The mass spectrum of octane

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 octane

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

Equations to explain the most abundant ion peaks of octane

Formation of m/z 85 ion: [CH₃(CH₂)₆CH₃]⁺  ===>  [CH₃(CH₂)₅]⁺  +  CH₂CH₃

C-C bond scission, molecular ion loses ethyl group, 114 - 29 = 85

Subsequent proton loss gives the m/z 84 ion.

Formation of m/z 71 ion: [CH₃(CH₂)₆CH₃]⁺  ===>  [CH₃(CH₂)₄]⁺  +  CH₂CH₂CH₃

or from other fragments e.g.: [CH₃(CH₂)₅]⁺  ===>  [CH₃(CH₂)₄]⁺  +  CH₂

Formation of m/z 57 ion: [CH₃(CH₂)₆CH₃]⁺  ===>  [CH₃(CH₂)₃]⁺  +  CH₂CH₂CH₂CH₃

C-C bond scission splitting the molecule in half, 114 - 57 = 57.

Formation of m/z 43 ion: [CH₃(CH₂)₆CH₃]⁺  ===>  [CH₃CH₂CH₂]⁺  +  C₅H₁₁

or from other fragments e.g.: [CH₃(CH₂)₄]⁺  ===>  [CH₃CH₂CH₂]⁺  +  CH₂=CH₂

The m/z 43 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 29 ion: [CH₃(CH₂)₆CH₃]⁺  ===>  [CH₂CH₃]⁺  +  CH₃(CH₂)₅

or from other fragments e.g.:  [CH₃(CH₂)₅]⁺  ===>  [CH₂CH₃]⁺  +  C₄H₈

All involving C-C bond scission of the alkyl chain.

Note:

(i) For alkanes that you often get a series of peaks separated by 14 mass units, due to successive loss of a CH₂ grouping.

(ii) You also get ionised fragments formed by the loss of an alkene molecule e.g. C₂H₄, C₃H₆ and C₄H₈.

(iii) With alkane molecules the largest fragment in each group of peaks is usually formed by C-C chain bond scission, rather than proto loss (bond enthalpies: C-C 336 kJ/mol and C-H 416 kJ/mol).