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

Interpreting the mass 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 mass spectra of E-but-2-ene and Z-but-2-ene

mass spectrum of E-but-2-ene C4H8 CH3CH=CHCH3 E-2-butene Z-2-butene trans-but-2-ene cis-but-2-ene trans-2-butene cis-2-butene image diagram doc brown's advanced organic chemistry revision notes 

mass spectrum of Z-but-2-ene C4H8 CH3CH=CHCH3 E-2-butene Z-2-butene trans-but-2-ene cis-but-2-ene trans-2-butene cis-2-butene image diagram doc brown's advanced organic chemistry revision notes

 alkenes 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)

Interpreting the fragmentation pattern of the mass spectrum of E-but-2-ene and Z-but-2-ene

[M]+ is the parent molecular ion peak (M) with an m/z of 56 corresponding to [C4H8]+, the original E-but-2-ene and Z-but-2-ene molecule minus an electron, [CH3CH=CHCH3]+.

The small M+1 peak at m/z 57, corresponds to an ionised E-but-2-ene and Z-but-2-ene molecule with one 13C atom in it i.e. an ionised E-but-2-ene and Z-but-2-ene molecule of formula [13C12C3H8]+

Carbon-13 only accounts for ~1% of all carbon atoms (12C ~99%), but the more carbon atoms in the molecule, the greater the probability of observing this 13C M+1 peak.

E-but-2-ene and Z-but-2-ene have 4 carbon atoms, so on average, ~1 in 25 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (E-but-2-ene and Z-but-2-ene) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of E-but-2-ene and Z-but-2-ene.

alkeneUnless otherwise indicated, assume the carbon atoms in E-but-2-ene and Z-but-2-ene are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of but-2-ene.

(fragmentation data table applies to both E/Z isomers of but-2-ene)

The parent molecular ion peak is from the m/z 56 ion: [H3C-CH=CHCH3]+.

m/z value of [fragment]+ 56 55 53 51 50 41
[molecular fragment]+ [C4H8]+ [C4H7]+ [C4H5]+ [C4H3]+ [C4H2]+ [C3H5]+
m/z value of [fragment]+ 39 29 28 27 26 15
[molecular fragment]+ [C3H3]+ [C2H5]+ [C2H4]+ [C2H3]+ [C2H2]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of E-but-2-ene and Z-but-2-ene

Atomic masses: H = 1;  C = 12

Bond enthalpies = kJ/mol: C-C = 348;  C-H = 412;  C=C = 612

The fragmentation patterns are very similar for both E/Z isomers of but-2-ene (2-butene).

I can't see any particular characteristic ion peak that readily distinguishes these two E/Z (cis/trans) isomers.

Possible equations to explain some of the most abundant ion peaks in the mass spectrum of E-but-2-ene and Z-but-2-ene (tabulated above)

Formation of m/z 55 ion:

[CH3CH=CHCH3]+  ===>  [C4H7]+  +  H

C-H bond scission of the parent molecular ion with loss of a hydrogen atom, mass change 56 - 1 = 55.

Further loss of hydrogen atom/molecule creates m/z ions of 55 down to 50 (see table of ions).

Formation of m/z 41 ion:

[CH3CH=CHCH3]+  ===>  [C3H5]+  +  CH3

C-H bond scission of the parent molecular ion with loss of a methyl group, mass change 56 - 15 = 41.

The m/z 41 ion gives the base ion peak, the most abundant 'stable' ion formed from the but-ene isomeric molecules.

Further loss of hydrogen atom/molecule creates m/z ions of 40 down to 37 (see table of ions).

Formation of m/z 29 ion:

[C4H7]+  ===>  [C2H5]+  +  C2H2

e.g. C-C bond scission in a larger fragment and proton rearrangement, mass change 55 - 26 = 29.

Further loss of hydrogen atom/molecule creates m/z ions of 28 down to 25 (see table of ions).

Formation of m/z 15 ion:

[CH3CH=CHCH3]+  ===>  [CH3]+  +  C3H5

C-H bond scission of the parent molecular ion with loss of a C3H5 group, mass change 56 - 41 = 15, but much less likely than the formation of the m/z 41 ion. The m/z 15 ion can also be formed from other fragments.


Key words & phrases: isomer of molecular formula C4H8 CH3CH=CHCH3 image diagram on how to interpret and explain the mass spectrum of E-but-2-ene and Z-but-2-ene m/z m/e base peaks, image and diagram of the mass spectrum of E-but-2-ene and Z-but-2-ene, details of the mass spectroscopy of E-but-2-ene and Z-but-2-ene,  low and high resolution mass spectrum of E-but-2-ene and Z-but-2-ene, prominent m/z peaks in the mass spectrum of E-but-2-ene and Z-but-2-ene, comparative mass spectra of E-but-2-ene and Z-but-2-ene, the molecular ion peak in the mass spectrum of E-but-2-ene and Z-but-2-ene, analysing and understanding the fragmentation pattern of the mass spectrum of E-but-2-ene and Z-but-2-ene, characteristic pattern of peaks in the mass spectrum of E-but-2-ene and Z-but-2-ene, relative abundance of mass ion peaks in the mass spectrum of E-but-2-ene and Z-but-2-ene, revising the mass spectrum of E-but-2-ene and Z-but-2-ene, revision of mass spectroscopy of E-but-2-ene and Z-but-2-ene, most abundant ions in the mass spectrum of E-but-2-ene and Z-but-2-ene, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of E-but-2-ene and Z-but-2-ene, how to analyse the mass spectrum of E-but-2-ene and Z-but-2-ene, how to describe explain the formation of fragmented ions in the mass spectra of E-but-2-ene and Z-but-2-ene equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of E-but-2-ene and Z-but-2-ene recognising the base ion peak of E-but-2-ene and Z-but-2-ene interpreting interpretation the mass spectrum of E-but-2-ene and Z-but-2-ene functional group alkene cis-but-2-ene cis-2-butene trans-but-2-ene trans-2-butene

Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of E-but-2-ene Z-but-2-ene trans-2-butene cis-2-butene. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of E-but-2-ene Z-but-2-ene trans-2-butene cis-2-butene. The m/e m/z value of the molecular ion peak in the mass spectrum of E-but-2-ene Z-but-2-ene trans-2-butene cis-2-butene.  The m/e m/z value of the base ion peak in the mass spectrum of E-but-2-ene Z-but-2-ene trans-2-butene cis-2-butene. Possible examples of equations showing the formation of the ionised fragments in E-but-2-ene Z-but-2-ene trans-2-butene cis-2-butene. Revision notes on the mass 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 mass spectrum. Mass spectroscopy of alkenes, mass 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 infrared 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)

Mass spectroscopy index

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