Advanced Organic Chemistry: Mass spectrum of 2-methylbut-1-ene

The mass spectrum of 2-methylbut-1-ene

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-methylbut-1-ene

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Mass spectroscopy - spectra index

mass spectrum of 2-methylbut-1-ene fragmentation pattern of m/z m/e ions for analysis and identification of 2-methylbut-1-ene image diagram 2-methyl-1-butene doc brown's advanced organic chemistry revision notes 

2-methylbut-1-ene C5H10, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of 2-methylbut-1-ene

[M]+ is the molecular ion peak (M) with an m/z of 70 corresponding to [C5H10]+, the original 2-methylbut-1-ene molecule minus an electron, [H2C=C(CH3)CH2CH3]+.

The small M+1 peak at m/z 71, corresponds to an ionised 2-methylbut-1-ene molecule with one 13C atom in it i.e. an ionised 2-methylbut-1-ene molecule of formula 13C12C4H10

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2-methylbut-1-ene.

m/z value of [fragment]+ 56 55 53 42
[molecular fragment]+ [C4H8]+ [H2C=C(CH3)CH2]+ [C4H5]+ [C3H6]+
m/z value of [fragment]+ 41 39 29 27
[molecular fragment]+ [C3H5]+ [C3H3]+ [CH3CH2]+ [C2H3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-methylbut-1-ene

Equations to explain the most abundant ion peaks of 2-methylbut-1-ene

Formation of m/z 55 ion: [H2C=C(CH3)CH2CH3]+  ===>  [H2C=C(CH3)CH2]+  +  CH3

The m/z 55 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formed by C-C bond scission and loss of methyl group (70 - 15 = 55).

Formation of m/z 41 ion: [H2C=C(CH3)CH2CH3]+  ===>  [C3H5]+  +  CH2CH3

Formed by C-C bond scission and loss of non-ionised ethyl group (70 - 29 = 41).

Formation of m/z 29 ion: [H2C=C(CH3)CH2CH3]+  ===>  [CH2CH3]+  +  C3H5

Formed by C-C bond scission and loss of ionised ethyl group (70 - 41 = 29).

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