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Advanced Organic Chemistry: Mass spectrum of methanol

The mass spectrum of methanol

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 methanol

CH3OH mass spectrum of methanol fragmentation pattern of m/z m/e ions for analysis and identification of methanol methyl alcohol image diagram doc brown's advanced organic chemistry revision notes 

Methanol  CH4O alcohols and ether structure and naming (c) doc b alcohols and ether structure and naming (c) doc b alcohols and ether structure and naming (c) doc b alcohols and ether structure and naming (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of methanol

[M]+ is the molecular ion peak (M) with an m/z of 32 corresponding to [CH4O]+, the original methanol molecule minus an electron, [CH3OH]+.

The small M+1 peak at m/z 33, corresponds to an ionised methanol molecule with a 13C atom in it i.e. an ionised methanol molecule of formula [13CH3OH]+

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.

Methanol has 1 carbon atom, so on average, ~1 in 100 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (methanol) is usually given an arbitrary 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 methanol.

m/z value of [fragment]+ 31 30 29 28 18 ? 15 14 13
[molecular fragment]+ [CH2OH]+ [CH2O]+ [CHO]+ [CO]+ [H2O]+ [CH3]+ [CH2]+ [CH]+

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

Atomic masses: C = 12;  O = 16; H = 1

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-O = 360;  O-H = 463

Equations to explain the most abundant ion peaks of methanol

Formation of m/z 31 ion:

[CH3OH]+  ===>  [CH2OH]+  +  H

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

This is formed by proton loss from the scission of a C-H bond, not the stronger O-H bond (see list above)

Formation of m/z 30 ion:

[CH2OH]+  ===>  [CHOH]+  +  H

Proton loss from the m/z 31 ion.

Formation of m/z 29 ion:

[CHOH]+  ===>  [CHO]+  +  H

Proton loss from the m/z 30 ion.

Formation of m/z 15 ion:

[CH3OH]+  ===>  [CH3]+  +  OH

Scission of C-O bond.


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