mass spectrum of methyl ethanoate C3H6O2 CH3COOCH3 fragmentation pattern of m/z m/e ions for analysis and identification of methyl acetate image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of methyl ethanoate

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Interpreting the mass spectrum of methyl ethanoate (methyl acetate)

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 ethanoate

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Mass spectroscopy - spectra index

mass spectrum of methyl ethanoate C3H6O2 CH3COOCH3 fragmentation pattern of m/z m/e ions for analysis and identification of methyl acetate image diagram doc brown's advanced organic chemistry revision notes

methyl ethanoate (methyl acetate), , , ,

Interpreting the fragmentation pattern of the mass spectrum of methyl ethanoate

[M]⁺ is the molecular ion peak (M) with an m/z of 74 corresponding to [C₃H₆O₂]⁺, the original methyl ethanoate molecule minus an electron, [CH₃COOCH₃]⁺.

The small M+1 peak at m/z 75, corresponds to an ionised methyl ethanoate molecule with one ¹³C atom in it i.e. an ionised methyl ethanoate 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.

Methyl ethanoate has 3 carbon atoms, so on average, ~1 in 3 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (methyl ethanoate) 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 methyl ethanoate.

Unless otherwise indicated, assume the carbon atoms in methyl ethanoate are the ¹²C isotope.

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

m/z value of [fragment]⁺	59	43	42	29 ?	28	15
[molecular fragment]⁺	[CH₃CO₂]⁺	[CH₃CO]⁺	[CH₂CO]⁺	[CHO]⁺	[CO]⁺	[CH₃]⁺

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of methyl ethanoate

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 methyl ethanoate (tabulated above)

Formation of m/z 59 ion:

[CH₃COOCH₃]⁺ ===> [CH₃CO₂]⁺ + CH₃

Scission of C-O bond, loss of methyl group, mass change 74 - 15 = 59.

Formation of m/z 43 ion:

[CH₃COOCH₃]⁺ ===> [CH₃CO]⁺ + CH₃O

C-O bond scission, mass change 74 - 31 = 43.

The m/z 42 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Proton loss gives the m/z 42 ion.

Formation of m/z 29 ion:

[?]⁺ ===> [?]⁺ + ?

Formation of m/z 28 ion:

[CH₃CO]⁺ ===> [CO]⁺ + CH₃

C-C bond scission of the m/z 43 ion.

Formation of m/z 15 ion:

[CH₃COOCH₃]⁺ ===> [CH₃]⁺ + COOCH₃

[CH₃COOCH₃]⁺ ===> [CH₃]⁺ + CH₃COO

[CH₃CO]⁺ ===> [CH₃]⁺ + CO

C-C or C-O bond scission in the parent molecular ion, or for larger fragment ions (e.g. m/z 43 ion).

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