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Interpreting
and explaining the mass
spectrum of 2-bromo-2-methylpropane
(tert-butyl bromide)
[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
spectroscopy analysis of
2-bromo-2-methylpropane (mass spectra)
[spectra
page updated
April 1st 2026 *]
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mass spectrum of
(CH3)3CBr
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Links associated with 2-bromo-2-methylpropane
Mass spectrometry - spectra index
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See also
comparing
infrared, mass, 1H NMR & 13C NMR spectra of 4 halogenoalkane isomers of C4H9Br
Introductory note on the mass spectrum of 2-bromo-2-methylpropane
Students and teachers please note
my explanation of the mass spectrum of 2-bromo-2-methylpropane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2-bromo-2-methylpropane 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 2-bromo-2-methylpropane.
I've included a stick diagram and
table of m/z ions for the mass spectrum of 2-bromo-2-methylpropane
and doing the mass spectrum analysis under standard conditions, databases can be
compiled based complex fingerprint patterns, often involving
the relative intensities of many fragment ions, and used to identify compounds including
2-bromo-2-methylpropane.
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 2-bromo-2-methylpropane. 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
2-bromo-2-methylpropane,
but the mass spectrometer software does!
2-bromo-2-methylpropane (tert-butyl bromide),
C4H9Br,
(CH3)3C-Br
The molecular structure and naming of haloalkanes
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 actual isotopic composition for
naturally occurring bromine is
79Br
(50.7%) and 81Br (49.3%), which is why the
molecular/fragment ions with the heavier bromine-81 isotope have
slightly less intensity peaks - see the small peaks for m/z ions
121 and 123.
Theoretically there are very tiny M+1 and M+3 peaks at m/z 137 and
139, corresponds to an ionised
2-bromo-2-methylpropane
molecule with one 13C atom in it i.e. an ionised 2-bromo-2--methylpropane 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-bromo-2-methylpropane has 4 carbon atoms, so on
average, ~1 in 25 molecules will contain a 13C atom.
However, fragments containing a 13C atom
are observed e.g. the m/z 58 ion
[13C12C3H9]+
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.
The base ion peak for
the mass spectrum of 2-bromo-2-methylpropane is m/z 57
[C4H9]+
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]+ |
[13C12C3H9]+ |
[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
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
(~1% 13); 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.
Where R is alkyl, the double RBr peaks
of roughly 1 : 1 abundance ratio are characteristic of
organo-bromine compounds (one m/z ion peak is slightly shorter than
the other, technically 50.7 : 49.3).
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 56 and
57 ions:
[(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.
There is also a real,
but low, possibility of elimination of hydrogen bromide from the
parent molecular ions to also give the m/z 56 ion.
[C4H9Br]+ ===> [C4H8]+
+ HBr
Mass change 136/138 - 80/82 = 56.
Therefore, as above for bromine atoms, there is a low probability that the
hydrogen bromide molecules 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
|
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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|
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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
How do you interpret the mass spectrum of
2-bromo-2-methylpropane How to interpret
the mass spectrum of 2-bromo-2-methylpropane Explanatory diagram of the mass spectrum of the
2-bromo-2-methylpropane molecule in
terms of its molecular structure.
Table listing data of the m/z ion prominent main peaks in the mass spectrum of
2-bromo-2-methylpropane. How to explain the mass spectrum of
2-bromo-2-methylpropane. The m/z value of the
molecular ion peak in the mass spectrum of 2-bromo-2-methylpropane. Identifying
2-bromo-2-methylpropane from
its mass spectrum pattern. The m/z m/e peak analysis interpretation
diagram of the mass
spectrum of the 2-bromo-2-methylpropane molecule. The uses of the mass spectrum of the
2-bromo-2-methylpropane molecule. The distinctive features of the mass spectrum of
the 2-bromo-2-methylpropane molecule explained. explaining the fragmentation pattern of the mass spectrum of
2-bromo-2-methylpropane equations showing the
formation of the ionised fragments in the mass spectrum of
2-bromo-2-methylpropane
what does the mass spectrum tell you about the structure and
properties of the 2-bromo-2-methylpropane molecule? Data table of ionised fragments in
the mass spectrum of 2-bromo-2-methylpropane and equations for their formation in the
fragmentation of the ionised 2-bromo-2-methylpropane molecule.
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 spectrometry index
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