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Interpreting the mass
spectrum of 1-chloro-2-methylpropane
(CH3)2CHCH2Cl
[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
1-chloro-2-methylpropane
[updated
Mar 11th 2026 *]
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mass spectrum of
(CH3)2CHCH2Cl
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The
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Mass spectrometry
- introduction and spectra index
See also
Comparing infrared, mass, 1H NMR & 13C NMR
spectra of the 4 structural isomers of C4H9Cl
Introductory note on the mass spectrum of 1-chloro-2-methylpropane
Students and teachers please note
my explanation of the mass spectrum of 1-chloro-2-methylpropane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
1-chloro-2-methylpropane 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 1-chloro-2-methylpropane and only the formation of singly charged
positive are considered for the mass spectrum of
1-chloro-2-methylpropane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
1-chloro-2-methylpropane
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
1-chloro-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 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!
 ,
 ,
 ,
 ,
1-chloro-2-methylpropane
For more see
Molecular structure, classification and
naming of
halogenoalkanes (haloalkanes)
Interpreting the fragmentation pattern of the mass spectrum of
1-chloro-2-methylpropane
[M]+ is the tiny
molecular ion peak
(M) with an
m/z of
92 corresponding to [C4H9Cl]+, the original 1-chloro-2-methylpropane molecule minus an electron,
[(CH3)2CHCH235Cl]+
Since this ion is so unstable, there is
less chance of observing the
m/z 94 M+2 ion
[(CH3)2CHCH237Cl]+
(see note below on isotopes).
Since chlorine has two common isotopes of
35Cl
and 37Cl in the approximate ratio of 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 of
1-chlorobutane.
Two examples of this
are quoted in the table below for m/z values of 79 and 77, and 51
and 49, you can see they are roughly in a ratio 3 : 1 in the mass spectrum diagram for
1-chloro-2-methylpropane above - look out for fragments with a
chlorine atom in them.
The very tiny M+1 peak at m/z 93, corresponds to an ionised
1-chloro-2-methylpropane
molecule with one 13C atom in it i.e. an ionised
1-chloro-2-methylpropane molecule of
formula [13C12C3H935Cl]+
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-chloro-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-chloro-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.
Base ion peak for the
mass spectrum of 1-chloro-2-methylpropane is the m/z 43 ion
[CH3CHCH3]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 1-chloro-2-methylpropane.
Unless otherwise indicated, assume the carbon atoms in
1-chloro-2-methylpropane are the 12C isotope.
The parent molecular ions are
m/z ions 92 and 94
[(CH3)2CHCH235Cl]+
and [(CH3)2CHCH237Cl]+
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of 1-chloro-2-methylpropane.
|
m/z value of
[fragment]+ |
79 |
77 |
57 |
56 |
51 |
49 |
|
[molecular fragment]+ |
[CH3CHCH237Cl]+ |
[CH3CHCH235Cl]+ |
[(CH3)2CHCH2]+ |
[C4H8]+ |
[CH237Cl]+ |
[CH235Cl]+ |
|
m/z value of
[fragment]+ |
43 |
42 |
41 |
39 |
29 |
28 |
27 |
|
[molecular fragment]+ |
[CH3CHCH3]+ |
[C3H6]+ |
[C3H5]+ |
[C3H3]+ |
[CH3CH2]+ |
[C2H4]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 1-chloro-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); Cl = 35 or 37 (ratio ~3:1)
Bond enthalpies kJ/mol: C-C = 348; C-Cl = 338;
C-H = 412
Possible equations to explain some of the most abundant ion peaks
in the mass spectrum of 1-chloro-2-methylpropane
Note the molecular ion peaks (M and M+2) are very small indicating
the parent molecular ion of 1-chloro-2-methylpropane fragments very easily.
Formation of m/z 77 and 79 ions:
[(CH3)2CHCH235Cl]+ ===> [CH3CHCH235Cl]+
or [CH3CHCH237Cl]+
+ CH3
Either way the m/z 77 and 79 ions correspond to
[C3H6Cl]+.
C-C bond scission in the parent molecular ion to
free an end methyl group.
Low probability due to strength of C-C bond,
scission of the weaker C-Cl bond more likely.
Mass loss 92 - 15 = 77 and 94 - 15 =
79.
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 57 ion:
[(CH3)2CHCH2Cl]+ ===> [(CH3)2CHCH2]+
+ Cl
Formed by the scission of the C-Cl bond, the weakest
bond in the 1-chloro-2-methylpropane molecule.
Mass change 92 - 35 = 57 or 94 - 37 = 57.
Formation of m/z 56 ion:
[(CH3)2CHCH2Cl]+ ===> [C4H8]+
+ H35Cl or H37Cl
Elimination of hydrogen chloride from the parent
molecular ion to form an ionised butene fragment.
Mass loss 92 - 36 = 56 or 94 - 38 = 56.
Formation of m/z 49 and 51 ions:
[(CH3)2CHCH2Cl]+ ===> [CH2Cl]+
+ C3H7
C-C bond scission in the parent molecular ion.
Mass changes 94 - 43 = 51 or 92 - 43 =
49.
