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

Advanced Organic Chemistry: Mass spectrum of 2-methylpropene

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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

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Mass spectroscopy - spectra index

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

2-methylpropene, , , alkenes structure and naming (c) doc b ,

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

ALL SPECTROSCOPY INDEXES

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