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Advanced Organic Chemistry: Mass spectrum of 2,2-dimethylbutane

The mass spectrum of 2,2-dimethylbutane

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 2,2-dimethylbutane

mass spectrum of 2,2-dimethylbutane fragmentation pattern of m/z m/e ions for analysis and identification of 2,2-dimethylbutane image diagram doc brown's advanced organic chemistry revision notes 

 2,2-dimethylbutane C6H14, alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of 2,2-dimethylbutane

[M]+ is the molecular ion peak (M) with an m/z of 86 corresponding to [C6H14]+, the original 2,2-dimethylbutane molecule minus an electron, [(CH3)3CCH2CH3]+, BUT it doesn't seem to show up!

The minute M+1 peak at m/z 87, corresponds to an ionised 2,3-dimethylbutane molecule with one 13C atom in it i.e. an ionised 2,2-dimethylbutane molecule of formula 13C12C5H14, BUT this doesn't seem to show up either!

The molecular ion of 2,2-dimethylbutane must be very unstable.

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2,2-dimethylbutane.

m/z value of [fragment]+ 72 71 70 58 57 56 55
[molecular fragment]+ [C5H12]+ [C5H11]+ [C5H10]+ [C4H10]+ [(CH3)3C]+ [C4H8]+ [C4H9]+
m/z value of [fragment]+ 43 42 41 39 29 27
[molecular fragment]+ [(CH3)2CH]+ [C3H6]+ [C3H5]+ [C3H3]+ [CH3CH2]+ [C2H5]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2,2-dimethylbutane

Equations to explain the most abundant ion peaks of 2,2-dimethylbutane

Formation of m/z 71 ion: [(CH3)3CCH2CH3]+  ===>  [C5H11]+  +  CH3

 C-C bond chain scission, methyl fragment lost from parent molecular ion (86 - 15 = 71)

Several possibilities for structure of m/z ion e.g. [CH3)3CCH2]+ or [(CH3)2CCH2CH3]+.

Formation of m/z 57 ion: [(CH3)3CCH2CH3]+  ===>  [(CH3)3C]+  +  CH2CH3

 C-C bond chain scission, ethyl fragment lost.

The 2nd most abundant ion, a relatively stable tertiary carbocation

Formation of m/z 43 ion: [(CH3)3CCH2CH3]+  ===>  [(CH3)2CH]+  +  C3H

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

Formation of m/z 29 ion: [(CH3)3CCH2CH3]+  ===>  [CH2CH3]+  +  (CH3)3C

 C-C bond chain scission, ethyl fragment ionised.

m/z sequences like 43, 42, 41 or 29, 28, 27, indicate successive proton loss.


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