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Advanced Organic Chemistry: Mass spectrum of 3-hydroxybutanone

Interpreting the mass spectrum of 3-hydroxybutanone  (acetoin)

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 3-hydroxybutanone

mass spectrum of 3-hydroxybutanone C4H8O2 CH3COCH(OH)CH3 fragmentation pattern of m/z m/e ions for analysis and identification of acetoin image diagram doc brown's advanced organic chemistry revision notes 

C4H8O2, 3-hydroxybutanone 3-hydroxybutan-2-one acetoin CH3COCH(OH)CH3 C4H8O2, 3-hydroxybutanone, 3-hydroxybutan-2-one, 'acetoin', CH3COCH(OH)CH3

Interpreting the fragmentation pattern of the mass spectrum of 3-hydroxybutanone

[M]+ is the molecular ion peak (M) with an m/z of 88 corresponding to [C4H8O2]+, the original 3-hydroxybutanone molecule minus an electron, [CH3COCH(OH)CH3]+.

The small M+1 peak at m/z 89, corresponds to an ionised 3-hydroxybutanone molecule with one 13C atom in it i.e. an ionised 3-hydroxybutanone molecule of formula [13C12C3H8O2]+

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.

3-hydroxybutanone has 4 carbon atoms, so on average, ~1 in 4 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (3-hydroxybutanone) 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 3-hydroxybutanone.

Unless otherwise indicated, assume the carbon atoms in 3-hydroxybutanone are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of 3-hydroxybutanone.

m/z value of [fragment]+ 72 71 57 45 43 29 27 18 15
[molecular fragment]+ [?]+ [C3H7C=O]+ [?]+ [CH3CO]+ [CH3CHOH]+ [C2H5]+ [C2H3]+ [H2O]+ [CH3]+

In some mass spectrum diagrams of 3-hydroxybutanone, the m/z 45 ion is shown as the base peak ion, obviously both are the most abundant relatively stable ions.

Both the m/z 43 and 45 ions are formed via the same bond scission of the parent molecular ion.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 3-hydroxybutanone

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 3-hydroxybutanone (tabulated above)

Formation of m/z 71 ion:

[CH3COCH(OH)CH3]+  ===>  [CH3COCHCH3]+  +  OH

C-O bond scission, loss of hydroxyl group, mass change 88 - 17 = 71.

Formation of m/z 45 ion:

[CH3COCH(OH)CH3]+  ===>  [CH3CHOH]+  +  CH3CO

C-C bond scission of the parent molecular ion, mass change 88 - 43 = 45.

Formation of m/z 43 ion:

[CH3COCH(OH)CH3]+  ===>  [CH3CO]+  +  CH3CHOH

C-C bond scission of the parent molecular ion, mass change 88 - 45 = 43.

The m/z 43 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 15 ion:

[CH3COCH(OH)CH3]+  ===>  [CH3]+  +  C3H5O2  ?

C-C bond scission of the parent molecular ion, mass change 88 - 55 = 43, or from C-C bond scission in one of the larger fragments e.g. the m/z 43 or 45 ions.


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Links associated with 3-hydroxybutanone

The chemistry of ALCOHOLS revision notes INDEX

The chemistry of ALDEHYDES and KETONES revision notes INDEX

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