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Interpreting the mass
spectrum of 2-bromopropane
CH3CHBrCH3
[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
spectrometry - analysing the mass spectrum of 2-bromopropane
[updated
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mass spectrum of
CH3CHBrCH3
LINKS associated
with 2-bromopropane
The
chemistry of organic halogen compounds
This is a BIG website, please take time to explore it
Mass spectrometry
- introduction and spectra index
Introductory note on the mass spectrum of 2-bromopropane
Students and teachers please note
my explanation of the mass spectrum of 2-bromopropane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2-bromopropane molecule, the initial ionisation to give the molecular ion is:
M(g) +
high KE e- ==> [M•]+(g) + 2e-
and for 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-bromopropane and only the formation of singly charged
positive are considered for the mass spectrum of 2-bromopropane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
2-bromopropane
and doing the mass spectrum analysis under standard conditions,
databases can be compiled based on complex fingerprint patterns, often involving
the relative intensities of many fragment ions, and used to identify compounds including
2-bromopropane.
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 the accurate ion
masses, 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,
but the mass spectrometer software does!
2-bromopropane,
C3H7Br,
CH3CHBrCH3,
 ,
The molecular structure and naming of haloalkanes
Interpreting the fragmentation pattern of the mass spectrum of
2-bromopropane
(a secondary haloalkane)
[M]+ is the molecular ion peaks (M) with an m/z of
122 and 124 corresponding to [C3H7Br]+, the original 2-bromopropane molecule minus an electron,
[CH3CHBrCH3]+
There are two molecular ion peaks because
bromine as two isotopes, 50.6% 79Br and 49.4% 81Br.
m/z ion 122
[CH3CH79BrCH3]+
is the slightly higher intensity M peak because of the slightly higher
% of the bromine-79 isotope, and considered the
M ion
m/z ion 124
[CH3CH81BrCH3]+
is the slightly lesser intensity
M+2 peak
because of the slightly lesser % of the bromine-81 isotope, and
considered the
M+2 ion
Their average relative isotopic mass is ~80, so the relative
molecular mass for 2-bromopropane is ~123.
However, this means any fragment carrying a bromine
atom should show up as twin peaks, two mass units apart and
approximately of equal height (intensities), .
You might see very tiny M+1
and M+3 peaks at m/z 123 and 125, corresponding to an ionised
2-bromopropane
molecule with one 13C atom in it i.e. an ionised 2-bromopropane molecule of
formula [13C12C2H779/81Br]+
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+1/3
peaks.
2-bromopropane has 3 carbon atoms, so on
average, ~1 in 33 molecules will contain a 13C atom.
The most abundant ion of the molecule under mass
spectrometry investigation (2-bromopropane) is usually given an arbitrary value of
100, called the base ion peak, and all other abundances
('intensities') are measured against it.
The base ion peak for
2-bromopropane is the m/z 43 ion
[C3H7]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2-bromopropane.
The parent molecular ion
peaks are m/z 122 and 124 ions:
[CH3CH79BrCH3]+
and [CH3CH81BrCH3]+
Unless otherwise indicated, assume the carbon atoms in
2-bromopropane are the 12C isotope.
|
m/z value of
[fragment]+ |
124 |
122 |
109 |
107 |
82 |
81 |
80 |
79 |
|
[molecular fragment]+ |
[C3H781Br]+ |
[C3H779Br]+ |
[C2H481Br]+ |
[C2H479Br]+ |
[H81Br]+ |
[81Br]+ |
[H79Br]+ |
[79Br]+ |
|
m/z value of
[fragment]+ |
44 |
43 |
42 |
41 |
39 |
27 |
15 |
|
[molecular fragment]+ |
[13C12C2H7]+ |
[C3H7]+ |
[C3H6]+ |
[C3H5]+ |
[C3H3]+ |
[C2H3]+ |
[CH3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2-bromopropane
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: C = 12
(~1% 13); H = 1; O = 16; Br = 79 or 81 (ratio ~1 : 1)
Bond enthalpies kJ/mol: C-C = 348; C-H = 412;
C-Br 276
Equations to explain the most abundant ion peaks of
2-bromopropane
Formation of m/z 107 and 109 ions:
[CH3CHBrCH3]+ ===> [C2H4Br]+
+ CH3
C-C bond scission to lose an end methyl group.
Mass loss = 124 - 15 = 109 and 122 - 15
= 107
Very small peaks since C-C bond scission is
much less likely than C-Br scission - the latter having a
significantly lower bond enthalpy.
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 50.6 : 49.4).
Formation of m/z 80 and 82 ions:
[CH3CHBrCH3]+ ===> [H79Br]+
or [H81Br]+ + C3H6
Elimination of hydrogen bromide from the parent
molecular ion.
