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

Interpreting the mass spectrum of 2-methylpropan-2-ol (tert-butyl 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-2-ol

C4H10O (CH3)3COH mass spectrum of 2-methylpropan-2-ol fragmentation pattern of m/z m/e ions for analysis and identification of tert-butyl alcohol image diagram doc brown's advanced organic chemistry revision notes 

2-methylpropan-2-ol   C4H10alcohols 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-2-ol

[M]+ is the molecular ion peak (M) with an m/z of 74 corresponding to [C4H10O]+, the original 2-methylpropan-2-ol molecule minus an electron, [(CH3)3COH]+.

In the case of 2-methylpropan-2-ol, it is tiny, indicating the molecular ion is very unstable.

The most abundant ion of the molecule under mass spectrometry investigation (2-methylpropan-2-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-2-ol.

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

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

m/z value of [fragment]+ 60 59 57 43 41 39
[molecular fragment]+ [C3H8O]+ [(CH3)2COH]+ [(CH3)3C]+ [C3H7]+ [C3H5]+ [C3H3]+
m/z value of [fragment]+ 31 29 27 17 15
[molecular fragment]+ [CH2OH]+ [C2H5]+ [C2H3]+ [OH]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-methylpropan-2-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-2-ol (tabulated above)

Formation of m/z 59 and 60 ions:

[(CH3)3COH]+  ===>  [(CH3)2COH]+  +  CH3

C-C bond scission to lose a methyl group, mass change 74 - 15 = 59.

The m/z 59 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The [C3H7O]+ ion has an m/z of 60 if one of the carbon atoms is a 13C isotope i.e. [13C12C2H7O]+

Carbon-13 only accounts for ~1% of all carbon atoms (12C ~99%), but the more carbon atoms in the molecule/fragment, the greater the probability of observing this 13C M+1 peak.

The [C3H7O]+ fragment ion has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a 13C atom.

Formation of m/z 57 ion:

[(CH3)3COH]+  ===>  [(CH3)3C]+  +  OH

Scission of C-O bond, mass change 74 - 17 = 57.

This is a tertiary carbocation, relatively stable.

Formation of m/z 43 ion:

[(CH3)3COH]+  ===>  [C3H7]+  +  CH2OH

C-C bond scission and proton loss the CH2OH fragment?

Mass change = 74 - 31 = 43.

Formation of m/z 31 ion:

[(CH3)3COH]+  ===>  [CH2OH]+  +  C3H7

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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