 to 400 cm^-1 is considered the different fingerprint regions for the identification of E-but-2-ene and Z-but-2-ene and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of E-but-2-ene and Z-but-2-ene.

 , structural formula of but-2-ene , but doesn't show the two different spatial arrangements possible due to a high energy barrier to rotation about the double bond, known as the E/Z stereoisomers  (trans-but-2-ene and cis-but-2-ene isomers).

The molecular structure and naming of alkenes

Interpretation of the infrared spectrum of E-but-2-ene and Z-but-2-ene

The most prominent infrared absorption lines of E-but-2-ene and Z-but-2-ene

There are prominent bands of C-H stretching/deformation vibration absorptions peaking at wavenumbers ~3040 to 3010 cm^-1, and also 730 to 665 (Z/cis but-2-ene isomer) and 980 to 960 cm^-1 (E/trans but-2-ene isomer), associated with the C-H bonds in a R₁CH=CHR₂ grouping e.g. as in but-2-ene.

Note different wavenumber regions for the =C-H stretching vibrations.

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-2-ene ...

... BUT this absorption band is only seen in the Z/cis but-2-ene isomer, , NOT the E/trans isomer, , because of its particular symmetry - there is no change in the dipolar character of the molecule on C=C vibration - a condition necessary for infrared wave absorption - full explanation is university level.

Note that the >C=C< absorption vibration is also absent from the even more symmetrical 2,3-dimethylbut-2-ene (2,3-dimethyl-2-butene) for the same reason.

As you can see on comparison, the infrared spectra of Z-but-2-ene and E-but-2-ene have significantly different fingerprint regions.

Around wavenumbers ~1470 to 1370 cm^-1 are absorptions due to C-H deformation vibrations of the C-CH₃ grouping in but-2-ene.

The absence of other specific functional group bands will show that a particular functional group is absent from the E-but-2-ene and Z-but-2-ene molecular structure.

Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of Doc Brown's pre-university advanced level chemistry website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial. These organic chemistry revision notes on spectroscopy (on the infrared spectra of E/Z isomers of but-2-ene) suitable for use of pre-university students studying AQA advanced level chemistry, Edexcel advanced level chemistry, OCR advanced level chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level chemistry, CIE advanced level chemistry, CCEA advanced level chemistry, US grade 11-12 AP honors chemistry courses and they will also prove useful to 1st year undergraduate students of chemistry.