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

Interpreting the mass spectrum of 1,2-dichloroethane

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-dichloroethane

mass spectrum of 1,2-dichloroethane C2H4Cl2 CH2ClCH2Cl fragmentation pattern of m/z m/e ions for analysis and identification of 1,2-dichloroethane image diagram doc brown's advanced organic chemistry revision notes 

1,2-dichloroethane,  C2H4Cl2,  ClCH2-CH2Cl

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

[M]+ is the molecular ion peak, theoretically with an m/z values of 98, 100 and 102 corresponding to [C2H4Cl2]+, the original 1,2-dichloroethane molecule minus an electron, [ClCH2CH2Cl]+.

The reason for 3 possible molecular ions is due to the isotopes of chlorine (35Cl and 37Cl in a 3:1 ratio).

(i) M = 98 for C2H435Cl2,  (ii) M = 100 for C2H435Cl37Cl  and (iii) M = 102 for C2H437Cl2

Because of the greater abundance of 35Cl, (i) and (ii) are most likely to be observed (and expect (i) > (ii) and a much lower probability of observing (iii) - and this is what you see in the spectrum, (i) > (ii) >> (iii)

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

Unless otherwise indicated, assume the carbon atoms in 1,2-dichloroethane are the 12C isotope, but chlorine atoms can be 35Cl and 37Cl (3:1 ratio), which you observe in ionised fragments containing a Cl atom.

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

m/z value of [fragment]+ 65 64 63 62 51 49
[molecular fragment]+ [C2H437Cl]+ [C2H337Cl]+ [C2H435Cl]+ [C2H335Cl]+ [37ClCH2]+ [35ClCH2]+
m/z value of [fragment]+ 38 37 36 35 28 27 26 25
[molecular fragment]+ [H37Cl]+ [35Cl]+ [H35Cl]+ [35Cl]+ [C2H4]+ [C2H3]+ [C2H2]+ [C2H]+

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

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

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

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

Formation of m/z 62 and 64 ions:

[ClCH2CH2Cl]+  ===>  [C2H3Cl]+  +  HCl

Elimination of hydrogen chloride from the parent molecular ion.

Here you clearly see the 3:1 ratio of these two ions containing just one chlorine atom.

The m/z 62 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 63 and 65 ions:

[ClCH2CH2Cl]+  ===>  [ClCH2CH2]+  +  Cl

C-Cl bond scission and loss of chlorine radical from the parent molecular ion.

Here you clearly see the 3:1 ratio of these two ions containing just one chlorine atom.

Formation of m/z 49 and 51 ions:

[ClCH2CH2Cl]+  ===>  [ClCH2]+  +  CH2Cl

C-C bond scission of the parent molecular ion.

Here you clearly see the 3:1 abundance ratio of these two ions containing just one chlorine atom.

Formation of m/z 35 to 38 ions:

These correspond to the ionised chlorine atoms (m/z 35 and 37) and ionised hydrogen chloride molecules (m/z 36 and 38).

Note the expected 3:1 ratio for the relative abundance of m/z 35 and 37 ions AND similarly for the relative abundance of 36 and 38 ions.

Formation of m/z 27 ion 63 and 65 ions ?:

[ClCH2CH2]+  ===>  [C2H3]+  +  HCl

Elimination of HCl from the m/z ? and ? ions ?


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