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Advanced Organic Chemistry: Mass spectrum of but-1-ene

Interpreting the mass spectrum of but-1-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 but-1-ene

mass spectrum of but-1-ene C4H8 CH3CH2CH=CH3 fragmentation pattern of m/z m/e ions for analysis and identification of 1-butene image diagram doc brown's advanced organic chemistry revision notes 

But-1-ene, alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b , alkene

Interpreting the fragmentation pattern of the mass spectrum of but-1-ene

[M]+ is the parent molecular ion peak (M) with an m/z of 56 corresponding to [C4H8]+, the original but-1-ene molecule minus an electron, [H2C=CHCH2CH3]+.

The small M+1 peak at m/z 57, corresponds to an ionised but-1-ene molecule with one 13C atom in it i.e. an ionised but-1-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.

But-1-ene has 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 (but-1-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 but-1-ene.

Unless otherwise indicated, assume the carbon atoms in but-1-ene are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of but-1-ene.

The parent molecular ion peak is from the m/z 56 ion: [H2C=CHCH2CH3]+.

m/z value of [fragment]+ 55 53 51 50 42 42 41 40
[molecular fragment]+ [C4H7]+ [C4H5]+ [C4H3]+ [C4H2]+ [13C12C2H5]+ [12C3H6]+ [C3H5]+ [C3H4]+
m/z value of [fragment]+ 39 38 37 29 28 27 26 15
[molecular fragment]+ [C3H3]+ [C3H2]+ [C3H]+ [C2H5]+ [C2H4]+ [C2H3]+ [C2H2]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of but-1-ene

Atomic masses: H = 1;  C = 12

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

Possible equations to explain the most abundant ion peaks of but-1-ene (tabulated above)

Formation of m/z 55 to 50 ions:

[H2C=CHCH2CH3]+  ===>  [C4H7]+  +  H

[H2C=CHCH2CH3]+  ===>  [C4H6]+  +  H2

C-H bond scission in the parent molecular ion, loss of hydrogen atom of elimination of a hydrogen molecule, mass changes 56 - 1 = 55 or 56 - 2 = 54.

Further loss of hydrogen atom/molecule from these fragments can give m/z ions from 53 down to 49.

They could also undergo C-C bond scission to give smaller ionised fragments containing 1-3 carbon atoms.

Formation of m/z 41 ion:

[H2C=CHCH2CH3]+  ===>  [C3H5]+  +  CH3

C-C bond scission in the parent molecular ion, loss methyl group, mass change 56 - 15 = 41.

The m/z 41 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Further loss of hydrogen atom/molecule can give m/z ions of 40 down to 36.

Formation of m/z 29 ion:

[H2C=CHCH2CH3]+  ===>  [CH3CH2]+  +  C2H3

C-C bond scission in the parent molecular ion, mass change 56 - 27 = 29.

Further hydrogen loss gives m/z ions from 28 down to 25 e.g.

Formation of m/z 27 ion:

[C2H5]+  ===>  [C2H3]+  +  H2

Elimination of a hydrogen molecule from the m/z 29 ion, mass change 29 - 2 = 27.

Formation of m/z 15 ion:

[H2C=CHCH2CH3]+  ===>  [CH3]+  +  H2C=CHCH2

C-C bond scission, mass change 56 - 41 = 15, but this time the methyl group carries the positive charge.


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Links associated with but-1-ene

The chemistry of ALKENES revision notes INDEX

The infrared spectrum of but-1-ene

The H-1 NMR spectrum of but-1-ene  (Please read 8 points at the top of the 1H NMR index page)

The C-13 NMR spectrum of but-1-ene

Mass spectroscopy index

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