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Interpreting the fragmentation pattern of the mass spectrum of 1-methoxypropane

[M]⁺ is the molecular ion peak (M) with an m/z of 74 corresponding to [C₄H₁₀O]⁺, the original 2-methoxypropane molecule minus an electron, [CH₃OCH₂CH₂CH₃]⁺.

The small M+1 peak at m/z 75, corresponds to an ionised 1-methoxypropane molecule with one ¹³C atom in it i.e. an ionised 1-methoxypropane molecule of formula [¹³C¹²C₃H₁₀O]⁺

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-methoxypropane has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a ¹³C atom.

You can also get an m/z 75 (M+1) ion from a collision between a hydrogen radical and the molecular ion.

[M]⁺  +  H•  ==>  [MH]⁺

The most abundant ion of the molecule under mass spectrometry investigation (1-methoxypropane) 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 1-methoxypropane.

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

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

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

Atomic masses: H = 1; C = 12; O = 16

Bond enthalpies kJ/mol: C-C = 348; C-H = 412;  C-O = 360

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

Formation of m/z 73 ion:

[CH₃OCH₂CH₂CH₃]⁺  ===>  [C₄H₉O]⁺  +  H

C-H bond fission leading to H atom loss, mass change 74 - 1 = 73.

The m/z 73 ion most likely has the structure [CH₂OCH₂CH₂CH₃]⁺.

Formation of m/z 45 ion:

[CH₃OCH₂CH₂CH₃]⁺  ===>  [CH₃OCH₂]⁺  +  CH₂CH₃

C-C bond scission, mass change 74 - 29 = 45.

The m/z 45 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 43 ion:

[CH₃OCH₂CH₂CH₃]⁺  ===>  [CH₃CH₂CH₂]⁺  +  CH₃O

C-O bond scission to give an ionised propyl fragment, loss of methoxy group, mass change 74 - 31 = 43.

Formation of m/z 29 ion:

[CH₃OCH₂CH₂CH₃]⁺  ===>  [CH₂CH₃]⁺  +  CH₃OCH₂

C-C bond scission, mass change 74 - 45 = 29.

Formation of m/z 15 ion:

[CH₃OCH₂CH₂CH₃]⁺  ===>  [CH₃]⁺  +  OCH₂CH₂CH₃

C-O bond scission, loss of propoxy fragment, mass change 74 - 59 = 15.

The methyl ion can also be formed from other fragments by C-C or C-O bond scission.

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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