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

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

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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

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H-1 proton NMR spectroscopy - spectra index

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 ?

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