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

Interpreting the mass spectrum of 1,1-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 1,1-dichloroethane

mass spectrum of 1,1-dichloroethane C2H4Cl2 CH3CHCl2 fragmentation pattern of m/z m/e ions for analysis and identification of 1,1-dichloroethane image diagram doc brown's advanced organic chemistry revision notes 

1,1-dichloroethane,  C2H4Cl2,  CH3-CHCl2

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

[M]+ is the molecular ion peak, theoretically with an m/z values of of 98, 100 and 102 corresponding to [C2H4Cl2]+, the original 1,1-dichloroethane molecule minus an electron, [CH3CHCl2]+.

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 (1,1-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 1,1-dichloroethane.

Unless otherwise indicated, assume the carbon atoms in 1,1-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 1,1-dichloroethane.

m/z value of [fragment]+ 85 83 65 63 61
[molecular fragment]+ [CH35Cl37Cl]+ [CH35Cl2]+ [CH3CH37Cl]+ [CH3CH35Cl]+ [C2H235Cl]+
m/z value of [fragment]+ 47 ? 36 35 27 26 25
[molecular fragment]+ [C35Cl]+ [H35Cl]+ [35Cl]+ [C2H3]+ [C2H2]+ [C2H]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1,1-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 1,1-dichloroethane (tabulated above)

Formation of m/z 83 and 85 ions:

[CH3CHCl2]+  ===>  [CHCl2]+  +  CH3

C-C bond scission, loss of methyl group from parent molecular ion.

They occur in a similar ratio to the molecular ion peaks of m/z 98 and 100.

Formation of m/z 63 and 65 ions:

[CH3CHCl2]+  ===>  [CH3CHCl]+  +  Cl

C-Cl bond scission and loss of chlorine radical.

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

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

These two ions can lose hydrogen atoms to give lower m/z values e.g. 60-62 and 64.

Formation of m/z 27 ion ?:

[CH3CHCl]+  ===>  [C2H3]+  +  HCl

Elimination of hydrogen chloride from the m/z 63 and 65 ions ?


Key words & phrases: C2H4Cl2 CH3CHCl2 Cl2CHCH3 CHCl2CH3 image diagram on how to interpret and explain the mass spectrum of 1,1-dichloroethane m/z m/e base peaks, image and diagram of the mass spectrum of 1,1-dichloroethane, details of the mass spectroscopy of 1,1-dichloroethane,  low and high resolution mass spectrum of 1,1-dichloroethane, prominent m/z peaks in the mass spectrum of 1,1-dichloroethane, comparative mass spectra of 1,1-dichloroethane, the molecular ion peak in the mass spectrum of 1,1-dichloroethane, analysing and understanding the fragmentation pattern of the mass spectrum of 1,1-dichloroethane, characteristic pattern of peaks in the mass spectrum of 1,1-dichloroethane, relative abundance of mass ion peaks in the mass spectrum of 1,1-dichloroethane, revising the mass spectrum of 1,1-dichloroethane, revision of mass spectroscopy of 1,1-dichloroethane, most abundant ions in the mass spectrum of 1,1-dichloroethane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of 1,1-dichloroethane, how to analyse the mass spectrum of 1,1-dichloroethane, how to describe explain the formation of fragmented ions in the mass spectra of 1,1-dichloroethane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of 1,1-dichloroethane recognising the base ion peak of 1,1-dichloroethane interpreting interpretation the mass spectrum of 1,1-dichloroethane halide chloride chloro functional group


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