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Interpreting
and explaining the mass
spectrum of 1,1,2-trichloroethane
[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 1,1,2-trichloroethane
[spectra page updated
April 3rd 2026 *]
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Cl2CHCH2Cl
or CH2ClCHCl2
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Mass spectrometry - spectra index
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Introductory note on the mass spectrum of 1,1,2-trichloroethane
Students and teachers please note
my explanation of the mass spectrum of 1,1,2-trichloroethane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
1,1,2-trichloroethane 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 1,1,2-trichloroethane and only the formation of singly charged
positive are considered for the mass spectrum of
1,1,2-trichloroethane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
1,1,2-trichloroethane
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
1,1,2-trichloroethane.
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 1,1,2-trichloroethane. 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
1,1,2-trichloroethane,
but the mass spectrometer software does!
1,1,2-trichloroethane Cl2CHCH2Cl
or CHCl2CH2Cl
The molecular structure and naming of haloalkanes
Interpreting the fragmentation pattern of the mass spectrum of
1,1,2-trichloroethane
[M]+ is the molecular ion peak (M) with an m/z of
132, 134 or 136 (and theoretically 138) corresponding to
[C2H3Cl3]+, the original 1,1,2-trichloroethane molecule minus an electron,
[Cl2CHCH2Cl]+,
depending on the chlorine isotopic composition of the 1,1,2-trichloroethane
molecule.
Since chlorine consists of two isotopes, 35Cl
and 37Cl (in a 3:1 ratio), there are four possibilities
for the 'isotopic' molecular formula of 1,1,2-trichloroethane
molecule, therefore theoretically four possible molecular ions.
(i) M = m/z ion 132 for
C2H335Cl3
(designate M, the most abundant parent molecular ion)
(ii) M+2 = ion m/z 134 for
C2H335Cl237Cl,
(iii) M+4 = ion m/z 136 for
C2H335Cl37Cl2
(iv) M+6 =
ion m/z 138 for C2H337Cl3
(lowest isotope combination probability in molecular ion)
the increments of two are due the mass difference between
35Cl and 37Cl.
Because 35Cl is more abundant, ions
of (i) and (ii) are the most likely to be observed.
There is a smaller chance of the m/z ion 136 and
a very low probability of the m/z 138 ion.
This is what you observe in the spectrum, the
abundances being (i) > (ii) >> (iii) >> (iv).
The most abundant ion of the molecule under mass
spectrometry investigation (1,1,2-trichloroethane) 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 1,1,2-trichloroethane is m/z 97
[C2H335Cl2]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 1,1,2-trichloroethane.
Unless otherwise indicated, assume the carbon atoms in
1,1,2-trichloroethane are the 12C isotope, but the chlorine
atoms maybe 35Cl or 37Cl and this makes the mass
spectrum even more complex to interpret.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of 1,1,2-trichloroethane.
|
m/z value of
[fragment]+ |
101 |
100 |
99 |
98 |
97 |
|
[molecular fragment]+ |
[C2H337Cl2]+ |
[C2H237Cl2]+ |
[C2H335Cl37Cl]+ |
[C2H235Cl37Cl]+ |
[C2H335Cl2]+ |
|
m/z value of
[fragment]+ |
96 |
95 |
87 |
85 |
83 |
|
[molecular fragment]+ |
[C2H235Cl2]+ |
[C2H335Cl2]+ |
[CH37Cl2]+ |
[CH35Cl37Cl]+ |
[CH35Cl2]+ |
|
m/z value of
[fragment]+ |
64 |
63 |
62 x-ref 64 |
61
x-ref 63 |
60 |
|
[molecular fragment]+ |
[C2H337Cl]+ |
[C2H237Cl]+ |
[C2H335Cl]+ |
[C2H235Cl]+ |
[C2H35Cl]+ |
|
m/z value of
[fragment]+ |
51 |
49 |
47 |
36 |
35 |
27 |
26 |
25 |
|
[molecular fragment]+ |
[CH237Cl]+ |
[CH235Cl]+ |
[C35Cl]+
|
[H35Cl]+ |
[35Cl]+ |
[C2H3]+ |
[C2H2]+ |
[C2H]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 1,1,2-trichloroethane
This is a very complex
pattern because of three chlorine atoms and their two isotopes, so I've
picked out a few 'obvious' patterns for the mass spectrum of
1,1,2-trichloroethane!
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)
Bond enthalpies = kJ/mol: C-C = 348; C-H = 412;
C-Cl = 338
Possible
suggested equations to explain the most abundant ion peaks of 1,1,2-trichloroethane
(tabulated above)
Formation of m/z 97, 99 and 101 ions
(see m/z ion table above):
[Cl2CHCH2Cl]+ ===> [C2H3Cl2]+
+ Cl
C-Cl bond scission (weakest bond), loss of chlorine
radical from the parent molecular ion.
The m/z 97 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
The increments of 2
are a common pattern in the ion m/z because of 35Cl and
37Cl.
Alternatively, but much less probable, the Cl is
ionised to give the m/z 35 and 37 ions.
[Cl2CHCH2Cl]+ ===> [Cl]+
+ C2H3Cl2
You can see roughly a 3:1 ratio for the m/z 35
and 37 ions.
Formation of m/z 96, 98 and 100 ions
(see m/z ion table above):
[Cl2CHCH2Cl]+ ===> [C2H2Cl2]+
+ HCl
Elimination of hydrogen chloride from the parent
molecular ion.
Possible structures of ion [CHCl=CHCl]+
or [Cl2C=CH2]+.
Alternatively, but very much less probable, the HCl
is ionised to give the m/z 36 and 38 ions.
You can see roughly a 3:1 ratio for the m/z 36
and 38 ions.
