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Advanced Organic Chemistry: Mass spectrum of bromoethane

The mass spectrum of bromoethane

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 bromoethane

C2H5Br CH3CH2Br mass spectrum of bromoethane fragmentation pattern of m/z m/e ions for analysis and identification of bromoethane ethyl bromide image diagram doc brown's advanced organic chemistry revision notes 

Bromoethane  (c) doc b  (c) doc b  (c) doc b  (c) doc b  (c) doc b 

Interpreting the fragmentation pattern of the mass spectrum of bromoethane

[M]+ is the molecular ion peak (M) with an m/z of 108 and 110 corresponding to [C2H579Br]+ and [C2H579Br]+ the original bromoethane molecule minus an electron, [CH3CH2Br]+.

There are two molecular ion peaks because bromine as two isotopes, 50.5% 79Br and 50.5% 81Br.

Their average relative mass is ~80, so the relative molecular mass for bromoethane is ~109.

However, this means any fragment carrying a bromine atom should show up as twin peaks, two mass units apart and approximately of equal height (intensities).

The small M+1 and M+2 peaks at m/z 109 and 111, corresponds to an ionised bromoethane molecule with one 13C atom in it i.e. an ionised bromoethane molecule of formula [13C12CH579Br]+ and [13C12CH581Br]+.

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 these 13C peaks.

Bromoethane has 2 carbon atoms, so on average, ~1 in 50 molecules will contain a 13C atom.

They seem to show up clearly because the parent molecular ions (m/z 108 and 110) are relatively stable, their intensities are second only to the base ion peak of m/z 29.

The most abundant ion of the molecule under mass spectrometry investigation (bromoethane) 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 bromoethane.

Unless otherwise indicated, assume the carbon atoms in the bromoethane molecule are the 12C isotope.

m/z value of [fragment]+ 111 110 109 108 95 93
[molecular fragment]+ [13C12CH581Br]+ [C2H581Br]+ [13C12CH579Br]+ [C2H579Br]+ [CH281Br]+ [CH279Br]+
m/z value of [fragment]+ 81 79 29 28 27 26 25
[molecular fragment]+ [81Br]+ [79Br]+ [C2H5]+ [C2H4]+ [C2H3]+ [C2H2]+ [C2H]+

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

Atomic masses: C = 12;  H = 1; Br = 79 or 81

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-Br = 276;

Equations to explain the most abundant ion peaks of bromoethane

Formation of m/z 93 and 95 ions:

[CH3CH2Br]+  ===>  [CH2Br]+  +  CH3

C-C bond scission with mass loss of methyl group, 108 - 15 = 93, 110 - 15 = 95

Formation of m/z 79 and 81 ion:

[CH3CH2Br]+  ===>  [79Br]+  or  [81Br]  +  CH2CH3

C-Br bond scission with mass loss of ethyl group, 108 - 29 = 79, 110 - 29 = 81

Formation of m/z 29 ion:

[CH3CH2Br]+  ===>  [CH3CH2]+  +  Br

C-Br bond scission and the ethyl fragment carries the positive charge.

This is breaking the weakest bond in the bromoethane molecule.

The m/z 29 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The ethyl cation loses successive protons to generate m/z ions of masses 28 => 27 => 26 => 25.


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