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Interpreting and explaining the mass
spectrum of 2-chloropropane
[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-chloropropane
[updated
Mar 12th 2026 *]
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analysis mass spectrum of
CH3CHClCH3
Links associated
with 2-chloropropane
The
chemistry of organic halogen compounds
This is a BIG
chemistry website, please take time to explore it
Mass spectrometry
- introduction and spectra index
Introductory note on the mass spectrum of 2-chloropropane
Students and teachers please note
my explanation of the mass spectrum of 2-chloropropane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2-chloropropane 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-chloropropane and only the formation of singly charged
positive are considered for the mass spectrum of
2-chloropropane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
2-chloropropane
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-chloropropane.
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-chloropropane C3H7Cl
The molecular structure and naming of haloalkanes
Interpreting the fragmentation pattern of the mass spectrum of
2-chloropropane
There are two molecular ion
peaks
[M]+ observed in the mass spectrum of 2-chloropropane.
The molecular ion peaks
of M and M+2 with a m/z
values of 78 and 80 corresponding to
[C3H7Cl]+, the original
2-chloropropane molecule minus an electron
M peak of m/z 78 ion
[CH3CH35ClCH3]+
M+2 peak of m/z 80 ion
[CH3CH37ClCH3]+
Since chlorine has two common isotopes of
35Cl
and 37Cl in the ratio 3 : 1, you should observe double peaks
in the intensity ratio 3 : 1, two mass units apart for molecular
fragments containing a chlorine atom from the fragmentation pattern of
2-chlorobutane.
This also applies to the molecular ion, so two
molecular ions are observed at m/z 78 and 80.
The tiny M+1 peak at m/z 79, corresponds to an ionised
2-chloropropane
molecule with one 13C atom in it i.e. an ionised 1-chloropropane molecule of
formula [13C12C2H735Cl]+
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.
2-chloropropane 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-chloropropane) 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 in
the mass spectrum of 2-chloropropane is the m/z 43 ion
[C3H7]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2-chloropropane.
P arent
molecular ion peaks are the m/z 78 ion
[CH3CH35ClCH3]+
and m/z 80 ion
[CH3CH37ClCH3]+
Unless otherwise indicated, assume the carbon atoms in
2-chloropropane are the 12C isotope.
|
m/z value of
[fragment]+ |
65 |
63 |
44 |
43 |
42 |
41 |
|
[molecular fragment]+ |
[CH3CH37Cl]+ |
[CH3CH35Cl]+ |
[C3H8]+ |
[C3H7]+ |
[C3H6]+ |
[C3H5]+ |
|
m/z value of
[fragment]+ |
40 |
39 |
29 |
28 |
27 |
26 |
|
[molecular fragment]+ |
[C3H4]+ |
[C3H3]+ |
[C2H5]+ |
[C2H4]+ |
[C2H3]+ |
[C2H2]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2-chloropropane
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; Cl = 35 or 37 (3:1)
Bond enthalpies kJ/mol: C-C = 348; C-Cl = 338;
C-H = 412
Equations to explain the most abundant ion peaks of
2-chloropropane
Formation of m/z 63 and 65 ions:
[CH3CHClCH3]+ ===> [CH3CH35Cl]+
or [CH3CH37Cl]+ +
CH3
C-C bond fission where the end methyl group has
broken off.
Low probability because of the strong C-C bond
enthalpy.
Note the expected 3:1 ratio of chlorine containing
fragments.
Where R is alkyl, the double RCl m/z ion
peaks of roughly 3 : 1 abundance ratio are characteristic of
organo-chlorine compounds i.e. caused by the 3 : 1 isotope ratio of
35Cl : 37Cl.
Formation of m/z 44 ion:
[CH3CHClCH3]+
===> [C3H6]+
+ HCl
Elimination of hydrogen chloride from the parent
molecular ion (but see below).
Formation of m/z 43 ion:
[CH3CHClCH3]+ ===> [CH3CHCH3]+
+ Cl
Bond scission of the C-Cl bond, the weakest bond in
the molecule.
The m/z 43 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
The m/z 44 ion is probably formed in the same way, but containing a
13C carbon isotope atom i.e.
[13C12C2H7]+
and not
[C3H8]+
An accurate mass
spectrometer sorts this out, measuring relative fragment ion
masses to four decimal places e.g. using v ery accurate relative isotopic masses,
12C
= 12.0000 13C = 13.0034, 1H = 1.0078
from which you can calculate
(predict) that the accurate relative ion masses are:
For m/z 44: [C3H8]+
= 44.0624 and
[13C12C2H7]+ = 44.058,
a difference of 0.0044 in relative ion mass.
The m/z 43 ion can lose hydrogen atoms to give the
m/z 39, 41 and 42 ions (see data table).
Formation of m/z 42 ion:
[CH3CHClCH3]+ ===> [CH3CHCH3]+
+ HCl
Elimination of hydrogen chloride.
Formation of m/z 41 ion:
[?]+ ===> [C3H5]+
+ ?
