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Advanced Organic Chemistry: Mass spectrum of 1,2-dibromoethane

The mass spectrum of 1,2-dibromoethane

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 1,2-dibromoethane

C2H4Br2 BrCH2CH2Br mass spectrum of 1,2-dibromoethane fragmentation pattern of m/z m/e ions for analysis and identification of 1,2-dibromoethane image diagram doc brown's advanced organic chemistry revision notes

1,2-dibromoethaneC2H4Br2 BrCH2CH2Br2

Interpreting the fragmentation pattern of the mass spectrum of 1,2-dibromoethane

[M]+ are the molecular ion peaks (M) with m/z values of 186, 188 and 190 corresponding to [C2H4Br2]+, the original 1,2-dibromoethane molecule minus an electron.

The three possibilities (and in a 1:2:1 ratio, see below) are:

m/z 186 [79BrCH2CH279Br]+, m/z 188 [79BrCH2CH281Br]+, and m/z 190 [81BrCH2CH281Br]+

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

Their average relative isotopic mass is ~80, so the relative molecular mass for 1,2-dibromoethane is ~188.

 There are four possible bromine permutations in the 1,2-dibromoethane molecule:

79Br79Br, 79Br81Br,  81Br79Br and 81Br81Br (think of a 2 x 2 random probability square).

However, this means any fragment carrying a bromine atom should show up as three peaks, two mass units apart and approximately in a ratio of 1:2:1 in heights (intensities).

The tiny M+1 peaks at m/z 187 and 189, corresponds to an ionised 1,2-dibromoethane molecule with one 13C atom in it i.e. an ionised 1,2-dibromoethane molecule of formula [13C12CH4Br2]+

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.

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

The most abundant ion of the molecule under mass spectrometry investigation (1,2-dibromoethane) is usually given an arbitrary abundance 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 1,2-dibromoethane.

Unless otherwise indicated, assume the carbon atoms in 1,2-dibromoethane are the 12C isotope.

3 m/z values of [M]+ 190 188 186
[molecule M]+ [79BrCH2CH279Br]+ [79BrCH2CH281Br]+ [81BrCH2CH281Br]+
m/z value of [fragment]+ 109 107 95 93
[molecular fragment]+ [C2H481Br]+ [C2H479Br]+ [CH281Br]+ [CH279Br]+
m/z value of [fragment]+ 82 81 80 79 28 27 26
[molecular fragment]+ [H81Br]+ [81Br]+ [H79Br]+ [79Br]+ [C2H4]+ [C2H3]+ [C2H2]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1,2-dibromoethane

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

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

Equations to explain the most abundant ion peaks of 1,2-dibromoethane

The three molecular ions exist due to the permutations of the two bromine isotopes.

Their occurrence and ~ 1:2:1 ratio have already been described above.

Formation of m/z 107 and 109 ions:

[BrCH2CH2Br]+  ===>  [C2H4Br]+  +  Br

C-Br bond scission, two possible fragments [C2H479Br]+ and  [C2H481Br]+ in approximately a 1:1 ratio.

The m/z 107 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The weaker C-Br bond is more likely to be broken than the stronger C-C or C-H bonds.

In fact, and as a consequence, apart from the m/z 27 ion peak, all other ion peaks, formation described below, are of a much lower intensity, but I offer some suggestions of how they may be formed.

Formation of m/z 93 and 95 ions:

[BrCH2CH2Br]+  ===>  [CH2Br]+  +  CH2Br

C-C bond scission of parent molecular ion (C-C bond scission of m/z 107 and 109 ions ?).

Two possible ionised fragments:

[CH279Br]+ and [CH281Br]+ in approximately 1:1 ratio.

Formation of m/z 79 and 81 ions:

[BrCH2CH2Br]+  ===>  [79Br]  or  [81Br]+  +  BrCH2CH2

C-Br bond scission of parent molecular ion (or other bromine containing fragment).

Formation of m/z 80 and 82 ions:

[BrCH2CH2Br]+  ===>  [H79Br]+  or  [H81Br]+  +  C2H3Br

Elimination of hydrogen bromide from parent molecular ion.

Formation of m/z 28 ion:

[C2H4Br]+  ===>  [C2H4]+  +  Br

C-Br bond scission of the m/z 107 or 109 ions to give an ionised ethene molecule.

Formation of m/z 27 ion:

[C2H4Br]+  ===>  [C2H3]+  +  HBr

Elimination of hydrogen bromide from the m/z 107 and 109 ions.


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