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Advanced Organic Chemistry: Mass spectrum of 2,2,3-trimethylbutane

The mass spectrum of 2,2,3-trimethylbutane

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 2,2,3-trimethylbutane

C7H16 mass spectrum of 2,2,3-trimethylbutane fragmentation pattern of m/z m/e ions for analysis and identification of 2,2,3-trimethylbutane image diagram doc brown's advanced organic chemistry revision notes 

2,2,3-trimethylbutane  C7H16 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 2,2,3-trimethylbutane

[M]+ is the molecular ion peak (M) with an m/z of 100 corresponding to [C7H16]+, the original 2,2,3-trimethylbutane molecule minus an electron, [(CH3)3CCH(CH3)2]+.

The molecular ion peak of 2,2,3-trimethylbutane is very small, suggesting it is a very unstable ion.

You might detect an M+1 peak at m/z 100, corresponds to an ionised 2,2,3-trimethylbutane molecule with one 13C atom in it i.e. an ionised 2,2,3-trimethylbutane molecule of formula [13C12C6H16]+

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.

2,2,3-trimethylbutane has 7 carbon atoms, so on average, ~1 in 14 molecules will contain a 13C atom.

You are highly unlikely to detect this ion, since the molecular ion is so unstable.

The most abundant ion of the molecule under mass spectrometry investigation (2,2,3-trimethylbutane) is usually given an arbitrary 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 2,2,3-trimethylbutane.

m/z value of [fragment]+ 86 85 69 58 57 56
[molecular fragment]+ [C6H16]+ [C6H13]+ [C5H9]+ [C4H10]+ [C4H9]+ [C4H8]+
m/z value of [fragment]+ 55 43 41 39 29 27
[molecular fragment]+ [C4H7]+ [C3H7]+ [C3H5]+ [C3H3]+ [C2H5]+ [C2H3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2,2,3-trimethylbutane

Atomic masses: C = 12  H = 1

Equations to explain the most abundant ion peaks of 2,2,3-trimethylbutane

Formation of m/z 85 ion:

[(CH3)3CCH(CH3)2]+  ===>  [C6H13]+  +  CH3

C-C bond scission to release a methyl group from the parent molecular ion.

Ion mass change: 100 - 15 = 85

Formation of m/z 69 ion:

[?]+  ===>  [C5H9]+  +  ?

85 - 28

Formation of m/z 57 ion:

[(CH3)3CCH(CH3)2]+  ===>  [(CH3)3C]+  +  C3H7

The m/z 57 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Ion mass change: 100 - 43 = 57

also 85 - 28 = 57

Formation of m/z 56 ion:

[?]+  ===>  [C4H8]+  +  ?

Formation of m/z 43 ion:

[(CH3)3CCH(CH3)2]+  ===>  [CH(CH3)2]+  +  (CH3)3C

Ion mass change: 100 - 57 = 43

Note this is the same C-C bond fission as the m/z 57 ion formation, but the other fragment is ionised.

Formation of m/z 41 ion:

[?]+  ===>  [C3H5]+  +  ?


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