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Students and teachers please note my explanation of the mass spectrum of 1,2-dichloroethane is designed for advanced, but pre-university, chemistry courses.

If M represents the 1,2-dichloroethane molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and fragmentation equations assume [M]⁺ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]⁺ ==> [X]⁺  +  R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions that might be formed in the fragmentation pattern for the mass spectrum of 1,2-dichloroethane and only the formation of singly charged positive are considered for the mass spectrum of 1,2-dichloroethane.

I've included a stick diagram and table of m/z ions for the mass spectrum of 1,2-dichloroethane and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including 1,2-dichloroethane.

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated and compared the accurate ion masses if appropriate for 1,2-dichloroethane. BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this for 1,2-dichloroethane, but the mass spectrometer software does!

1,2-dichloroethane,  C₂H₄Cl₂,  ClCH₂-CH₂Cl

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

The molecular structure and naming of haloalkanes

[M]⁺ is the molecular ion peak M, theoretically with 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 isotopic ratio).

(1) M = m/z 98 for [C₂H₄³⁵Cl₂]⁺

(2) M+2 peak = m/z 100 for [C₂H₄³⁵Cl³⁷Cl]⁺ [³⁵ClCH₂CH₂³⁷Cl]⁺  or  [³⁷ClCH₂CH₂³⁵Cl]⁺

(3) M+4 = m/z 102 for [C₂H₄³⁷Cl₂]⁺

(1)-(3) apply to ions containing 2Cl atoms.

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)

These will occur in the ratio of 9 : 6 : 1 from 16 permutations. See Mass spectra m/z ion ratios based on isotopes of chlorine and/or bromine in organic halogen compounds

You can see the 9 : 6 : 1 ratio on the mass spectrum diagram above.

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.

The base ion peak for the mass spectrum of 1,2-dichloroethane is m/z 98 [C₂H₄³⁵Cl₂]⁺

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, which makes the mass spectrum of 1,2-dichloroethane a bit more complicated..

There are three possible parent molecular ions

m/z 98 [C₂H₄³⁵Cl₂]⁺, m/z [C₂H₄³⁵Cl³⁷Cl]⁺  and m/z 102 [C₂H₄³⁷Cl₂]⁺

The most abundant (tallest peak intensity) is for m/z 98, so it is designated the 'M' ion peak.

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

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

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

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.

mass change 98/100 = 36/38 = 62/64

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.

Where R is alkyl, the double RCl m/z ion peaks of roughly 3 : 1 abundance ratio are characteristic of organo-chlorine compounds i.e. caused by the 3 : 1 isotope ratio of ³⁵Cl : ³⁷Cl.

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.

Again, you can 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) which are .produced from elimination of hydrogen chloride or C-Cl bond scission to give ionised chlorine atoms.

 [³⁵Cl]⁺ & [³⁷Cl]⁺  and  [H³⁵Cl]⁺ & [H³⁷Cl]⁺ respectively.

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:

[ClCH₂CH₂]⁺  ===>  [C₂H₃]⁺  +  HCl

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

The m/z 27 ion can lose protons to give the m/z 26 and 25 ions.

The infrared spectrum of 1,2-dichloroethane

The H-1 NMR spectrum of 1,2-dichloroethane

The C-13 NMR spectrum of 1,2-dichloroethane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

Mass spectrometry index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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