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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.

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.

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

Mass spectroscopy index

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