Although small, t he
double HBr peaks of roughly 1 : 1 ratio are very characteristic
of organo-bromine compounds (one m/z ion peak is slightly
shorter than the other, technically 50.6 : 49.4).
Formation of m/z 79 and 81 ions:
[CH3CHBrCH3]+ ===> [79Br]+
or [81Br]+ + C3H7
C-Br bond scission, but the alkyl fragment is more
likely to carry the positive charge (see below).
Again, although small, t he
double Br peaks of roughly 1 : 1 ratio are very characteristic of
organo-bromine compounds (one m/z ion peak is slightly shorter than
the other, technically 50.6 : 49.4).
Formation of m/z 43 ion:
[CH3CHBrCH3]+ ===> [C3H7]+
+ Br
C-Br bond fission to give the ionised alkyl ion
(bond weaker than C-C).
Mass loss 124 - 81 = 43 and 122 - 79 =
43
The m/z 43 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
It undergoes success proton loss e.g. 43 => 42 => 41
=> 40 => 39
Formation of m/z 41 ion:
[C3H7]+ ===> [C3H5]+
+ H2
Loss of hydrogen, mass change 43 - 2 = 41
Formation of m/z 27 ion:
[C2H4Br]+ ===> [C2H3]+
+ HBr
Elimination of hydrogen bromide.
Mass loss 109 - 82 = 27 and 107 - 80
= 27
Formation of m/z 15 ion:
[CH3CHBrCH3]+ ===> [CH3]+
+ C2H4Br
C-C bond scission.
Mass loss 124 - 109 = 15 and 122 -
107 = 15
Summary of the
mass
spectrum of 2-bromopropane and extra comments
The mass spectrum of 2-bromopropane (CH3–CHBr–CH3)
with exam precision and clarity.
Key Features of
2-Bromopropane of relevance to its mass spectrum
- Molecular formula: C3H7Br
- Relative molecular mass: ~123
- Contains bromine, which has two major isotopes: ⁷⁹Br
and ⁸¹Br (nearly 1:1 ratio)
- Fragmentation: Typical alkyl cleavage and halogen loss
Prominent m/z Ions
and Their Origins
in the mass
spectrum of 2-bromopropane
|
m/z |
Ion |
Origin /
Fragmentation |
Notes |
| 122 |
[M]⁺ (with ⁷⁹Br) |
Molecular ion with ⁷⁹Br isotope |
One of the
twin molecular ion peaks |
| 124 |
[M+2]⁺ (with ⁸¹Br) |
Molecular ion with ⁸¹Br isotope |
Equal intensity to m/z 122 |
| 43 |
C3H7⁺ |
Propyl cation from cleavage of C–Br |
Often the
base ion peak |
| 41 |
C3H5⁺ |
Allyl-type fragment (loss of H2) |
Resonance-stabilized |
| 27 |
C2H3⁺ |
Vinyl cation or rearranged fragment |
Seen in many halogenoalkanes |
Common
Misconceptions about
the mass
spectrum of 2-bromopropane
(see below too)
- Confusing M and M+2 peaks: Students may expect a single
molecular ion peak. Bromine causes two peaks (m/z 122 and
124) of equal intensity due to isotopes.
- Expecting a peak at average mass (~123): Mass
spectrometry shows actual isotopic masses, not averages.
- Misidentifying base peak: The most intense peak is
often not the molecular ion but a stable fragment like
C3H7⁺ (m/z 43).
- Overinterpreting small peaks: Peaks like m/z 27 may be
misattributed unless fragmentation pathways are understood.
Exam Tips for
questions involving
the mass
spectrum of 2-bromopropane
(see above too)
- Always mention bromine isotopes: “Twin molecular ion
peaks at m/z 122 and 124 due to ⁷⁹Br and ⁸¹Br.”
- Identify base peak clearly: “The base peak at m/z ~43
corresponds to the propyl cation.”
- Use correct ion notation: [M]⁺ for molecular ion, not
just “parent peak.”
- Avoid average mass references: Quote actual m/z values,
not calculated averages.
- Link fragmentation to structure: “Loss of Br gives a
stable alkyl cation at m/z ~43.”
Practice Question:
Mass Spectrum of 2-Bromopropane
Compound: 2-bromopropane (C3H7Br)
A student analyses the mass spectrum of 2-bromopropane. The spectrum shows
two prominent molecular ion peaks at m/z 122 and m/z 124, and
a base peak at m/z 43.
Question:
- Explain why the molecular ion peak appears as a pair at m/z 122
and m/z 124.
- Identify the fragment ion responsible for the base peak at m/z
43 and explain its formation.
