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Advanced Organic Chemistry: Mass spectrum of ethylbenzene

The mass spectrum of ethylbenzene

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 ethylbenzene

C8H10 mass spectrum of ethylbenzene fragmentation pattern of m/z m/e ions for analysis and identification of ethylbenzene image diagram doc brown's advanced organic chemistry revision notes 

Ethylbenzene, C8H10 , C6H5CH2CH3 , (c) doc b , (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of ethylbenzene

[M]+ is the molecular ion peak (M) with an m/z of 106 corresponding to [C8H10]+, the original ethylbenzene molecule minus an electron, [C6H5CH2CH3]+.

The small M+1 peak at m/z 10, corresponds to an ionised ethylbenzene molecule with one 13C atom in it i.e. an ionised ethylbenzene molecule of formula 13C12C7H10O

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of ethylbenzene.

m/z value of [fragment]+ 105 91 79 78 77
[molecular fragment]+ [C6H5CH2CH2]+ [C6H5CH2]+ [C6H7]+ [C6H6]+ [C6H5]+
m/z value of [fragment]+ 65 51 39 27 17
[molecular fragment]+ [C5H5]+ [C4H3]+ [C3H3]+ [C2H3]+ [CH5]+

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

Equations to explain the most abundant ion peaks of ethylbenzene

Formation of m/z 105 ion: [C6H5CH2CH3]+  ===>  [C6H5CH2CH2]+  +  H

106 - 1 = 105, proton loss from the parent molecular ion of ethylbenzene.

Formation of m/z 91 ion: [C6H5CH2CH3]+  ===>  [C6H5CH2CH2]+  +  CH3

or: [C6H5CH2CH2]+  ===>  [C6H5CH2CH2]+  +  CH2

The m/z 91 ion is the base peak ion, the most abundant and 'stable' ion fragment.

106 - 15 = 91 or 105 - 14 = 91, loss of CH3 from the parent molecular ion or loss of CH2 from the m/z 91 ion. Both involve C-C bond scission in the alkyl side-chain of ethylbenzene.

Formation of m/z 77 ion: [C6H5CH2CH3]+  ===>  [C6H5]+  +  CH2CH3

or: [C6H5CH2CH2]+  ===>  [C6H5]+  +  CH2=CH2

C-C bond fission from the parent molecular ion or the m/z 91 ion, again, both involve C-C bond scission of the benzene ring from the alkyl side-chain of ethylbenzene.

The observation of an m/z ion of mass 77 is very characteristic of (aromatic) benzene compounds.

It is actually a phenyl radical carrying a positive charge (the free radical cation, [C6H5]+.

The m/z 17 ion: [CH5]+  seems very strange in terms of conventional valence bond theory, but it is formed in mass spectrometers!


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