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

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Interpreting the mass spectrum of ethyl methanoate (ethyl formate)

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Mass spectroscopy - spectra index

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

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.

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.

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	45	45	43	31
[molecular fragment]⁺	[C₃H₆O]⁺	[C₃H₄O]⁺	[?]⁺	[?]⁺	[CH₃CH₂O]⁺	[HCO₂]⁺	[?]⁺	[CH₃O]⁺

m/z value of [fragment]⁺	30	29	29	28	28	27	26	18	15
[molecular fragment]⁺	[CH₂O]⁺	[HCO]⁺	[CH₃CH₂]⁺	[C₂H₄]⁺	[CO]⁺	[C₂H₃]⁺	[C₂H₂]⁺	[H₂O]⁺	[CH₃]⁺

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

Atomic masses: H = 1; C = 12; 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]⁺ ===> [COOCH₂CH₃]⁺ + H

C-H bond scission and loss of hydrogen atom.

Formation of m/z 56 ion:

[HCOOCH₂CH₃]⁺ ===> [C₃H₄O]⁺ + H₂O

Elimination of water , mass change 74 - 18 = 56.

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

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.

Formation of m/z 28 ion:

[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?

Formation of m/z 15 ion:

[HCOOCH₂CH₃]⁺ ===> [CH₃]⁺ + C₂H₃O₂

C-C bond scission of the parent molecular ion, mass change 74 - 59 = 15, or from one of the larger fragment ions

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