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

Advanced Organic Chemistry: Mass spectrum of 1,1-dichloroethane CH₃CHCl₂

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

[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectrometry - analysing the mass spectra of 1,1-dichloroethane [spectra page updated April 3rd 2026 *]

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Introductory note on the mass spectrum of 1,1-dichloroethane

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

If M represents the 1,1-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 1,1-dichloroethane.

I've included a stick diagram and table of m/z ions for the mass spectrum of 1,1-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 1,1-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 1,1-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 1,1-dichloroethane, but the mass spectrometer software does!

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

The molecular structure and naming of haloalkanes

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 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 occur in a ~3:1 ratio).

(2) m/z ion 98 is [C₂H₄³⁵Cl₂]⁺ (M ion peak, most abundant molecular ion)

(2) m/z ion 100 is [C₂H₄³⁵Cl³⁷Cl]⁺ (M+2 ion peak) [CH₃CH³⁵Cl³⁷Cl]⁺ or [CH₃CH³⁷Cl³⁵Cl]⁺

(3) m/z ion 102 [C₂H₄³⁷Cl₂]⁺ (M+4 ion peak)

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

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

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) in terms of peak intensity.

The presence of the two naturally occurring chlorine isotopes makes the interpretation of the mass spectrum of 1,2-dichloroethane a bit more complicated.

BUT, 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 (1,1-dichloroethane) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

m/z ion 63 is [CH₃CH³⁵Cl]⁺ and the base ion peak.

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.

The are three possible parent molecular ions of m/z 98, 100 and 102

From above they are: [C₂H₄³⁵Cl₂]⁺ [C₂H₄³⁵Cl³⁷Cl]⁺ [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 1,1-dichloroethane.

m/z value of [fragment]⁺	85	85	83	65 xref 63
[molecular fragment]⁺	[CH₃³⁵Cl₂]⁺	[CH³⁵Cl³⁷Cl]⁺	[CH³⁵Cl₂]⁺	[CH₃CH³⁷Cl]⁺

m/z value of [fragment]⁺	64 x-ref 62	64 x-ref 63	63	62
[molecular fragment]⁺	[C₂H₃³⁷Cl]⁺	[¹³C¹²CH₄³⁵Cl]⁺	[CH₃CH³⁵Cl]⁺	[C₂H₃³⁵Cl]⁺

m/z value of [fragment]⁺	61	60	47 ?	36	35	27	26	25
[molecular fragment]⁺	[C₂H₂³⁵Cl]⁺	[C₂H³⁵Cl]⁺	[C³⁵Cl]⁺	[H³⁵Cl]⁺	[³⁵Cl]⁺	[C₂H₃]⁺	[C₂H₂]⁺	[C₂H]⁺

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1,1-dichloroethane (because of the chlorine isotopes, there are more possible ions, but only producing tiny peaks e.g. fragments containing the less abundant ³⁷Cl isotope)

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 (~1% 13); 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, 85 and 87 ions:

[CH₃CHCl₂]⁺ ===> [CHCl₂]⁺ + CH₃

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

mass change 98/100 - 15 = 83/85 (M-15 ion peak)

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

There are three possibilities and two are clearly seen on the mass spectrum.

These 'dichloro' ions will have a ratio of 9 : 6 : 1 in abundance. See Mass spectra ion ratios based on isotopes of chlorine and/or bromine in organic halogen compounds

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.

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.

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

Formation of m/z 36 and 35 ions:

Hydrogen chloride from elimination or chlorine atom from C-Cl bond scission can be ionised instead of the carbon based fragment, though low probability judging by the small size of these ion peaks. [H³⁵Cl]⁺ and [³⁵Cl]⁺ respectively.

Formation of m/z 27 ion ?:

[CH₃CHCl]⁺ ===> [C₂H₃]⁺ + HCl

Elimination of hydrogen chloride 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.

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The infrared spectrum of 1,1-dichloroethane

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

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

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