[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 - analysing the mass spectra of methyl propanoate [spectra page updated April 4th 2026 *]

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

If M represents the methyl propanoate 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 that might be formed in the fragmentation pattern for the mass spectrum of methyl propanoate and only the formation of singly charged positive are considered for the mass spectrum of methyl propanoate.

I've included a stick diagram and table of m/z ions for the mass spectrum of methyl propanoate and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including methyl propanoate.

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated and compared the accurate ion masses if appropriate for methyl propanoate. BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this for methyl propanoate, but the mass spectrometer software does!

Interpreting the fragmentation pattern of the mass spectrum of methyl propanoate

[M]⁺ is the molecular ion peak (M) with an m/z of 88 corresponding to [C₄H₈O₂]⁺, the original methyl propanoate molecule minus an electron, [CH₃CH₂COOCH₃]⁺

The small M+1 peak at m/z 89, corresponds to an ionised methyl propanoate molecule with one ¹³C atom in it i.e. an ionised methyl propanoate molecule of formula [¹³C¹²C₃H₈O₂]⁺

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.

methyl propanoate 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 (methyl propanoate) 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 methyl propanoate is the m/z 57 ion [C₃H₅O]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of methyl propanoate.

Unless otherwise indicated, assume the carbon atoms in methyl propanoate are the ¹²C isotope.

The parent molecular ion peak for the mass spectrum of methyl propanoate is the m/z 88 ion corresponding to [C₄H₈O₂]⁺ or [CH₃CH₂COOCH₃]⁺

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of methyl propanoate.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of methyl propanoate

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;  C=O

Possible equations to explain the most abundant ion peaks of methyl propanoate (tabulated above)

Formation of m/z 87 ion:

[CH₃CH₂COOCH₃]⁺  ===>  [C₄H₇O₂]⁺  +  H

C-H bond scission, hydrogen atom lost,

mass change 88 - 1 = 87 (M-1 ion peak)

Formation of m/z 59 ion:

[CH₃CH₂COOCH₃]⁺  ===>  [COOCH₃]⁺  +  C₂H₅

C-C bond scission of the parent molecular ion, loss of ethyl group,

mass change 88 - 29 = 59 (M-29 ion peak)

Formation of m/z 57 ion:

[CH₃CH₂COOCH₃]⁺  ===>  [CH₃CH₂CO]⁺  +  OCH₃

C-O bond scission of the parent molecular ion,

mass change 88 - 31 = 57 (M-31 ion peak)

The m/z 57 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 57 ion can lose hydrogen atoms to give the m/z 55 and 56 ions (see table).

From the molecular structure of methyl propanoate the m/z 58 ion unlikely to be [C₃H₆O]⁺ and more likely to be [¹³C¹²C₂H₅O]⁺ which would be formed in the same way as the m/z 57 ion.

I'm not sure which fragment species predominates for some pairs of ions with the same integer m/z value, which might be formed by other fragmentation reactions, BUT an accurate mass spectrometer can sort them out, and can measure relative fragment ion masses to four decimal places.

Using accurate relative isotopic masses you can calculate and predict an accurate relative ion mass:

¹H = 1.0078  ¹²C = 12.0000  ¹³C = 13.0034 ¹⁶O = 15.9949

 [C₃H₆O]⁺ = 58.0417 and [¹³C¹²C₂H₅O]⁺ = 58.0373, a difference of 0.0044 in relative ion mass.

Formation of m/z 31 ion:

[CH₃CH₂COOCH₃]⁺  ===>  [OCH₃]⁺  +  CH₃CH₂CO

C-O bond scission of the parent molecular ion,

mass change 88 - 57 = 31 (M-57 ion peak)

Formation of m/z 29 ion:

[CH₃CH₂COOCH₃]⁺  ===>  [C₂H₅]⁺  +  COOCH₃

C-C bond scission of the parent molecular ion,

mass change 88 - 59 = 29 (M-59 ion peak)

The m/z 29 ion can lose hydrogen atoms to give the m/z 26 to 28 ions.

The m/z 30 ion is likely to formed in the same way as the m/z 29 ion, but containing a ¹³C carbon isotope atom i.e. [¹³C¹²CH₅]⁺ rather than a [C₂H₆]⁺ ion.

Again, using accurate relative isotopic masses you can calculate and predict an accurate relative ion mass:

¹H = 1.0078  ¹²C = 12.0000  ¹³C = 13.0034

For m/z 30: [¹³C¹²CH₅]⁺ = 30.0424, [C₂H₆]⁺ = 30.0468, a difference of 0.0044 in relative ion mass

Formation of m/z 15 ion:

[C₄H₈O₂]⁺  ===>  [CH₃]⁺  +  C₃H₅O₂

C-C or C-O bond scission to form a methyl ion, mass change 88 - 73 = 15.

The infrared spectrum of methyl propanoate (methyl propionate)

The H-1 NMR spectrum of methyl propanoate (methyl propionate)

The C-13 NMR spectrum of methyl propanoate (methyl propionate)

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