[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of 1-methoxypropane (mass spectra) [spectra page updated April 1st 2026 *]

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Introductory note on the mass spectrum of 1-methoxypropane

Students and teachers please note my explanation of the mass spectrum of 1-methoxypropane is designed for advanced, but pre-university, chemistry courses.

If M represents the 1-methoxypropane molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and fragmentation equations assume [M]⁺ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]⁺ ==> [X]⁺  +  R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions might be formed in the fragmentation pattern for 1-methoxypropane.

I've included stick diagram and table of m/z ions for the mass spectrum of 1-methoxypropane and conducting the mass spectrum analysis under standard conditions, a database can be built up based complex fingerprint patterns, often involving relative intensities of many fragment ions, that can be used to identify compounds including 1-methoxypropane.

1-methoxypropane   C₄H₁₀O        

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.

The base ion peak for the mass spectrum of 1-methxypropane is the m/z 43 ion [C₂H₅O]⁺

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.

The parent molecular ion of 1-methoxypropane m/z 74 ion [CH₃OCH₂CH₂CH₃]⁺

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

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

Atomic masses: H = 1; C = 12 (~1% 13); 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 (M-1 ion peak)

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

Formation of m/z 45 ion:

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

C-C bond scission and loss of ethyl group,

mass change 74 - 29 = 45 (M-29 ion peak)

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 (M-31 ion peak)

Formation of m/z 31 ion:

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

C-O bond scission to give an ionised methoxy fragment, loss of propyl group,

mass change 74 - 43 = 31 (M-43 ion peak)

Formation of m/z 29 ion:

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

C-C bond scission,

mass change 74 - 45 = 29 (M-45 ion peak)

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 larger fragments by C-C or C-O bond scission.

The infrared spectrum of 1-methoxypropane (methyl propyl ether)

The H-1 NMR spectrum of 1-methoxypropane (methyl propyl ether)

The C-13 NMR spectrum of 1-methoxypropane (methyl propyl ether)

Mass spectrometry index

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