- Suggest why the molecular ion peak is less intense than the base peak.
- Predict the relative intensities of the m/z 122 and m/z
124 peaks and justify your answer using isotopic abundances.
- Explain how the fragmentation pattern helps distinguish 2-bromopropane
from its isomer 1-bromopropane.
Model Answer
a) Isotopic Pair at m/z 122 and 124
- Bromine has two major isotopes: ⁷⁹Br and ⁸¹Br.
- 2-bromopropane contains one bromine atom, so the molecular ion exists in
two forms:
-
C3H7⁷⁹Br → m/z 122
-
C3H7⁸¹Br → m/z 124
- These peaks are two mass units apart due to the isotopic difference.
b) Fragment Ion at m/z 43
- The m/z 43 peak corresponds to the isopropyl cation (CH(CH3)2⁺).
- It forms when the Br atom is lost as a neutral radical:
-
C3H7Br →
CH(CH3)2⁺ (m/z 43) + Br•
- This is the most stable carbocation formed from 2-bromopropane due to
tertiary-like stabilization.
c) Molecular Ion Peak Intensity
- The molecular ion peak is less intense because:
- The C–Br bond is relatively weak and easily cleaved.
- The isopropyl cation is highly stable, so fragmentation is favoured.
- Many molecules fragment before reaching the detector intact.
d) Relative Intensities of m/z 122
and 124
- Natural abundance of bromine isotopes:
- ⁷⁹Br ≈ 50.6%
- ⁸¹Br ≈ 49.4%
- Therefore, the m/z 122 and m/z 124 peaks will be
approximately equal in intensity, forming a near 1:1 ratio.
e) Distinguishing from 1-Bromopropane
- Both compounds show molecular ion peaks at m/z 122 and 124 due
to Br isotopes.
- However, their fragmentation differs:
- 1-bromopropane forms a straight-chain propyl cation (m/z
43) with less stability.
- 2-bromopropane forms a branched isopropyl cation (m/z 43)
which is more stable and dominant.
- The relative intensity and stability of the m/z 43 peak is
higher in 2-bromopropane, making it the base peak.
Key words & phrases: C3H7Br CH3CHBrCH3 image diagram on how to interpret and explain the mass spectrum of
2-bromopropane m/z m/e base peaks, image and diagram of the mass spectrum of
2-bromopropane, details of the mass spectroscopy of 2-bromopropane, low and high resolution mass
spectrum of 2-bromopropane, prominent m/z peaks in the mass spectrum of
2-bromopropane, comparative
mass spectra of 2-bromopropane, the molecular ion peak in the mass spectrum of
2-bromopropane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2-bromopropane, characteristic pattern of peaks in the mass spectrum of
2-bromopropane, relative
abundance of mass ion peaks in the mass spectrum of 2-bromopropane, revising the mass
spectrum of 2-bromopropane, revision of mass spectroscopy of 2-bromopropane, most abundant ions in the
mass spectrum of 2-bromopropane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2-bromopropane, how to analyse the mass
spectrum of 2-bromopropane, how to describe explain the formation of fragmented ions in the
mass spectra of 2-bromopropane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2-bromopropane recognising the
base ion peak of 2-bromopropane interpreting
interpretation the mass spectrum of 2-bromopropane isopropyl bromide
How do you interpret the mass spectrum of
2-bromopropane How to interpret
the mass spectrum of 2-bromopropane Explanatory diagram of the mass spectrum of the
2-bromopropane molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
2-bromopropane. How to explain the mass spectrum of 2-bromopropane. The m/z value of the
molecular ion peak in the mass spectrum of 2-bromopropane. Identifying
2-bromopropane from
its mass spectrum pattern. The m/z m/e peak analysis of the mass
spectrum of the 2-bromopropane molecule. The uses of the mass spectrum of the
2-bromopropane molecule. The distinctive features of the mass spectrum of
the 2-bromopropane molecule explained. explaining the fragmentation pattern of the mass spectrum of
2-bromopropane equations showing the
formation of the ionised fragments in the mass spectrum of
2-bromopropane
what does the mass spectrum tell you about the structure and
properties of the 2-bromopropane molecule? Data table of ionised
fragments in the mass spectrum of 2-bromopropane and equations for their
formation in the fragmentation of 2-bromopropane molecules
Links associated
with
2-bromopropane
The infrared spectrum of
2-bromopropane
The H-1 NMR spectrum of 2-bromopropane
The C-13 NMR spectrum of
2-bromopropane
The chemistry of HALOGENOALKANES (haloalkanes)
revision notes INDEX
Mass spectrometry - introduction and spectra index
ALL SPECTROSCOPY INDEXES
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