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Advanced Organic Chemistry: Mass spectrum of 2-methylpropan-1-ol

Interpreting the mass spectrum of 2-methylpropan-1-ol (isobutyl alcohol)

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-methylpropan-1-ol

mass spectrum of 2-methylpropan-1-ol C4H10O (CH3)2CHCH2OH fragmentation pattern of m/z m/e ions for analysis and identification of isobutyl alcohol image diagram doc brown's advanced organic chemistry revision notes 

2-methylpropan-1-ol   C4H10O   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 2-methylpropan-1-ol

[M]+ is the molecular ion peak (M) with an m/z of 74 corresponding to [C4H10O]+, the original 2-methylpropan-1-ol molecule minus an electron, [(CH3)2CHCH2OH]+.

The very tiny M+1 peak at m/z 75, corresponds to an ionised 2-methylpropan-1-ol molecule with one 13C atom in it i.e. an ionised 2-methylpropan-1-ol molecule of formula [13C12C3H10O]+

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-methylpropan-1-ol 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-methylpropan-1-ol) 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-methylpropan-1-ol.

Unless otherwise indicated, assume the carbon atoms in 2-methylpropan-1-ol are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of 2-methylpropan-1-ol.

m/z value of [fragment]+ 73 59 56 55 45 44 43 42
[molecular fragment]+ [C4H9O]+ [C3H7O]+ [C4H8]+ [C4H7]+ [C2H5O]+ [C3H8]+ [C3H7]+ [C3H6]+
m/z value of [fragment]+ 41 39 33 31 29 28 27 17 ? 15
[molecular fragment]+ [C3H5]+ [C3H3]+ [?]+ [CH2OH]+ [C2H5]+ [C2H4]+ [C2H3]+ [OH]+ [?]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-methylpropan-1-ol

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

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

Possible equations to explain the most abundant ion peaks of 2-methylpropan-1-ol (tabulated above)

Formation of m/z 73 ion:

[(CH3)2CHCH2OH]+  ===>  [(CH3)2CHCHOH]+  +  H

C-H bond scission to lose a proton.

Formation of m/z 59 ion:

[(CH3)2CHCH2OH]+  ===>  [C3H7O]+  +  CH3

C-C bond scission, loss of methyl group, mass change = 74 - 15

Formation of m/z 55 ion:

[(CH3)2CHCHOH]+  ===>  [C4H8]+  +  H2O

Elimination of water from the m/z 73 ion to give an ionised butene molecule, mass change 73 - 18 = 55.

Formation of m/z 56 ion:

[(CH3)2CHCH2OH]+  ===>  [C4H8]+  +  H2O

Elimination of water to give an ionised butene molecule, mass change 74 - 18 = 56

Formation of m/z 43 ion:

[(CH3)2CHCH2OH]+  ===>  [C3H7]+  +  CH2OH

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

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

The m/z 43 ion can lose a proton to give the m/z 42 ion (ionised propene molecule), further proton loss to give m/z 41 and 39 ions.

Formation of m/z 31 ion:

[(CH3)2CHCH2OH]+  ===>  [CH2OH]+  +  C3H7CH2OH

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

Note this is the same C-C bond scission in the m/z 43 ion formation - quite often either fragment can be ionised, but only one of the two fragments can carry the positive charge.


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