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 , 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)

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.

Unless 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.

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

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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