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

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Advanced Organic Chemistry: Mass spectrum of methyl methanoate

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The mass spectrum of methyl methanoate

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing mass spectra of methyl methanoate

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Mass spectroscopy - spectra index

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 ,

For more see Structure and naming of CARBOXYLIC ACIDS and DERIVATIVES, including isomers

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: [HCOOCH₃]⁺

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

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.

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.

Formation of m/z 28.,30 31 ions:

[H₃CO]⁺ ===> [H₂CO]⁺ + H

Hydrogen atom loss from the m/z 31 ion.

This could lose hydrogen to give the m/z 28 ion [CO]⁺.

Formation of m/z 15 ion:

[HCOOCH₃]⁺ ===> [CH₃]⁺ + HCO₂

From C-O bond scission, mass change 60 - 45 = 15 .

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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