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

Advanced Organic Chemistry: Mass spectrum of ethylbenzene

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The mass spectrum of ethylbenzene

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Mass spectroscopy - spectra index

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 , ,

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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