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Interpreting the mass
spectrum of 1-bromo-2-methylpropane
(CH3)2CHCH2Br
(re-edit)
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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-methylpropane (isobutyl
bromide)
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Mass spectroscopy - spectra index
Links associated with 1-bromo-2-methylpropane
See also
comparing
infrared, mass, 1H NMR & 13C NMR spectra of 4 halogenoalkane isomers of C4H9Br
1-bromo-2-methylpropane
(isobutyl bromide),
C4H9Br,
(CH3)2CHCH2Br
Interpreting the fragmentation pattern of the mass spectrum of
1-bromo-2-methylpropane
[M]+ is the parent molecular ion peak (M) with an m/z of
136 or 138 corresponding to [C4H9Br]+, the original 1-bromo-2-methylpropane molecule minus an electron,
[(CH3)2CHCH2Br]+.
There are two possibilities because bromine consists
of two isotopes, 79Br and 81Br in the ratio ~1
: 1.
Therefore the molecular ion can be [(CH3)2CHCH279Br]+
or [(CH3)2CHCH281Br]+,
which should, and does, show up as a double peak of ~equal heights
(~equal abundance).
These are referred to as the
M and
M+2
peaks
respectively, emphasising the two mass unit difference due to the
bromine isotopes in the two molecular ions of
1-bromo-2-methylpropane.
The two bromine isotopes also account for the tiny
'twin peaks' of m/z ions 93 & 95 and 121 & 132.
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 93 and
95, and 121 and 123, plus small molecular ion peaks of m/z values 136 and 138
(but still, all pairs of ~equal height!) in the mass spectrum of
1-bromo-2-methylpropane.
The small M+1 peak at m/z 137, corresponds to an ionised
1-bromo-2-methylpropane
molecule with one 13C atom in it i.e. an ionised
1-bromo-2-methylpropane molecule of
formula [13C12C4H979Br]+
Carbon-13 only accounts for ~1% of all carbon atoms
(12C ~99%), but the more carbon atoms in the molecule,
the greater the probability of observing this 13C M+1
peak.
1-bromo-2-methylpropane has 4 carbon atoms, so on
average, ~1 in 25 molecules will contain a 13C atom.
The most abundant ion of the molecule under mass
spectrometry investigation (1-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
(including M and M+2 ions) in the fragmentation pattern of 1-bromo-2-methylpropane.
Unless otherwise indicated, assume the carbon atoms in
1-bromo-2-methylpropane are the 12C isotope.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of 1-bromo-2-methylpropane.
The parent molecular ion M of
1-bromo-2-methylpropane m/z 136:
[(CH3)2CHCH2Br]+
| m/z value of
[fragment]+ |
138 |
136 |
123 |
121 |
109 |
107 |
| [molecular fragment]+ |
[C4H981Br]+ |
[C4H979Br]+ |
[C3H681Br]+ |
[C3H679Br]+ |
[C2H481Br]+ |
[C2H479Br]+ |
| m/z value of
[fragment]+ |
trace of 95 |
trace of 93 |
58, with 13C
atom |
57, all 12C
atoms |
56 |
55 |
| [molecular fragment]+ |
[CH281Br]+ |
[CH279Br]+ |
[C4H9]+ |
[C4H9]+ |
[C4H8]+ |
[C4H7]+ |
| m/z value of
[fragment]+ |
43 |
42 |
41 |
40 |
39 |
29 |
28 |
27 |
26 |
15 |
| [molecular fragment]+ |
[C3H7]+ |
[C3H6]+ |
[C3H5]+ |
[C3H2]+ |
[C3H3]+ |
[C2H4]+ |
[C2H4]+ |
[C2H3]+ |
[C2H2]+ |
[CH3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 1-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
1-bromo-2-methylpropane
(tabulated above)
Formation of m/z 121 and 123 ions:
[(CH3)2CHCH2Br]+ ===> [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 93 and 95 ions:
[(CH3)2CHCH2Br]+
===> [CH2Br]+
+ C3H7
C-C bond scission in the parent molecular ion, mass
change 136/138 - 43 = 93/95.
Note the twin ~1:1 peaks due to the two bromine
isotopes.
The C-Br bond is the weakest bond in the molecule,
hence the most likely bond scission is C-Br with 1-bromo-2-methylpropane (see
below).
Formation of m/z ions of mass 79 to 82
There are a set of tiny peaks associated with the
bromine atoms of the parent molecule, and note there are only two
stable isotopes of bromine 79Br and 81Br (50%
abundance for each, giving a ratio of 1:1).
[R-Br]+ ===> [R] +
79Br+ or 81Br+
These two ions of mass 79 and 81 (ratio 1:1),
derived from the ionised molecule/fragment are very tiny, but
characteristic peaks of the two isotopes of bromine, observed in the
mass spectrum of any organo-bromine compound like
1-bromo-2-methylpropane.
