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Interpreting the mass
spectrum of 2-bromobutane
(sec-butyl bromide)
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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 2-bromobutane
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Mass spectroscopy - spectra index
See also
comparing
infrared, mass, 1H NMR & 13C NMR spectra of 4 halogenoalkane isomers of C4H9Br
2-bromobutane, C4H9Br,
CH3CHBrCH2CH3,
CH3-CH(Br)-CH2-CH3
Secondary haloalkane/halogenoalkane and
old names: sec-butyl bromide, sec-butyl bromide (secondary alkyl halide)
Interpreting the fragmentation pattern of the mass spectrum of 2-bromobutane
[M]+ is the parent molecular ion peaks (M) have an m/z of
136 and 138 corresponding to [C4H9Br]+, the original 2-bromobutane molecule minus an electron,
[CH3CHBrCH2CH3]+.
There are two possibilities because bromine consists
of two isotopes, 79Br and 81Br in the ratio ~1
: 1.
Therefore the molecular ion can be [CH3CH79BrCH2CH3]+
or
[CH3CH81BrCH2CH3]+,
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 2-bromobutane.
The two bromine isotopes also account for the tiny 'twin
peaks' of m/z ions 107 and 109 (details discussed later).
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 107 and
109 and the
tiny molecular ion peaks of m/z values 136 and 138 (but still of ~equal
height!) in the mass spectrum of 2-bromobutane.
Theoretically there are very tiny M+1 and M+3 peaks at m/z 137 and
139, corresponds to an ionised
2-bromobutane
molecule with one 13C atom in it i.e. an ionised 2-bromobutane molecule of
formula [13C12C3H9Br]+
However, they are barely observed because the parent
molecular ions are highly unstable with very low abundances i.e.
very small peaks at m/z values 136 and 138.
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 these 13C M+1
peaks.
2-bromobutane 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 (2-bromobutane) 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-bromobutane.
Unless otherwise indicated, assume the carbon atoms in
2-bromobutane are the 12C isotope.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of 2-bromobutane.
The parent molecular ions of 2-bromobutane m/z
136:
[C4H979Br]+
and m/z 138
[C4H981Br]+
| m/z value of
[fragment]+ |
138 |
136 |
109 |
107 |
58, with 13C
atom |
57, all 12C
atoms |
56 |
55 |
| [molecular fragment]+ |
[C4H981Br]+ |
[C4H979Br]+ |
[C2H481Br]+ |
[C2H479Br]+ |
[C4H9]+ |
[C4H9]+ |
[C4H8]+ |
[C4H7]+ |
| m/z value of
[fragment]+ |
79 |
80 |
81 |
82 |
m/z
ions 79 to 82 have a very low abundance, just tiny peaks in the mass spectrum
of 2-bromobutane. |
| [molecular fragment]+ |
[79Br]+ |
[H79Br]+ |
[81Br]+ |
[H81Br]+ |
| 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 2-bromobutane
Atomic masses: H = 1; C = 12; Br
= 79 or 81 (1:1 isotope abundance ratio)
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-bromobutane
(tabulated above)
Formation of m/z 107 and 109 ion:
[CH3CHBrCH2CH3]+ ===> [C2H4Br]+
+ C2H5
C-C bond scission in the parent molecular ion and
proton rearrangement, mass
change 136/138 - 29 = 107/109.
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 2-bromobutane (see
below).
The C-Br bond is the weakest bond in the molecule,
hence the most likely bond scission, but the alternative ionisation
(below) is more likely with 2-bromobutane, since only tiny traces of
these ions are observed.
Formation of m/z 57 ion:
[CH3CHBrCH2CH3]+ ===> [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 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:
[CH3CHBrCH2CH3]+ ===> [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 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:
[CH3CHBrCH2CH3]+ ===> [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:
C4H9Br CH3CHBrCH2CH3 CH3CH2CHBrCH3 image diagram on how to interpret and explain the mass spectrum of
2-bromobutane m/z m/e base peaks, image and diagram of the mass spectrum of
2-bromobutane, details of the mass spectroscopy of 2-bromobutane, low and high resolution mass
spectrum of 2-bromobutane, prominent m/z peaks in the mass spectrum of
2-bromobutane, comparative
mass spectra of 2-bromobutane, the molecular ion peak in the mass spectrum of
2-bromobutane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2-bromobutane, characteristic pattern of peaks in the mass spectrum of
2-bromobutane, relative
abundance of mass ion peaks in the mass spectrum of 2-bromobutane, revising the mass
spectrum of 2-bromobutane, revision of mass spectroscopy of 2-bromobutane, most abundant ions in the
mass spectrum of 2-bromobutane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2-bromobutane, how to analyse the mass
spectrum of 2-bromobutane, how to describe explain the formation of fragmented ions in the
mass spectra of 2-bromobutane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2-bromobutane recognising the base ion
peak of 2-bromobutane interpreting interpretation the mass spectrum of
2-bromobutane sec-butyl
bromide sec-butyl bromide functional group alkyl bromide alkyl halide haloalkane
halogenoalkane Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
2-bromobutane (sec-butyl bromide). Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 2-bromobutane (sec-butyl bromide). The m/e m/z value of the molecular ion peak in the
mass spectrum of 2-bromobutane (sec-butyl bromide). The m/e m/z value of the base ion peak in the
mass spectrum of 2-bromobutane (sec-butyl bromide). Possible examples of equations showing the formation
of the ionised fragments in 2-bromobutane (sec-butyl bromide). Revision notes on the mass spectrum of
2-bromobutane (sec-butyl bromide).
Matching and deducing the structure of the 2-bromobutane (sec-butyl bromide) molecule from its mass
spectrum.
Links associated
with 2-bromobutane
The chemistry of HALOGENOALKANES (haloalkanes)
revision notes INDEX
The infrared spectrum of 2-bromobutane
(sec-butyl bromide)
The H-1
NMR spectrum of 2-bromobutane (sec-butyl bromide)
The
C-13 NMR spectrum of 2-bromobutane (sec-butyl bromide)
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
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