mass spectrum of methyl methanoate fragmentation pattern of m/z m/e ions for analysis and identification of methyl formate image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of methyl methanoate HCOOCH₃

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Interpreting & explaining the mass spectrum of methyl methanoate

[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 methyl methanoate [spectra page updated Mar 25th 2026 *]

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Introductory note on the mass spectrum of methyl methanoate

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

If M represents the methyl methanoate molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^- ==> [M•]⁺_(g) + 2e^- and for 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 methanoate and only the formation of singly charged positive are considered for the mass spectrum of methyl methanoate.

I've included a stick diagram and table of m/z ions for the mass spectrum of methyl 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 methyl 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 the accurate ion masses, 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 methanoate, but the mass spectrometer software does!

mass spectrum of methyl methanoate fragmentation pattern of m/z m/e ions for analysis and identification of methyl formate image diagram doc brown's advanced organic chemistry revision notes

Methyl methanoate (methyl formate) C₂H₄O₂, , (c) doc b ,

The molecular structure and naming of carboxylic acids and derivatives

Interpreting the fragmentation pattern of the mass spectrum of methyl methanoate

[M]⁺ is the molecular ion peak (M) with an m/z of 60 corresponding to [C₂H₄O₂]⁺, the original methyl methanoate molecule minus an electron, [HCOOCH₃]⁺.

The minute M+1 peak at m/z 61, corresponds to an ionised methyl methanoate molecule with one ¹³C atom in it i.e. an ionised methyl methanoate molecule of formula ¹³C¹²CH₄O₂

Carbon consists of ~1% ¹³C atoms, ethanoic acid has 2 carbon atoms, so about 1 in 50 molecules or fragments may contain a carbon-13 atom.

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

Parent molecular ion is the m/z 60 ion [C₂H₄O₂]⁺ or [HCOOCH₃]⁺

The m/z 31 ion is the base peak ion [H₃CO]⁺ for the mass spectrum of methyl methanoate.

m/z value of [fragment]⁺	32 ?	31	30	29	28	15
[molecular fragment]⁺	[CH₄O]⁺	[H₃CO]⁺	[H₂C=O]⁺	[CHO]⁺	[CO]⁺	[CH₃]⁺

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of methyl 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; O = 16

Bond enthalpies = kJ/mol: C-H = 412; C-O = 360; C=O = 743

Possible equations to explain the most abundant ion peaks of methyl methanoate

Formation of m/z 31 ion:

[HCOOCH₃]⁺ ===> [H₃CO]⁺ + HCO

C-O bond scission of the parent molecular ion,

mass change 60 - 29 = 31 (M-29 ion peak)

The m/z 31 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 29 ion:

[HCOOCH₃]⁺ ===> [CHO]⁺ + OCH₃

C-O bond scission in the parent molecular ion,

mass change 60 - 31 = 29 (M-31 ion)

Formation of m/z 28 and 30 ions:

[H₃CO]⁺ ===> [H₂CO]⁺ + H

Hydrogen atom loss from the m/z 31 ion gives the m/z 30 ion.

The m/z 30 ion could lose hydrogen to give the m/z 28 ion [CO]⁺

[H₂CO]⁺ ===> [CO]⁺ + H₂

Formation of m/z 15 ion:

[HCOOCH₃]⁺ ===> [CH₃]⁺ + HCO₂

From C-O bond scission, mass change 60 - 45 = 15 (M-45 ion peak)

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 isomers of C₂H₄O₂ NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original ethanoic acid (acetic acid) and methyl methanoate (methyl formate) image sizes.

INFRARED SPECTRA: Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm^-1, the most striking difference is the broad O-H stretching band ~3200 cm^-1, found in the infrared spectrum of carboxylic acids, but absent in the infrared spectrum of esters.

MASS SPECTRA: Apart from the m/z of 60 for the parent molecular ion and m/z 15 ion [CH₃]⁺, both ethanoic acid and methyl methanoate show few similarities in their mass spectra. Their base ion peaks are quite different - for ethanoic acid it is m/z 43 and for methyl methanoate it is m/z 31. The mass spectrum of methyl methanoate has very prominent peaks for m/z ions 31 and 32 and a complete absence of m/z ions 42, 43 and 45.

1H NMR SPECTRA: The 1H NMR spectra of ethanoic acid and methyl methanoate are similar with two 'wide-apart' chemical shift singlet peaks in the integrated proton ratio of 3:1 (meaning 2 different ¹H chemical environments).

13C NMR SPECTRA: The 13C NMR spectra of ethanoic acid and methyl methanoate are similar, both molecules give two C-13 NMR spectral lines (meaning 2 different ¹³C chemical environments).

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What next? links associated with methyl methanoate

The infrared spectrum of methyl methanoate (methyl formate)

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

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

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