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Interpreting the fragmentation pattern of the mass spectrum of 1-bromo-2-chloroethane

[M]⁺ is the molecular ion peak (M) with m/z values of 142, 144 and 146 corresponding to [C₂H₄BrCl]⁺, the original 1-bromo-2-chloroethane molecule minus an electron, [BrCH₂CH₂Cl]⁺.

This is quite a tricky mass spectrum to interpret because of the two sets of isotopes of bromine and chlorine.

Since chlorine has two common isotopes of ³⁵Cl and ³⁷Cl in the ratio 3 : 1, you should observe double peaks in the intensity ratio 3 : 1, two mass units apart for molecular fragments containing a chlorine atom from the fragmentation of 1-chlorobutane (if the fragment has no bromine atom).

BUT, this is complicated by the isotopes of bromine!

Bromine as two isotopes, 50.5% ⁷⁹Br and 50.5% ⁸¹Br, which any fragment carrying a bromine atom should show up as twin peaks of intensity ratio ~1:1, two mass units apart (if the fragment has no chlorine atom).

So, what you observe in terms of abundances, is the result of two ratios, and therefore I'm keeping the interpretation as simple as I can (at pre-university level!).

However, you should be able to work out what the four possible molecular ions consist of:

m/z 142 [⁷⁹BrCH₂CH₂³⁵Cl]⁺, m/z 144 [⁷⁹BrCH₂CH₂³⁷Cl]⁺, m/z 144 [⁸¹BrCH₂CH₂³⁵Cl]⁺ and m/z 146[⁸¹BrCH₂CH₂³⁷Cl]⁺, and in terms of probability the ratio would be: 3:1:3:1,

There is one Br and one Cl in each molecule of 1-bromo-2chloroethane.

Cross 79 and 81 with 35, 35, 35 and 37 in a 2 x 4 probability chart to get the above ratios.

The most abundant ion of the molecule under mass spectrometry investigation (1-bromo-2-chloroethane) 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-bromo-2-chloroethane.

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

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1-bromo-2-chloroethane

Atomic masses: H = 1; C = 12; Cl = 35 or 37 (3:1);  Br = 79 or 81 (1:1);

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412; C-Cl = 338; C-Br 276

Equations to explain the most abundant ion peaks of 1-bromo-2-chloroethane

Formation of m/z 107 and 109 ions:

[BrCH₂CH₂Cl]⁺  ===>  [⁷⁹BrCH₂CH₂]⁺  or  [⁸¹BrCH₂CH₂]⁺  +  Cl

Scission of C-Cl bond, but weaker C-Br bond more likely to be broken (see below)

Formation of m/z 93 and 95 ions:

[BrCH₂CH₂Cl]⁺  ===>  [CH₂⁷⁹Br]⁺]⁺  or  [CH₂⁸¹Br]⁺]⁺  +  CH₂Cl

Scission of C-C bond, strong bond, low probability.

Formation of m/z 63 and 65 ions:

[BrCH₂CH₂Cl]⁺  ===>  [CH₂CH₂³⁵Cl]⁺  or  [CH₂CH₂³⁷Cl]⁺   +  Br

Scission of the C-Br bond, the weakest bond in the molecule.

The m/z 63 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 49 ion:

[BrCH₂CH₂Cl]⁺  ===>  [CH₂³⁵Cl]⁺  +  BrCH₂

Scission of C-C bond, strong bond, low probability.

Small peak at m/z 51 corresponding to  [CH₂³⁷Cl]⁺

Formation of m/z 28 ion:

[C₂H₄Cl]⁺  ===>  [C₂H₄]⁺  +  Cl

[C₂H₄Br]⁺  ===>  [C₂H₄]⁺  +  Br

C-Br bond scission of the m/z 63/65 or 107/109 pairs of ions to give an ionised ethene molecule.

Formation of m/z 27 ion:

[C₂H₄Cl]⁺  ===>  [C₂H₄]⁺  +  Cl

[C₂H₄Br]⁺  ===>  [C₂H₃]⁺  +  HBr

Elimination of hydrogen chloride or hydrogen bromide from the m/z 63/65 or 107/109 ions.

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