mass spectrum of ethyl methanoate C3H6O2 HCOOCH2CH3 fragmentation pattern of m/z m/e ions for analysis and identification of ethyl formate image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of ethyl methanoate (ethyl formate) HCOOCH₂CH₃

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Interpreting and explaining the mass spectrum of ethyl methanoate (ethyl formate)

[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 ethyl methanoate [spectra page updated April 3rd 2026 *]

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Introductory note on the mass spectrum of ethyl methanoate

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

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

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

mass spectrum of ethyl methanoate C3H6O2 HCOOCH2CH3 fragmentation pattern of m/z m/e ions for analysis and identification of ethyl formate image diagram doc brown's advanced organic chemistry revision notes

Ethyl methanoate (ethyl formate) C₃H₆O₂ (c) doc b

The molecular structure and naming of carboxylic acids and derivatives

Interpreting the fragmentation pattern of the mass spectrum of ethyl methanoate

[M]⁺ is the molecular ion peak (M) with an m/z of 74 corresponding to [C₃H₆O₂]⁺, the original ethyl methanoate molecule minus an electron, [HCOOCH₂CH₃]⁺

The small M+1 peak at m/z 75, corresponds to an ionised ethyl methanoate molecule with one ¹³C atom in it i.e. an ionised ethyl methanoate molecule of formula [¹³C¹²C₂H₆O₂]⁺

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.

Ethyl methanoate has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (ethyl 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 ethylmethylamine is m/z 31 ion [CH₃O]⁺

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

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

The parent molecular ion is the m/z 74 ion corresponding to [C₃H₆O₂]⁺ or [HCOOCH₂CH₃]⁺

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

m/z value of [fragment]⁺	73	56	47 ?	46	46	45	45	43
[molecular fragment]⁺	[C₃H₅O₂]⁺	[C₃H₄O]⁺	[CH₃O₂]⁺	[CH₂O₂]⁺	[C₂H₆O]⁺	[C₂H₅O]⁺	[HCO₂]⁺	[C₂H₃O]⁺

m/z value of [fragment]⁺	31	30	29	29	28	28	27	26	18	15
[molecular fragment]⁺	[CH₃O]⁺	[CH₂O]⁺	[HCO]⁺	[C₂H₅]⁺	[C₂H₄]⁺	[CO]⁺	[C₂H₃]⁺	[C₂H₂]⁺	[H₂O]⁺	[CH₃]⁺

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of ethyl 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 ethyl methanoate (tabulated above)

Formation of m/z 73 ion:

[HCOOCH₂CH₃]⁺ ===> [C₃H₅O₂]⁺ + H

C-H bond scission and loss of hydrogen atom.

mass change 73 - 1 = 72 (M-1 ion peak)

Formation of m/z 56 ion (and m/z 18 ion):

[HCOOCH₂CH₃]⁺ ===> [C₃H₄O]⁺ + H₂O

Elimination of water,

mass change 74 - 18 = 56 (M-18 ion peak)

Much less likely is the ionisation of the eliminated water molecule to give the m/z 18 ion.

[HCOOCH₂CH₃]⁺ ===> [H₂O]⁺ + C₃H₄O

Formation of m/z 46 ion:

[HCOOCH₂CH₃]⁺ ===> [CH₂O₂]⁺ + C₂H₄

Elimination of an ethene molecule from the parent molecular ion,

mass change 74 - 28 = 46 (M-28 ion peak)

Not likely to be the [C₂H₆O]⁺ ion.

An accurate mass spectrometer sorts this out, measuring relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

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

[C₂H₆O]⁺ = 46.0417, [CH₂O₂]⁺ = 46.0054, a difference of 0.0363 in relative ion mass.

Formation of m/z 45 ion:

[HCOOCH₂CH₃]⁺ ===> [HCOO]⁺ + CH₂CH₃

[HCOOCH₂CH₃]⁺ ===> [OCH₂CH₃]⁺ + HCO

C-O bond scission of the parent molecular ion, two possibilities of bond breaking leading to two possibilities of ionised fragment,

both mass changes 74 - 29 = 45 (M-29 ion peak)

An accurate mass spectrometer sorts this out, measuring relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

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

[C₂H₆O]⁺ = 46.0417, [CH₂O₂]⁺ = 46.0054, a difference of 0.0363 in relative ion mass.

Formation of m/z 31 ion:

[OCH₂CH₃]⁺ ===> [CH₃O]⁺ + CH₂ ???

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

but I'm not sure how it is formed.

Formation of m/z 29 ion:

[HCOOCH₂CH₃]⁺ ===> [CHO]⁺ + OCH₂CH₃

[HCOOCH₂CH₃]⁺ ===> [CH₂CH₃]⁺ + HCOO

Two possibilities of C-O bond scission of the parent molecular ion.

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

An accurate mass spectrometer sorts this out, measuring relative fragment ion masses to four decimal places e.g. using very 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 29: [CHO]⁺ = 29.0027 and [C₂H₅]⁺ = 29.0390, a difference of 0.0363 in relative ion mass,

Formation of m/z 28 ion:

[HCOOCH₂CH₃]⁺ ===> [C₂H₄]⁺ + CH₂O₂

Elimination of an ethene molecule from the parent molecular ion,

mass change 74 - 46 = 28 (M-46 ion peak)

or ?

[CH₂CH₃]⁺ ===> [C₂H₄]⁺ + H

Hydrogen atom loss from ethyl ion (above), further hydrogen atom losses give the m/z 27 and 26 ions.

It may be possible the [CO]⁺ ion be formed from other fragment ions?

An accurate mass spectrometer sorts this out, measuring relative fragment ion masses to four decimal places e.g. using very 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 28: [CO]⁺ = 27.9949 and [C₂H₄]⁺ = 28.0312, difference of 0.0363 in relative ion mass

Formation of m/z 15 ion:

[HCOOCH₂CH₃]⁺ ===> [CH₃]⁺ + C₂H₃O₂

C-C bond scission of the parent molecular ion,

mass change 74 - 59 = 15 (M-59 ion peak)

or from one of the larger fragment ions like the m/z 29 [CH₂CH₃]⁺ ion.

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The infrared spectrum of ethyl methanoate (ethyl formate)

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

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

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