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Interpreting the mass
spectrum of 2-bromo-2-methylpropane
(tert-butyl bromide)
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 2-bromo-2-methylpropane
See also
comparing
infrared, mass, 1H NMR & 13C NMR spectra of 4 halogenoalkane isomers of C4H9Br
2-bromo-2-methylpropane (tert-butyl bromide),
C4H9Br,
(CH3)3C-Br
Interpreting the fragmentation pattern of the mass spectrum of
2-bromo-2-methylpropane
Theoretically, [M]+ is the parent molecular ion peaks
of M and M+2 with an m/z of
136 and 138.
However, these molecular ions, [C4H9Br]+,
are so unstable, they fragment immediately to give, predominantly, the
m/z 57 ion, [C4H9]+, whose formation is discussed in more detail below along with
a table of ions formed
in the fragmentation pattern of 2-bromo-2-methylpropane.
Bromine consists of two isotopes, 79Br and
81Br in roughly equal proportions, therefore any molecular
ion or fragment containing a bromine atom will show up as a double peak
of similar height (abundance) two mass units apart e.g. m/z ions 121 and 123 and the very
tiny molecular ion peaks of m/z values 136 and 138 (but still of ~equal
height!) in the mass spectrum of 2-bromo-2-methylpropane.
The most abundant ion of the molecule under mass
spectrometry investigation (2-bromo-2-methylpropane) 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 2-bromo-2-methylpropane.
Unless otherwise indicated, assume the carbon atoms in
2-bromo-2-methylpropane are the 12C isotope.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of 2-bromo-2-methylpropane.
The theoretical parent molecular ion of
2-bromo-2-methylpropane m/z 136/138: [C4H9Br]+
Data table of
some of the ions formed in the fragmentation pattern of the mass
spectrum of 2-bromo-2-methylpropane
| m/z value of
[fragment]+ |
138 (not seen) |
136 (not seen) |
123 |
121 |
| [molecular fragment]+ |
[C4H981Br]+ |
[C4H979Br]+ |
[C3H681Br]+ |
[C3H679Br]+ |
| m/z value of
[fragment]+ |
82 |
81 |
80 |
79 |
58, with 13C
atom |
57, all 12C
atoms |
56 |
55 |
| [molecular fragment]+ |
[H81Br]+ |
[81Br]+ |
[H79Br]+ |
[79Br]+ |
[C4H9]+ |
[C4H9]+ |
[C4H8]+ |
[C4H7]+ |
| m/z value of
[fragment]+ |
41 |
39 |
29 |
27 |
15 |
| [molecular fragment]+ |
[C3H5]+ |
[C3H3]+ |
[C2H4]+ |
[C2H3]+ |
[CH3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2-bromo-2-methylpropane
Atomic masses: H = 1; C = 12; Br = 79 or 81 (1:1
isotope abundance
ratio); I = 127
Bond enthalpies = kJ/mol: C-C = 348;
C-H = 412; C-Br 276
Possible
equations to explain some of the most abundant ion peaks of
2-bromo-2-methylpropane
(tabulated above)
Formation of m/z 121 and 123 ions:
[(CH3)3C-Br]+ ===> [C3H6Br]+
+ CH3
C-C bond scission in the parent molecular ions, mass
change 136/138 - 15 = 121/123..
Low probability as the C-Br is a much weaker bond,
hence more likely to break.
Note the twin ~1:1 peaks due to the two bromine
isotopes.
Formation of m/z 79 to 82 ions:
These are mentioned in conjunction with the
formation of the m/z 57 and 56 ions (see table of ions above and
fragmentation equations below for m/z 56 and 57 ions).
Formation of m/z 57 ion:
[(CH3)3C-Br]+ ===> [C4H9]+
+ Br
This ionisation is very
likely, C-Br bond scission in the parent molecular ion, mass change
136/138 - 79/81 = 57.
This ion is a relatively stable tertiary
carbocation, the most stable type of positive alkyl ions.
The m/z 57 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
The m/z 58 ion is likely to be [13C12C3H9]+
i.e. as above but with a 13C atom in the hydrocarbon
fragment.
The m/z 57 ion can lose a hydrogen atom/molecule to
give m/z ions 56 and 55.
There is a very low probability that the bromine
atom can also be ionised to give m/z 79 and 81 ions (see table) and you can just
about make out the tiny twin peaks.
Formation of m/z 56 ion:
[(CH3)3C-Br]+ ===> [C4H8]+
+ HBr
Elimination of hydrogen bromide from the parent
molecular ion can also give the m/z 56 ion.
Mass change 136/138 - 80/82 = 56.
There is a very low probability that the hydrogen
bromide molecule can also be ionised to give m/z 80 and 82 ions -
you can just about make out the tiny twin peaks.