(Note that R could represent the rest of the
original 1-bromo-2-methylpropane molecule or any smaller fragment
retaining a bromine atom).
There are two other tiny peaks, but a bit larger
than those for the ionised bromine atoms.
They have m/z values of 80 and 82, that is 1 more
mass unit than the ionised bromine atoms.
These two peaks can arise from the formation of
hydrogen bromide (HBr) by an elimination reaction.
[(CH3)2CHCH2Br]+
===> [H79Br] or [H81Br] +
(CH3)2CH==CH2
Again, note there are two peaks expected in the
expected 1:1 ratio from the mass spectrum of
1-bromo-2-methylpropane.
Formation of m/z 57 ion:
[(CH3)2CHCH2Br]+ ===> [C4H9]+
+ Br
This alternative ionisation to above is much more
likely, C-Br bond scission in the parent molecular ion, mass change
136/138 - 79/81 = 57.
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 - you can just
about make out the tiny twin peaks.
Formation of m/z 56 ion:
[(CH3)2CHCH2Br]+ ===> [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 29, 28 and 27 ions:
Possible reactions include:
[(CH3)2CHCH2Br]+ ===> [C2H5]+
+ CH2CH2Br
From bond scission in the parent molecular ion.
m/z 27: [C4H8]+ ===> [C2H3]+
+ C2H5
m/z 28: [C4H8]+ ===> [C2H4]+
+ C2H4
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:
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1-bromo-2-methylpropane, comparative
mass spectra of 1-bromo-2-methylpropane, the molecular ion peak in the mass spectrum of
1-bromo-2-methylpropane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 1-bromo-2-methylpropane, characteristic pattern of peaks in the mass spectrum of
1-bromo-2-methylpropane, relative
abundance of mass ion peaks in the mass spectrum of 1-bromo-2-methylpropane, revising the mass
spectrum of 1-bromo-2-methylpropane, revision of mass spectroscopy of
1-bromo-2-methylpropane, most abundant ions in the
mass spectrum of 1-bromo-2-methylpropane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 1-bromo-2-methylpropane, how to analyse the mass
spectrum of 1-bromo-2-methylpropane, how to describe explain the formation of fragmented ions in the
mass spectra of 1-bromo-2-methylpropane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 1-bromo-2-methylpropane recognising the base ion
peak of 1-bromo-2-methylpropane interpreting interpretation the mass spectrum of
1-bromo-2-methylpropane
isobutyl bromide alkyl halide alkyl bromide functional group haloalkane
halogenoalkane bromoalkane Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
1-bromo-2-methylpropane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 1-bromo-2-methylpropane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 1-bromo-2-methylpropane. The m/e m/z value of the base ion peak in the
mass spectrum of 1-bromo-2-methylpropane. Possible examples of equations showing the formation
of the ionised fragments in 1-bromo-2-methylpropane. Revision notes on the mass spectrum of
1-bromo-2-methylpropane.
Matching and deducing the structure of the 1-bromo-2-methylpropane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic halogenoalkanes bromoalkanes alkyl bromides,
mass spectra of 1-bromo-2-methylpropane, an isomer of molecular formula
C4H9Br Explanatory diagram of the mass spectrum of the 1-bromo-2-methylpropane molecule in terms of its molecular structure. Listing data of the prominent main peaks in the mass spectrum of 1-bromo-2-methylpropane. How to explain the mass spectrum of 1-bromo-2-methylpropane. The m/z value of the molecular ion peak in the mass spectrum of 1-bromo-2-methylpropane. Identifying 1-bromo-2-methylpropane from its mass spectrum pattern. The m/z m/e peak analysis of the mass spectrum of the 1-bromo-2-methylpropane molecule. The uses of the mass spectrum of the 1-bromo-2-methylpropane molecule. The distinctive features of the mass spectrum of the 1-bromo-2-methylpropane molecule explained. explaining the fragmentation pattern of the mass spectrum of 1-bromo-2-methylpropane equations showing the formation of the ionised fragments in the mass spectrum of 1-bromo-2-methylpropane what does the mass spectrum tell you about the structure and properties of the 1-bromo-2-methylpropane molecule?(CH3)2CHCH2Br
Links associated with 1-bromo-2-methylpropane
The chemistry of HALOGENOALKANES (haloalkanes)
revision notes INDEX
The infrared spectrum of
1-bromo-2-methylpropane (isobutyl bromide)
The H-1 NMR spectrum of
1-bromo-2-methylpropane (isobutyl bromide)
The C-13 NMR spectrum
of 1-bromo-2-methylpropane (isobutyl bromide)
Mass spectroscopy index
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