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

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing infrared spectra of E-but-2-ene and Z-but-2-ene

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Spectra obtained from a liquid film of E-but-2-ene and Z-but-2-ene. 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 fingerprint region 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 (cis/trans)

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.

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.