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Advanced Organic Chemistry: Mass spectrum of 2-methylpropene

Interpreting the mass spectrum of 2-methylpropene (2-methylprop-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 2-methylpropene

mass spectrum of 2-methylpropene C4H8 (CH3)2C=CH2 fragmentation pattern of m/z m/e ions for analysis and identification of 2-methylprop-1-ene image diagram doc brown's advanced organic chemistry revision notes 

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Interpreting the fragmentation pattern of the mass spectrum of 2-methylpropene

[M]+ is the parent molecular ion peak (M) with an m/z of 56 corresponding to [C4H8]+, the original 2-methylpropene molecule minus an electron, [(CH3)2C=CH2]+.

The small M+1 peak at m/z ?, corresponds to an ionised 2-methylpropene molecule with one 13C atom in it i.e. an ionised 2-methylpropene 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.

2-methylpropene 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 (2-methylpropene) 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 2-methylpropene.

Unless otherwise indicated, assume the carbon atoms in 2-methylpropene are the 12C isotope.

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

The parent molecular ion of 2-methylpropene m/z 56: [(CH3)2C=CH2]+

Data table of some of the ions formed in the fragmentation pattern of the mass spectrum of 2-methylpropene

m/z value of [fragment]+ 56 55 53 51 50 41 40
[molecular fragment]+ [C4H8]+ [C4H7]+ [C4H5]+ [C4H3]+ [C4H2]+ [C3H5]+ [C3H4]+
m/z value of [fragment]+ 39 38 29 28 27 26 15
[molecular fragment]+ [C3H3]+ [C3H2]+ [C2H5]+ [C2H4]+ [C2H3]+ [C2H2]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-methylpropene

Atomic masses: H = 1;  C = 12

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

Possible equations to explain some of the most abundant ion peaks in the mass spectrum of 2-methylpropene (tabulated above)

Formation of m/z 55 ion:

[(CH3)2C=CH2]+  ===>  [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 from 54 down to 50 (see table of ions).

Formation of m/z 41 ion:

[(CH3)2C=CH2]+  ===>  [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 2-methylpropene.

Further loss of hydrogen atom/molecule creates m/z ions from 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:

[(CH3)2C=CH2]+  ===>  [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.


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Links associated with 2-methylpropene

The chemistry of ALKENES revision notes INDEX

The infrared spectrum of 2-methylpropene (2-methylprop-1-ene)

The H-1 NMR spectrum of 2-methylpropene (2-methylprop-1-ene)

The C-13 NMR spectrum of 2-methylpropene (2-methylprop-1-ene)

Mass spectroscopy index

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