Formation of m/z 39 ion:
[?]+ ===> [C3H3]+
+ ?
Formation of m/z 27 ion:
[CH3CHCl]+ ===> [C2H3]+
+ HCl
Elimination of hydrogen chloride from the m/z 63
and 65 ions.
Summary of key points for the mass spectrum of 2-chloropropane plus
extra exam revision comments
The mass spectrum of 2-chloropropane (CH3CHClCH3)
with precision, exam-board alignment, and misconception-busting
clarity.
Key Features of
the
mass spectrum of 2-chloropropane
- Molecular ion peaks: Two distinct peaks due to
chlorine isotopes (35Cl and 37Cl).
- Fragmentation pattern: Dominated by cleavage of
the C–Cl bond and rearrangements forming stable carbocations.
- Base peak: Typically at m/z = 43,
representing the most stable and abundant fragment.
Prominent m/z Ions
and Their Origins
the mass spectrum of 2-chloropropane
|
m/z |
Fragment Ion |
Origin / Structure |
Notes |
| 78 |
[CH3CH35ClCH3]+ |
Molecular ion with 35Cl |
Main molecular ion M⁺ peak |
| 80 |
[CH3CH37ClCH3]+ |
Molecular ion with 37Cl |
M⁺ + 2 peak due to isotope |
| 43 |
[CH3CHCH3]+ |
Propyl cation (loss of Cl) |
Base peak ion
— most intense |
| 63 |
[CH3CHCl]+ |
Loss of CH3 |
Contains Cl — isotope-sensitive |
| 65 |
[CH3CH37Cl]+ |
Loss of CH3 with 37Cl |
M⁺ + 2 version of above |
| 27 |
[C2H3]+
or [CH2CH]+ |
Rearranged fragment |
Often misunderstood — see below |
Common
Misconceptions about
the
mass spectrum of 2-chloropropane
(see below too)
- Confusing M⁺ and M⁺ + 2 peaks: Students often
forget chlorine has two isotopes, leading to two molecular ion peaks
at m/z = 78 and 80 in a 3:1 ratio.
- Misidentifying the base peak: The most intense
peak (m/z = 43) is not the molecular ion but a fragment — a
common trap.
- Overinterpreting low m/z peaks: Peaks like
m/z = 27 may arise from rearrangements or secondary
fragmentation, not simple bond cleavage.
- Assuming all fragments retain the Cl atom: Many
do not — especially the base peak.
Exam Revision Tips
for questions involving
the
mass spectrum of 2-chloropropane
(AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB)
(see above too)
- Isotope awareness: Chlorine’s 3:1 ratio (35Cl:37Cl)
is a frequent exam point — expect questions on M⁺ and M⁺ + 2 peak
heights.
- Base peak ≠ molecular ion: Know how to identify
both — base peak is the tallest, molecular ion is the highest m/z
with isotopic pair.
- Fragmentation logic: Practice deducing
fragments from bond cleavage and carbocation stability.
- Compare spectra: Be ready to distinguish
halogenoalkanes from alcohols or ketones based on fragmentation and
isotope patterns.
- Use data tables: Most boards provide m/z
reference tables — use them to justify peak assignments.
- Sketch fragmentation routes: Especially helpful
for OCR and IB where mechanistic understanding is tested.
Practice questions based on the
mass spectrum of 2-chloropropane
Three varied, technically rich
multiple-choice questions on the
mass spectrum of 2-chloropropane , designed for advanced
pre-university chemistry students across AQA, Edexcel, OCR, WJEC, CCEA, CIE,
IB, and US AP/Honors curricula.
These questions go beyond simple ion identification and explore isotopic
patterns, fragmentation logic, and structural comparison with isomers.
Question 1:
Isotopic Pattern Recognition
based on the mass spectrum of 2-chloropropane
In the mass spectrum of 2-chloropropane, two molecular ion peaks
are observed at m/z 78 and m/z 80 in a 3:1 ratio.
What does this pattern indicate?
- The molecule contains one chlorine atom, which has two isotopes with a
3:1 natural abundance ratio.
- The molecule contains two chlorine atoms, each contributing to the
isotopic pattern.
- The molecule contains one bromine atom, which has a 3:1 isotope ratio.
- The molecule contains both chlorine and bromine atoms, producing
overlapping isotope peaks.
Correct Answer: A
Explanation:
- Chlorine has two major isotopes: ³⁵Cl (≈75%) and ³⁷Cl (≈25%)
- A molecule with one chlorine atom shows a molecular ion
(M⁺) and M+2 peak in a 3:1 ratio
- 2-chloropropane contains one Cl atom, so the peaks at
m/z 78 (³⁵Cl) and m/z 80 (³⁷Cl) confirm
this
Distractor Analysis:
| Option |
Why It’s Incorrect |
| B |
Two Cl atoms would give a 9:6:1
triplet pattern due to binomial distribution |
| C |
Bromine has a 1:1 isotope ratio (⁷⁹Br
and ⁸¹Br), not 3:1 |
| D |
2-chloropropane contains only
chlorine, not bromine |
Question 2:
Fragmentation Logic
based on the mass spectrum of 2-chloropropane
A prominent fragment ion in the mass spectrum of 2-chloropropane
appears at m/z 43. Which of the following best explains the formation of this
fragment?
