mass spectrum of cyclohexene C6H10 fragmentation pattern of m/z m/e ions for analysis and identification of cyclohexene image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of cyclohexene C₆H₁₀

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Interpreting the mass spectrum of cyclohexene C₆H₁₀

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 cyclohexene

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Mass spectroscopy - spectra index

mass spectrum of cyclohexene C6H10 fragmentation pattern of m/z m/e ions for analysis and identification of cyclohexene image diagram doc brown's advanced organic chemistry revision notes

cyclohexene , , alkenes structure and naming (c) doc b ,

Interpreting the fragmentation pattern of the mass spectrum of cyclohexene

[M]⁺ is the parent molecular ion peak (M) with an m/z of 82 corresponding to [C₆H₁₀]⁺, the original cyclohexene molecule minus an electron.

The small M+1 peak at m/z 83, corresponds to an ionised cyclohexene molecule with one ¹³C atom in it i.e. an ionised cyclohexene molecule of formula [¹³C¹²C₅H₁₀]⁺

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 ion of m/z 83.

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

This sort of argument also applies to fragment ions from the parent molecular ion of cyclohexene - though the ratio will be greater e.g. m/z 68 could be [C₅H₈]⁺ or [¹³C¹²C₄H₇]⁺.

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

Unless otherwise indicated, assume the carbon atoms in cyclohexene are the ¹²C isotope.

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

The parent molecular ion of cyclohexene m/z 82: [C₆H₁₀]⁺

Data table of some of the ions formed in the fragmentation pattern of the mass spectrum of cyclohexene

m/z value of [fragment]⁺	39	40	41	42	50	51	53	54	55
[molecular fragment]⁺	[C₃H₃]⁺	[C₃H₄]⁺	[C₃H₅]⁺	[C₃H₆]⁺	[C₄H₂]⁺	[C₄H₃]⁺	[C₄H₅]⁺	[C₄H₆]⁺	[C₄H₇]⁺

m/z value of [fragment]⁺	65	67	68	77	79	81	82	83
[molecular fragment]⁺	[C₅H₅]⁺	[C₅H₇]⁺	[C₅H₈]⁺	[C₆H₅]⁺	[C₆H₇]⁺	[C₆H₉]⁺	[C₆H₁₀]⁺	[¹³C¹²C₅H₁₀]⁺

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

Atomic masses: H = 1; C = 12 (13 for ~1 in 100)

Bond enthalpies = kJ/mol: C-C = 348; C-H = 412; C=C = 612

Possible equations to explain some of the most abundant ion peaks in the mass spectrum of cyclohexene (tabulated above)

Formation of m/z 81 ion:

[C₆H₁₀]⁺ ===> [C₆H₉]⁺ + H

Loss of a hydrogen atom from the parent molecular ion, mass change 82 - 1 = 81.

The m/z 81 or 82 ions can lose hydrogen atoms/molecule to give the m/z ions of 81 down to 77 (see data table and spectrum).

Formation of m/z 67 ion:

[C₆H₁₀]⁺ ===> [C₅H₇]⁺ + CH₃

Formed by the loss of CH₃ (CH₂ + adjacent H) by ring scission of the parent molecular ion, mass change 82 - 15 = 67.

The m/z 67 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 68 peak could have the same structure as the m/z ion, but be [¹³C¹²C₄H₇] with a carbon-13 atom in it.

Formation of m/z 54 ion

[C₆H₁₀]⁺ ===> [C₄H₆]⁺ + C₂H₄

Perhaps loss of an unionised ethene molecule from the parent molecular ion?, mass change 82 - 28 = 54. This is the 2nd most abundant fragment in the spectrum.

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