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

[M]⁺ is the molecular ion peak (M) with an m/z of 132, 134 or 136 (and theoretically 138) corresponding to [C₂H₃Cl₃]⁺, the original 1,1,2-trichloroethane molecule minus an electron, [Cl₂CHCH₂Cl]⁺, depending on the chlorine isotopic composition of the 1,1,2-trichloroethane molecule.

Since chlorine consists of two isotopes, ³⁵Cl and ³⁷Cl (in a 3:1 ratio), there are four possibilities for the 'isotopic' molecular formula of 1,1,2-trichloroethane molecule, therefore theoretically four possible molecular ions.

(i) M = 132 for C₂H₃³⁵Cl₃,  (ii) M = 134 for C₂H₃³⁵Cl₂³⁷Cl,  (iii) M = 136 for C₂H₃³⁵Cl³⁷Cl₂

and  (iv) M = 138 for C₂H₃³⁷Cl₃., the increments of two are due the mass difference between ³⁵Cl and ³⁷Cl.

Because ³⁵Cl is more abundant, ions of (i) and (ii) are the most likely to be observed.

There is a smaller chance of the m/z ion 136 and a very low probability of the m/z 138 ion.

This is what you observe in the spectrum, the abundances being (i) ~ (ii) > (iii) >> (iv).

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

Unless otherwise indicated, assume the carbon atoms in 1,1,2-trichloroethane are the ¹²C isotope, but the chlorine atoms maybe ³⁵Cl or ³⁷Cl and this makes the mass spectrum even more complex to interpret.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of 1,1,2-trichloroethane.

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

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

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

Possible equations to explain the most abundant ion peaks of 1,1,2-trichloroethane (tabulated above)

Formation of m/z 97, 99 and 101 ions:

[Cl₂CHCH₂Cl]⁺  ===>  [C₂H₃Cl₂]⁺  +  Cl

C-Cl bond scission (weakest bond), loss of chlorine radical from the parent molecular ion.

The m/z 97 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Alternatively, but much less probable, the Cl is ionised to give the m/z 35 and 37 ions.

[Cl₂CHCH₂Cl]⁺  ===>  [Cl]⁺  +  C₂H₃Cl₂

You can see roughly a 3:1 ratio for the m/z 35 and 37 ions.

Formation of m/z 96, 98 and 100 ions:

[Cl₂CHCH₂Cl]⁺  ===>  [C₂H₂Cl₂]⁺  +  HCl

Elimination of hydrogen chloride from the parent molecular ion.

Possible structures of ion [CHCl=CHCl]⁺ or [Cl₂C=CH₂]⁺.

Alternatively, but very much less probable, the HCl is ionised to give the m/z 36 and 38 ions.

You can see roughly a 3:1 ratio for the m/z 36 and 38 ions.

Formation of m/z 83, 85 and 87 ions:

[Cl₂CHCH₂Cl]⁺  ===>  [CHCl₂]⁺  +  CH₂Cl

C-C bond scission of the parent molecular ion of 1,1,2-trichloroethane.

Formation of m/z 62 and 64 ions ?:

[Cl₂CHCH₂Cl]⁺  ===>  [C₂H₃Cl]⁺  +  Cl₂

Elimination of chlorine molecule from the parent molecular ion ?

Structure of ion ? [CH₂=CHCl]⁺

However, more likely to be formed by the loss of a Cl radical from the m/z 97, 99 and 101 ions ...

[C₂H₃Cl₂]⁺  ===>  [C₂H₃Cl]⁺  +  Cl

... essentially a 2 stage process from the parent molecular ion.

Formation of m/z 61 and 63 ions:

[C₂H₃Cl₂]⁺  ===>  [C₂H₂Cl]⁺  +  HCl

Elimination of hydrogen chloride from the m/z 97 and 99 ions.

Formation of m/z 49 and 51 ions:

[Cl₂CHCH₂Cl]⁺  ===>  [CH₂Cl]⁺  +  CHCl₂

C-C bond scission.

You can see these ions are roughly in a 3:1 ratio.

Note this alternative ionisation compared to the formation of the m/z 83, 85 and 87 ions.

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