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Advanced Organic Chemistry: Infrared spectra of E-but-2-ene and Z-but-2-ene

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

infrared spectrum of E-but-2-ene C4H8 CH3CH=CHCH3 wavenumbers cm-1 functional group detection fingerprint pattern identification of trans-but-2-ene and Z-but-2-ene trans-2-butene doc brown's advanced organic chemistry revision notes

infrared spectrum of Z-but-2-ene C4H8 CH3CH=CHCH3 wavenumbers cm-1 functional group detection fingerprint pattern identification of cis-but-2-ene cis-2-butene doc brown's advanced organic chemistry revision notes  

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.

 alkenealkenes structure and naming (c) doc b, structural formula of but-2-ene alkenes structure and naming (c) doc b , 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, alkenes structure and naming (c) doc b , NOT the E/trans isomer, alkenes structure and naming (c) doc b , 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.


Key words & phrases: isomer of molecular formula C4H8 CH3CH=CHCH3 image and diagram explaining the infrared spectrum of E-but-2-ene and Z-but-2-ene, complete infrared absorption spectrum of E-but-2-ene and Z-but-2-ene, comparative spectra of E-but-2-ene and Z-but-2-ene, prominent peaks/troughs for identifying functional groups in the infrared spectrum of E-but-2-ene and Z-but-2-ene, important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum of E-but-2-ene and Z-but-2-ene, revision of infrared spectroscopy of E-but-2-ene and Z-but-2-ene, fingerprint region analysis of E-but-2-ene and Z-but-2-ene, how to identify E-but-2-ene and Z-but-2-ene from its infrared spectrum, identifying organic compounds like E-but-2-ene and Z-but-2-ene from their infrared spectrum, how to analyse the absorption bands in the infrared spectrum of E-but-2-ene and Z-but-2-ene detection of alkene functional groups in the E-but-2-ene and Z-but-2-ene molecule example of the infrared spectrum of a molecule like E-but-2-ene and Z-but-2-ene with a alkene functional group  interpreting interpretation of the infrared spectrum of E-but-2-ene and Z-but-2-ene shows presence of alkene functional group functional group alkene cis-but-2-ene cis-2-butene trans-but-2-ene trans-2-butene

Diagram of absorption of wavenumber peaks in the infrared spectrum of E-but-2-ene Z-but-2-ene trans-2-butene cis-2-butene. Characteristic peak wavenumbers in the infrared spectrum of E-but-2-ene Z-but-2-ene trans-2-butene cis-2-butene. Finger print identification pattern using the infrared spectrum of E-but-2-ene Z-but-2-ene trans-2-butene cis-2-butene. Revision notes on the infrared spectrum of E-but-2-ene Z-but-2-ene trans-2-butene cis-2-butene. Matching and deducing the structure of the E-but-2-ene Z-but-2-ene trans-2-butene cis-2-butene molecule from  its infrared spectrum. Infrared spectroscopy of alkenes, infrared spectra of E-but-2-ene Z-but-2-ene trans-2-butene cis-2-butene, an isomer of molecular formula C4H8


Links associated with E-but-2-ene and Z-but-2-ene

The chemistry of ALKENES revision notes INDEX

The mass spectra of the E/Z isomers of but-2-ene (cis/trans isomers of 2-butene)

The H-1 NMR spectrum of E/Z but-2-ene (cis/trans isomers of 2-butene)

The C-13 NMR spectra of the E/Z isomers of but-2-ene (cis/trans isomers of 2-butene)

Infrared spectroscopy index

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