- Loss of chlorine radical to form a primary carbocation
- Loss of a methyl group to form a secondary carbocation
- Formation of a propyl cation after complete loss of chlorine and
hydrogen
- Cleavage of the C–Cl bond to form CH3CH⁺CH3
Correct Answer: D
Explanation:
- 2-chloropropane fragments by cleaving the C–Cl bond,
forming a secondary carbocation: CH3CH⁺CH3
- This ion has a mass of:
- C2H5
= 29
- CH3
= 15
→ Total = 43
This is a stable secondary carbocation, often the
base peak in the spectrum.
Distractor Analysis:
| Option |
Why It’s Incorrect |
| A |
A primary carbocation would be less
stable and appear at m/z 29 |
| B |
Loss of a methyl group would give m/z
63, not 43 |
| C |
Complete loss of Cl and H would not
yield a stable ion at m/z 43 |
Question 3: Isomer
Differentiation by Fragmentation
based on the mass spectrum of 2-chloropropane
Which of the following best explains why the mass spectrum of
2-chloropropane differs from that of 1-chloropropane, even though both have
the same molecular formula?
- 2-chloropropane has a higher molecular ion peak than 1-chloropropane due
to its branched structure
- 2-chloropropane forms a more stable secondary carbocation upon
fragmentation, leading to a stronger base peak
- 1-chloropropane contains bromine, which alters its fragmentation pattern
- 2-chloropropane undergoes rearrangement to form an alkene, while
1-chloropropane does not
Correct Answer: B
Explanation:
- Both isomers have the same molecular ion peak (m/z 78/80), but
fragment differently
- 2-chloropropane forms a secondary carbocation
(CH3CH⁺CH3)
→ m/z 43, highly stable
- 1-chloropropane forms a primary carbocation (CH3CH2CH2⁺)
→ m/z 43, less stable and less intense
- This difference in carbocation stability leads to
distinct base peaks
Distractor Analysis:
| Option |
Why It’s Incorrect |
| A |
Both isomers have the same molecular
ion mass; branching doesn’t affect M⁺ peak mass |
| C |
Neither molecule contains bromine |
| D |
Rearrangement to alkene is not the
dominant fragmentation pathway in either isomer under EI conditions |
Key words & phrases: C3H7Cl CH3CHClCH3 image diagram on how to interpret and explain the mass spectrum of
2-chloropropane m/z m/e base peaks, image and diagram of the mass spectrum of
2-chloropropane, details of the mass spectroscopy of 2-chloropropane, low and high resolution mass
spectrum of 2-chloropropane, prominent m/z peaks in the mass spectrum of
2-chloropropane, comparative
mass spectra of 2-chloropropane, the molecular ion peak in the mass spectrum of
2-chloropropane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2-chloropropane, characteristic pattern of peaks in the mass spectrum of
2-chloropropane, relative
abundance of mass ion peaks in the mass spectrum of 2-chloropropane, revising the mass
spectrum of 2-chloropropane, revision of mass spectroscopy of 2-chloropropane, most abundant ions in the
mass spectrum of 2-chloropropane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2-chloropropane, how to analyse the mass
spectrum of 2-chloropropane, how to describe explain the formation of fragmented ions in the
mass spectra of 2-chloropropane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2-chloropropane recognising the
base ion peak of 2-chloropropane interpreting
interpretation the mass spectrum of 2-chloropropane
isopropyl chloride How do you interpret the mass spectrum of
2-chloropropane How to interpret
the mass spectrum of 2-chloropropane Explanatory diagram of the mass spectrum of the
2-chloropropane molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
2-chloropropane. How to explain the mass spectrum of 2-chloropropane. The m/z value of the
molecular ion peak in the mass spectrum of 2-chloropropane. Identifying
2-chloropropane from
its mass spectrum pattern. The m/z m/e peak analysis of the mass
spectrum of the 2-chloropropane molecule. The uses of the mass spectrum of the
2-chloropropane molecule. The distinctive features of the mass spectrum of
the 2-chloropropane molecule explained. explaining the fragmentation pattern of the mass spectrum of
2-chloropropane equations showing the
formation of the ionised fragments in the mass spectrum of
2-chloropropane
what does the mass spectrum tell you about the structure and
properties of the 2-chloropropane molecule? Data table of ionised
fragments in the mass spectrum of 2-chloropropane and equations for their
formation in the fragmentation of 2-chloropropane molecules
Links associated
with
2-chloropropane
The
infrared spectrum of 2-chloropropane
The
H-1 NMR spectrum of 2-chloropropane
The
C-13 NMR spectrum of 2-chloropropane
The chemistry of HALOGENOALKANES (haloalkanes)
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