[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 propyl methanoate [spectra page updated April 4th 2026 *]

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 Mass spectrometry - spectra index

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

If M represents the propyl methanoate 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 propyl methanoate and only the formation of singly charged positive are considered for the mass spectrum of propyl methanoate.

I've included a stick diagram and table of m/z ions for the mass spectrum of propyl methanoate 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 propyl methanoate.

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 propyl methanoate. 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 propyl methanoate, but the mass spectrometer software does!

Propyl methanoate (propyl formate)            

The molecular structure and naming of carboxylic acids and derivatives

Interpreting the fragmentation pattern of the mass spectrum of propyl methanoate

[M]⁺ is the molecular ion peak (M) with an m/z of 88 corresponding to [C₄H₈O₂]⁺, the original propyl methanoate molecule minus an electron, [HCOOCH₂CH₂CH₃]⁺

The molecular peak ion is very small, suggesting the ion is relatively unstable compared to many other smaller fragmentation ions.

Not here, but you might see an M+1 peak at m/z 89, corresponding to an ionised propyl methanoate molecule with one ¹³C atom in it i.e. an ionised propyl methanoate 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.

Propyl methanoate 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 (propyl methanoate) 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 peak ion for the mass spectrum of methyl methanoate is m/z 31 ion [CH₃O]⁺

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

Unless otherwise indicated, assume the carbon atoms in propyl methanoate are the ¹²C isotope.

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

The parent molecular ion is m/z 88  to [C₄H₈O₂]⁺  or  [HCOOCH₂CH₂CH₃]⁺

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of propyl methanoate

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  743

Possible equations to explain the most abundant ion peaks of propyl methanoate (tabulated above)

Formation of m/z 73 ion:

[HCOOCH₂CH₂CH₃]⁺  ===>  [C₃H₅O₂]⁺  +  CH₃

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

mass change 88 - 15 = 73 (M-15 ion peak)

Formation of m/z 60 ion:

[HCOOCH₂CH₂CH₃]⁺  ===>  [C₂H₄O₂]⁺  +  CO

Several bond scissions in the parent molecular ion,

mass change 88 - 28 = 60 (M-28 ion peak)

Formation of m/z 59 ion:

[HCOOCH₂CH₂CH₃]⁺  ===>  [OCH₂CH₂CH₃]⁺  +  HCO

[C₄H₈O₂]⁺  ===>  [C₃H₇O]⁺  +  HCO

C-O bond scission in the parent molecular ion, loss of methyl group,

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

or

[HCOOCH₂CH₂CH₃]⁺  ===>  [HCOOCH₂]⁺  +  CH₂CH₃

[C₄H₈O₂]⁺  ===>  [C₂H₃O₂]⁺  +  CH₂CH₃

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

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

Note that an accurate mass spectrometer can sort out (resolve) pairs of ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places,

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000  ¹⁶O = 15.9949: you can then calculate (predict) that the accurate relative ion masses are:

For m/z 59 [C₃H₇O]⁺ = 59.0495  and  [C₂H₃O₂]⁺ = 59.0132, a relative ion mass difference of 0.0363.

Formation of m/z 45 ion:

[HCOOCH₂CH₂CH₃]⁺  ===>  [HCOO]⁺  +  CH₂CH₂CH₃

C-O bond scission of parent molecular ion,

mass change 88 - 43 = 45 (M-43 ion peak)

Formation of m/z 43 ion:

[HCOOCH₂CH₂CH₃]⁺  ===>  [CH₂CH₂CH₃]⁺  +  HCOO

C-O bond scission of parent molecular ion,

mass change 88 - 45 = 43 (M-45 ion peak)

The m/z 43 ion can lose hydrogen atoms to give the m/z 42 down to 3 ions (see diagram and data table).

The m/z 43 ion could also be the [C₂H₃O]⁺ ion, but origin?

[?]⁺  ===> [C₂H₃O]⁺  +  ?

Note that an accurate mass spectrometer can sort out ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places.

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000  ¹⁶O = 15.9949  ¹⁴N = 14.0031, from which you can calculate (predict) that the accurate relative ion masses are:

For m/z 43: [C₂H₃O]⁺ = 43.0183  and [C₃H₇]⁺ = 43.0546, a difference of 0.0363 in relative ion mass.

Formation of m/z 42 ion:

[?]⁺  ===>  [C₃H₆]⁺  or  [C₂H₂O]⁺  +  ?

Propene ion formed in some way from the parent molecular ion of fragment ion?

Theoretically could be a [C₂H₂O]⁺ ion?

Note that an accurate mass spectrometer can sort out ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places.

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000  ¹⁶O = 15.9949, from which you can calculate (predict) that the accurate relative ion masses are:

For m/z 42: [C₃H₆]⁺  = 42.0468  and [C₂H₂O]⁺ = 42.0105, a difference of 0.0363 in relative ion mass.

Formation of m/z 31 ion:

[C₂H₃O₂]⁺  ===>  [CH₃O]⁺  +  CO  ???

The m/z 31 ion is the base peak ion, the most abundant and 'stable' ion fragment - not sure how it is formed?

Formation of m/z 29 ion:

[HCOOCH₂CH₂CH₃]⁺  ===>  [CH₂CH₃]⁺  +  C₂H₃O₂

[C₄H₈O₂]⁺  ===>  [C₂H₅]⁺  +  C₂H₃O₂

C-C bond scission in the parent molecular ion,

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

or

[HCOOCH₂CH₂CH₃]⁺  ===>  [HCO]⁺  +  OCH₂CH₂CH₃

C-O bond scission, ionisation of ethyl group,

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

Hydrogen atom/molecule loss from the ethyl cation gives m/z ions of 28, 27 and 26.

Note that an accurate mass spectrometer can sort out ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places.

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000  ¹⁶O = 15.9949, from which you can calculate (predict) that the accurate relative ion masses are:

For m/z 29: [CHO]⁺ = 29.0027  and [C₂H₅]⁺ = 29.0390, a difference of 0.0363 in relative ion mass.

Formation of m/z 15 ion:

[HCOOCH₂CH₂CH₃]⁺  ===>  [C₃H₅O₂]⁺  +  CH₃

C-C bond scission in the parent molecular ion,

mass change 88 - 73 = 15 (M-73 ion peak)

The infrared spectrum of propyl methanoate (propyl formate)

The H-1 NMR spectrum of propyl methanoate (propyl formate)

The C-13 NMR spectrum of propyl methanoate (propyl formate)

The chemistry of CARBOXYLIC ACIDS and DERIVATIVES revision notes INDEX

Mass spectrometry index

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