Formation of m/z 41 and 39 ions:
Possible reactions include:
m/z 41: [C4H8]+ ===> [C3H5]+
+ CH3
m/z 39: [C3H5]+ ===> [C3H3]+
+ H2
Formation of m/z 27 and 29 ions:
Possible reactions include:
From bond scission in the parent molecular ion.
m/z 27: [C4H8]+ ===> [C2H3]+
+ C2H5
m/z 29: [C4H9]+ ===> [C2H5]+
+ C2H4
m/z 27: [C2H5]+ ===> [C2H3]+
+ H2
m/z 29: [C4H8]+ ===> [C2H5]+
+ C2H3
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Comparing the
infrared, mass, 1H NMR and 13C NMR spectra of the 4
halogenoalkane isomers of C4H9Br
NOTE: The images are linked to their
original detailed spectral analysis pages AND can be doubled in
size with touch screens to
increase the definition to the original 1-bromobutane,
2-bromobutane, 1-bromo-2-methylpropane and 2-bromo-2-methylpropane
image sizes. These four molecules
are structural isomers of molecular formula C4H9Br
and
exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower
aliphatic halogenoalkanes (haloalkanes, alkyl halides,
bromoalkanes, alkyl bromides). |
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INFRARED SPECTRA
(above):
Apart from the significant differences in the fingerprint region at
wavenumbers 1500 to 400 cm-1, there are no other
great striking differences, but each could be identified from
its infrared spectrum. |
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MASS SPECTRA (above):
All four give the parent molecular ions of m/z 136 and 138, but it is
only a relatively tiny peak for 2-bromobutane and 2-bromo-2-methylpropane. All four
give the base ion peak of m/z 57. All four give prominent peaks
for m/z ions 27, 29, 39 and 41 and all give a tiny peak from an ionised
iodine atom at m/z 127. They look quite similar to me and lack a
clear fingerprint fragmentation pattern. There are small
differences in the relative abundances (peak heights) for pairs
of ions involving 79Br/81Br isotopes e.g.
m/z 93/95, 107/109 and 121/123. 1-bromo-2-methylpropane is the
only one of the four to have a prominent peak for the m/z 43
ion. |
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1H NMR SPECTRA
(above): The 1H NMR spectra of all four molecules give different
integrated proton ratios i.e.1-bromobutane
four peaks of ratio 3:2:2:2; 2-bromobutane four peaks of
ratio 3:3:2:1,
1-bromo-2-methylpropane three peaks of ratio 6:2:1 and
2-bromo-2-methylpropane gives just one peak '1' (effectively no ratio
involved), so all four molecular structures can be distinguished from each other by their
1H NMR spectra proton ratios, numbers of peaks and (n+1)
rule splitting patterns. |
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13C NMR SPECTRA
(above): The
13C NMR spectra of the four molecules show various numbers of
carbon-13 chemical environments i.e 1-bromobutane and
2-bromobutane show four 13C NMR resonances,
1-bromo-2-methylpropane three 13C NMR resonances and
2-bromo-2-methylpropane only two 13C resonances. Therefore
1-bromo-2-methylpropane and 2-bromo-2-methylpropane can be
distinguished from the other three by their number of resonances
in their 13C NMR spectra, but 1-bromobutane and 2-bromobutane
cannot be distinguished from each other from their number of 13C
NMR resonance lines - other data would be required. |
Key words & phrases: C4H9Br (CH3)3CBr image diagram on how to interpret and explain the mass spectrum of
2-bromo-2-methylpropane m/z m/e base peaks, image and diagram of the mass spectrum of
2-bromo-2-methylpropane, details of the mass spectroscopy of
2-bromo-2-methylpropane, low and high resolution mass
spectrum of 2-bromo-2-methylpropane, prominent m/z peaks in the mass spectrum of
2-bromo-2-methylpropane, comparative
mass spectra of 2-bromo-2-methylpropane, the molecular ion peak in the mass spectrum of
2-bromo-2-methylpropane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2-bromo-2-methylpropane, characteristic pattern of peaks in the mass spectrum of
2-bromo-2-methylpropane, relative
abundance of mass ion peaks in the mass spectrum of 2-bromo-2-methylpropane, revising the mass
spectrum of 2-bromo-2-methylpropane, revision of mass spectroscopy of
2-bromo-2-methylpropane, most abundant ions in the
mass spectrum of 2-bromo-2-methylpropane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2-bromo-2-methylpropane, how to analyse the mass
spectrum of 2-bromo-2-methylpropane, how to describe explain the formation of fragmented ions in the
mass spectra of 2-bromo-2-methylpropane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2-bromo-2-methylpropane recognising the base ion
peak of 2-bromo-2-methylpropane interpreting interpretation the mass spectrum of
2-bromo-2-methylpropane tert-butyl bromide alkyl
halide haloalkane functional group alkyl bromide bromoalkane
Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
2-bromo-2-methylpropane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 2-bromo-2-methylpropane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 2-bromo-2-methylpropane. The m/e m/z value of the base ion peak in the
mass spectrum of 2-bromo-2-methylpropane. Possible examples of equations showing the formation
of the ionised fragments in 2-bromo-2-methylpropane. Revision notes on the mass spectrum of
2-bromo-2-methylpropane.
Matching and deducing the structure of the 2-bromo-2-methylpropane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic halogenoalkanes bromoalkanes alkyl bromides,
mass spectra of 2-bromo-2-methylpropane, an isomer of molecular formula C4H9Br
Links associated
with 2-bromo-2-methylpropane
The chemistry of HALOGENOALKANES (haloalkanes)
revision notes INDEX
The
infrared spectrum of 2-bromo-2-methylpropane
(tert-butyl bromide)
The H-1 NMR spectrum of
2-bromo-2-methylpropane
(tert-butyl bromide)
The
C-13 NMR spectrum of 2-bromo-2-methylpropane
(tert-butyl bromide)
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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