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Advanced Organic Chemistry: Mass spectrum of phenol

Interpreting the mass spectrum of phenol

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 phenol

mass spectrum of phenol C6H6O C6H5OH fragmentation pattern of m/z m/e ions for analysis and identification of phenol image diagram doc brown's advanced organic chemistry revision notes 

PhenolC6H6OC6H5OH, (c) doc b(c) doc b

Interpreting the fragmentation pattern of the mass spectrum of phenol

[M]+ is the molecular ion peak with an m/z of 94 corresponding to [C6H6O]+, the original phenol molecule minus an electron, [C6H5OH]+.

The small M+1 peak at m/z 95, corresponds to an ionised phenol molecule with one 13C atom in it i.e. an ionised phenol molecule of formula [13C12C5H6O]+

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.

Phenol has 6 carbon atoms, so on average, ~1 in 17 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (phenol) 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 phenol.

Unless otherwise indicated, assume thecarbon atoms in phenol are the 12C isotope.

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

Parent molecular ion of phenol: [C6H5OH]+, m/z 94, which in this case, is also the base peak ion.

m/z value of [fragment]+ 66 65 63 55 ? 53 ?
[molecular fragment]+ [C5H6]+ [C5H5]+ [C5H3]+ [C3H3O]+ [C4H5]+
m/z value of [fragment]+ 51 ? 50 ? 47 ? 40 39 38
[molecular fragment]+ [C4H3]+ [C4H2]+ [?]+ [C3H4]+ [C3H3]+ [C3H2]+

I'm not sure on the identity of some of these ions?

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

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

Bond enthalpies = kJ/mol: = 518 (benzene);  C-H = 412;  C-O = 360;  O-H = 463

Possible equations to explain the most abundant ion peaks of phenol (tabulated above)

Formation of m/z 66 ion:

[C6H5OH]+  ===>  [C5H6]+  +  CO

A complex reaction (from the internet), mass change 94 - 28 = 66.

Loss of CO fragment from parent molecular ion.

Formation of m/z 65 ion:

[C6H5OH]+  ===>  [C5H5]+  +  CHO

A complex reaction (from the internet), mass change 94 - 29 = 65.

Loss of CHO fragment from parent molecular ion.

The m/z 65 ion is also formed by hydrogen loss from the m/z 66 ion.

The m/z 63 ion may be formed by H2 loss from the m/z 65 ion.


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