The mass
spectrum of 1-bromo-2-chloroethane
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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-bromo-2-chloroethane
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Mass spectroscopy - spectra index
1-bromo-2-chloroethane,
C2H4BrCl,
BrCH2CH2Cl
Interpreting the fragmentation pattern of the mass spectrum of
1-bromo-2-chloroethane
[M]+ is the molecular ion peak (M) with m/z
values of
142, 144 and 146 corresponding to [C2H4BrCl]+, the original 1-bromo-2-chloroethane molecule minus an electron,
[BrCH2CH2Cl]+.
This is quite a tricky mass spectrum to interpret
because of the two sets of isotopes of bromine and chlorine.
Since chlorine has two common isotopes of 35Cl
and 37Cl in the ratio 3 : 1, you should observe double peaks
in the intensity ratio 3 : 1, two mass units apart for molecular
fragments containing a chlorine atom from the fragmentation of
1-chlorobutane (if the fragment has no bromine atom).
BUT, this is complicated by the isotopes of bromine!
Bromine as two isotopes, 50.5% 79Br and 50.5%
81Br, which any fragment carrying a bromine
atom should show up as twin peaks of intensity ratio ~1:1, two mass
units apart (if the fragment has no chlorine atom).
So, what you observe in terms of abundances, is the
result of two ratios, and therefore I'm keeping the interpretation
as simple as I can (at pre-university level!).
However, you should be able to work out what the
four possible molecular ions consist of: m/z 142 [79BrCH2CH235Cl]+,
m/z 144 [79BrCH2CH237Cl]+,
m/z 144 [81BrCH2CH235Cl]+
and m/z 146[81BrCH2CH237Cl]+,
and in terms of probability the ratio would be: 3:1:3:1,
There is one Br and one Cl in each molecule of
1-bromo-2chloroethane.
Cross 79 and 81 with 35, 35, 35 and 37 in a 2 x 4 probability
chart to get the above ratios.
The most abundant ion of the molecule under mass
spectrometry investigation (1-bromo-2-chloroethane) 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-bromo-2-chloroethane.
Unless otherwise indicated, assume the carbon atoms in
1-bromo-2-chloroethane are the 12C isotope.
4 m/z values of
[M]+ |
142 |
144 |
144 |
146 |
[molecule M]+ |
[79BrCH2CH235Cl]+ |
[79BrCH2CH237Cl]+ |
[81BrCH2CH235Cl]+ |
[81BrCH2CH237Cl]+ |
m/z value of
[fragment]+ |
109 |
107 |
95 |
93 |
[molecular fragment]+ |
[81BrCH2CH2]+ |
[79BrCH2CH2]+ |
[CH281Br]+ |
[CH279Br]+ |
m/z value of
[fragment]+ |
65 |
63 |
49 |
31 ? |
28 |
27 |
[molecular fragment]+ |
[CH2CH237Cl]+ |
[CH2CH235Cl]+ |
[CH235Cl]+ |
[?]+ |
[C2H8]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 1-bromo-2-chloroethane
Atomic masses: H = 1; C = 12; Cl
= 35 or 37 (3:1); Br = 79 or 81 (1:1);
Bond enthalpies kJ/mol: C-C = 348; C-H = 412; C-Cl = 338; C-Br 276
Equations to explain the most abundant ion peaks of
1-bromo-2-chloroethane
Formation of m/z 107 and 109 ions:
[BrCH2CH2Cl]+
===>
[79BrCH2CH2]+
or [81BrCH2CH2]+
+ Cl
Scission of C-Cl bond, but weaker C-Br bond more
likely to be broken (see below)
Formation of m/z 93 and 95 ions:
[BrCH2CH2Cl]+ ===> [CH279Br]+]+
or [CH281Br]+]+
+ CH2Cl
Scission of C-C bond, strong bond, low probability.
Formation of m/z 63 and 65 ions:
[BrCH2CH2Cl]+
===>
[CH2CH235Cl]+
or
[CH2CH237Cl]+
+ Br
Scission of the C-Br bond, the weakest bond in the
molecule.
The m/z 63 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
Formation of m/z 49 ion:
[BrCH2CH2Cl]+ ===> [CH235Cl]+
+ BrCH2
Scission of C-C bond, strong bond, low probability.
Small peak at m/z 51 corresponding to [CH237Cl]+
Formation of m/z 28 ion:
[C2H4Cl]+ ===> [C2H4]+
+ Cl
[C2H4Br]+ ===> [C2H4]+
+ Br
C-Br bond scission of the m/z 63/65 or 107/109
pairs of ions to give an ionised ethene molecule.
Formation of m/z 27 ion:
[C2H4Cl]+ ===> [C2H4]+
+ Cl
[C2H4Br]+ ===> [C2H3]+
+ HBr
Elimination of hydrogen chloride or hydrogen
bromide from the m/z 63/65 or 107/109 ions.
Key words & phrases: C2H4BrCl BrCH2CH2Cl image diagram on how to interpret and explain the mass spectrum of
1-bromo-2-chloroethane m/z m/e base peaks, image and diagram of the mass spectrum of
1-bromo-2-chloroethane, details of the mass spectroscopy of
1-bromo-2-chloroethane, low and high resolution mass
spectrum of 1-bromo-2-chloroethane, prominent m/z peaks in the mass spectrum of
1-bromo-2-chloroethane, comparative
mass spectra of 1-bromo-2-chloroethane, the molecular ion peak in the mass spectrum of
1-bromo-2-chloroethane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 1-bromo-2-chloroethane, characteristic pattern of peaks in the mass spectrum of
1-bromo-2-chloroethane, relative
abundance of mass ion peaks in the mass spectrum of 1-bromo-2-chloroethane, revising the mass
spectrum of 1-bromo-2-chloroethane, revision of mass spectroscopy of
1-bromo-2-chloroethane, most abundant ions in the
mass spectrum of 1-bromo-2-chloroethane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 1-bromo-2-chloroethane, how to analyse the mass
spectrum of 1-bromo-2-chloroethane, how to describe explain the formation of fragmented ions in the
mass spectra of 1-bromo-2-chloroethane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 1-bromo-2-chloroethane
recognising the base ion peak of 1-bromo-2-chloroethane
interpreting interpretation the mass spectrum of 1-bromo-2-chloroethane
Links associated
with 1-bromo-2-chloroethane
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