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Advanced Organic Chemistry: Mass spectrum of 2-methylpropanoic acid

Interpreting the mass spectrum of 2-methylpropanoic acid (isobutyric acid)

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 2-methylpropanoic acid

mass spectrum of 2-methylpropanoic acid C4H8O2 (CH3)2CHCOOH fragmentation pattern of m/z m/e ions for analysis and identification of isobutyric acid image diagram doc brown's advanced organic chemistry revision notes 

2-methylpropanoic acid (isobutyric acid), C4H8O2  (c) doc b   (c) doc b   (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of 2-methylpropanoic acid

[M]+ is the molecular ion peak (M) with an m/z of 88 corresponding to [C4H8O2]+, the original 2-methylpropanoic acid molecule minus an electron, [(CH3)2CHCOOH]+.

The tiny M+1 peak at m/z 89, corresponds to an ionised 2-methylpropanoic acid molecule with one 13C atom in it i.e. an ionised 2-methylpropanoic acid 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.

2-methylpropanoic acid has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (2-methylpropanoic acid) 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 2-methylpropanoic acid.

Unless otherwise indicated, assume the carbon atoms in 2-methylpropanoic acid are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of 2-methylpropanoic acid.

m/z value of [fragment]+ 73 55 45 44 ? 43 42 41 39 29 27
[molecular fragment]+ [C3H5O2]+ [C3H3O]+ [COOH]+ [CO2]+ [C3H7]+ [C3H6]+ [C3H5]+ [C3H3]+ [C2H5]+ [C2H3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-methylpropanoic acid

Atomic masses: H = 1;  C = 12;  O = 16

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

Possible equations to explain the most abundant ion peaks of 2-methylpropanoic acid (tabulated above)

Formation of m/z 73 ion:

[(CH3)2CHCOOH]+  ===>  [CH3CHCOOH]+  +  CH3

C-C bond scission of the parent molecular ion, loss of methyl group, mass change 88 - 15 = 73.

Formation of m/z 55 ion:

[C3H5O2]+  ===>  [C3H3O]+  +  H2O

Elimination of water from the m/z 73 ion? not sure on this one

Formation of m/z 45 ion:

[(CH3)2CHCOOH]+  ===>  [COOH]+  +  (CH3)2CH

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

Formation of m/z 43 ion:

[(CH3)2CHCOOH]+  ===>  [(CH3)2CH]+  +  COOH

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.

The m/z 43 ion is a more stable secondary carbocation.

Further loss of hydrogen atoms/molecules will give rise to m/z ions of 42, 41 and 39.


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