Formation of m/z 83, 85 and 87 ions
(see m/z ion table above):
[Cl2CHCH2Cl]+ ===> [CHCl2]+
+ CH2Cl
C-C bond scission of the parent molecular ion of
1,1,2-trichloroethane.
Formation of m/z 62 and 64 ions
(see m/z ion table above):
[Cl2CHCH2Cl]+ ===> [C2H3Cl]+
+ Cl2
Elimination of chlorine molecule from the parent
molecular ion by two C-Cl bond scissions?
Structure of ion ? [CH2=CHCl]+
You can see roughly a 3:1 ratio for the m/z 62 and
64 ions.
However, more likely to be formed by the loss of a
Cl radical from the m/z 97, 99 and 101 ions? ...
[C2H3Cl2]+ ===> [C2H3Cl]+
+ Cl
... essentially a 2 stage process from the
parent molecular ion.
Formation of m/z 61 and 63 ions
(see m/z ion table above):
[C2H3Cl2]+ ===> [C2H2Cl]+
+ HCl
Elimination of hydrogen chloride from the m/z 97 and
99 ions.
You can see roughly a 3:1 ratio for the m/z 61 and
63 ions.
Formation of m/z 49 and 51 ions
(see m/z ion table above):
[Cl2CHCH2Cl]+ ===> [CH2Cl]+
+ CHCl2
C-C bond scission of
the parent molecular ion.
You can see these ions are roughly in a 3:1 ratio.
Note this alternative ionisation compared to the
formation of the m/z 83, 85 and 87 ions.
Formation of m/z 35 to
38 ions (see m/z ion table
above):
Whenever a chlorine atom (Cl) or hydrogen chloride
molecule (HCl) are generated in the fragmentation process of
1,1,2-trichloroethane, there is a low, but real, probability of them
becoming ionised.
Formation of m/z 25 to
27 ions (see m/z ion table
above):
Some of the fragments must lose all their chlorine
atoms, but retain the three hydrogen atoms to give
[C2H3]+, which can then lose
protons to give
[C2H2]+ and
[C2H2]+..
I can't figure out what the m/z 28 ion is, since
there are only three hydrogen atoms in the original parent molecule!
Key words & phrases: C2H3Cl3 Cl2CHCH2Cl CHCl2CH2Cl
ClCH2CHCl2 CH2ClCHCl2 image diagram on how to interpret and explain the mass spectrum of
1,1,2-trichloroethane m/z m/e base peaks, image and diagram of the mass spectrum of
1,1,2-trichloroethane, details of the mass spectroscopy of 1,1,2-trichloroethane, low and high resolution mass
spectrum of 1,1,2-trichloroethane, prominent m/z peaks in the mass spectrum of
1,1,2-trichloroethane, comparative
mass spectra of 1,1,2-trichloroethane, the molecular ion peak in the mass spectrum of
1,1,2-trichloroethane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 1,1,2-trichloroethane, characteristic pattern of peaks in the mass spectrum of
1,1,2-trichloroethane, relative
abundance of mass ion peaks in the mass spectrum of 1,1,2-trichloroethane, revising the mass
spectrum of 1,1,2-trichloroethane, revision of mass spectroscopy of
1,1,2-trichloroethane, most abundant ions in the
mass spectrum of 1,1,2-trichloroethane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 1,1,2-trichloroethane, how to analyse the mass
spectrum of 1,1,2-trichloroethane, how to describe explain the formation of fragmented ions in the
mass spectra of 1,1,2-trichloroethane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 1,1,2-trichloroethane recognising the base ion
peak of 1,1,2-trichloroethane interpreting interpretation the mass spectrum of
1,1,2-trichloroethane formula
old names
functional group How do you interpret the mass spectrum of
1,1,2-trichloroethane How to interpret
the mass spectrum of 1,1,2-trichloroethane Explanatory diagram of the mass spectrum of the
1,1,2-trichloroethane molecule in
terms of its molecular structure.
Table listing data of the m/z ion prominent main peaks in the mass spectrum of
1,1,2-trichloroethane. How to explain the mass spectrum of
1,1,2-trichloroethane. The m/z value of the
molecular ion peak in the mass spectrum of 1,1,2-trichloroethane. Identifying
1,1,2-trichloroethane from
its mass spectrum pattern. The m/z m/e peak analysis interpretation
diagram of the mass
spectrum of the 1,1,2-trichloroethane molecule. The uses of the mass spectrum of the
1,1,2-trichloroethane molecule. The distinctive features of the mass spectrum of
the 1,1,2-trichloroethane molecule explained. explaining the fragmentation pattern of the mass spectrum of
1,1,2-trichloroethane equations showing the
formation of the ionised fragments in the mass spectrum of
1,1,2-trichloroethane
what does the mass spectrum tell you about the structure and
properties of the 1,1,2-trichloroethane molecule? Data table of ionised fragments in
the mass spectrum of 1,1,2-trichloroethane and equations for their formation in the
fragmentation of the ionised 1,1,2-trichloroethane molecule.
Links associated
with
1,1,2-trichloroethane
The
infrared spectrum of 1,1,1-trichloroethane
The
infrared spectrum of 1,1,2-trichloroethane
The mass
spectrum of 1,1,1-trichloroethane
The mass
spectrum of 1,1,2-trichloroethane
The H-1
NMR spectrum of 1,1,1-trichloroethane
The H-1
NMR spectrum of 1,1,2-trichloroethane
The
C-13 NMR spectrum of 1,1,1-trichloroethane
The
C-13 NMR spectrum of 1,1,2-trichloroethane
The chemistry of HALOGENOALKANES (haloalkanes)
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