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Advanced Organic Chemistry: Mass spectrum of octane

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

C8H18 mass spectrum of octane fragmentation pattern of m/z m/e ions for analysis and identification of n-octane image diagram doc brown's advanced organic chemistry revision notes 

Octane, C8H18, CH3(CH2)6CH3, CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3, alkane

Interpreting the fragmentation pattern of the mass spectrum of octane

[M]+ is the molecular ion peak (M) with an m/z of 114 corresponding to [C6H13]+, the original octane molecule minus an electron, [CH3(CH2)6CH3]+.

The tiny M+1 peak at m/z 115, corresponds to an ionised octane molecule with one 13C atom in it i.e. an ionised octane molecule of formula 13C12C6H18

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.

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

m/z value of [fragment]+ 85 84 71 70 57 56 55
[molecular fragment]+ [C6H13]+ [C6H12]+ [C5H11]+ [C5H10]+ [C4H9]+ [C4H8]+ [C4H7]+
m/z value of [fragment]+ 43 42 41 39 29 27 -
[molecular fragment]+ [C3H7]+ [C3H6]+ [C3H5]+ [C3H3]+ [C2H5]+ [C2H3]+ -

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).


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