Note the expected 3:1 ratio of intensities
expected for chlorine containing fragment ions.
This ionisation is less likely than the ionisation
of the alkyl group lost - see m/z 43 ion
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.
below.
Formation of m/z 43 ion:
[(CH3)2CHCH2Cl]+ ===> [C3H7]+
+ CH2Cl
The m/z 43 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
Formed by C-C bond scission of the carbon chain in
the parent molecular ion.
The m/z 43 ion is a secondary carbocation, a stable
type of alkyl based ion, the positive charge is stabilised by the +I
(inductive) effect of the two alkyl groups.
One reason why the ionised fragments, not containing
chlorine, are more likely to be formed, is the more electronegative
chlorine tends to make the chlorine containing fragment retain the
electrons.
Note that the m/z
ion peak of 44 could correspond to the
ion [13C12C2H7]+
rather than
[C3H8]+
Note that an accurate mass
spectrometer can sort out (resolve) pairs of ions with the same
integer m/z value because they can measure relative fragment ion
masses to four decimal places,
e.g. using accurate relative isotopic masses:
1H
= 1.0078 12C
= 12.0000
13C = 13.0034: you can then 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
Formation of m/z 42 ion:
[C3H7]+ ===> [C3H6]+
+ H
[C4H9]+ ===> [C3H6]+
+ CH3
Ionised propene molecule formed.
Formation of m/z 41 ion:
[C3H7]+ ===> [C3H5]+
+ H2
Formation of m/z 39 ion:
[C3H5]+ ===> [C3H3]+
+ H2
Formation of m/z 29 ion:
[CH3CHClCH2CH3]+
===> [C2H5]+
+ CH3CHCl
C-C bond scission of the parent molecular ion.
The m/z 29 ion can
lose hydrogen atoms to give the m/z 28, 27 and 26 ions.
Formation of m/z 27 ion:
[C2H5]+ ===> [C2H3]+
+ H2
Summary of the
mass spectrum of 1-chloro-2-methylpropane and extra comments
The mass spectrum of 1-chloro-2-methylpropane
(isobutyl chloride) with exam-ready clarity.
Overview of the
fragmentation pattern displayed by the
mass
spectrum of 1-chloro-2-methylpropane
Mass spectrometry of alkyl halides like 1-chloro-2-methylpropane
typically involves:
- Electron ionization (EI) causing cleavage of C–C and
C–Cl bonds
- Formation of molecular ion (M⁺·) and
characteristic fragments
- Isotopic pattern due to chlorine (³⁵Cl and ³⁷Cl)
Prominent m/z Ions
and origins
for the mass spectrum of 1-chloro-2-methylpropane
|
m/z |
Fragment Ion (+) |
Origin / Description |
| 92 |
M⁺· (C4H9Cl) |
Molecular ion with ³⁵Cl isotope |
| 94 |
M⁺· + 2 |
Molecular ion with ³⁷Cl isotope (1:3
ratio) |
| 57 |
C4H9 |
Loss of Cl· radical (alkyl cation) |
| 43 |
C3H7
(isopropyl) |
Further fragmentation of alkyl chain,
base peak ion |
| 42 |
C3H6 |
Loss of H from
C3H7
or loss of CH3+ from
C4H9+ |
| 41 |
C3H5
(allyl-like) |
Rearranged fragment from alkyl
backbone |
| 35/37 |
Cl⁺ |
Chlorine cation (low intensity,
confirms halogen) |
Common
Misconceptions about
the
mass spectrum of 1-chloro-2-methylpropane
(see also below)
- Confusing M⁺· with base peak: The molecular ion
is often weak or absent in mass spectra; students may wrongly expect
it to dominate.
- Ignoring isotopic peaks: The ³⁵Cl/³⁷Cl pattern
is diagnostic — overlooking it can lead to misidentification.
- Assuming all fragments are stable cations: Some
peaks arise from rearranged or resonance-stabilized ions.
Exam Tips for
questions involving
the
mass spectrum of 1-chloro-2-methylpropane
(see also above)
- Highlight the Cl isotope pattern: A 2 m/z
spacing with a 3:1 intensity confirms chlorine.
- Annotate fragmentation pathways: Show how each
ion forms from bond cleavage or rearrangement.
- Compare with isomers: Isobutyl chloride versus
tert-butyl chloride gives different fragmentation due to branching.
- Use base peak wisely: Often the most stable
carbocation (e.g.
C4H9⁺)
— not necessarily the molecular ion.
Practice Question:
Mass Spectrum of 1-Chloro-2-Methylpropane
Compound: 1-chloro-2-methylpropane (C4H9Cl)
A student analyses the mass spectrum of 1-chloro-2-methylpropane.
The spectrum shows two molecular ion peaks at m/z 92 and m/z
94, and a base peak at m/z 57.
Question:
- Explain why the molecular ion peak appears as a pair at m/z 92
and m/z 94.
- Identify the fragment ion responsible for the base peak at m/z
57 and explain its formation.
- Predict the relative intensities of the m/z 92 and m/z
94 peaks and justify your answer using isotopic abundances.
- Suggest why the molecular ion peak is less intense than the base peak.
- Explain how the fragmentation pattern helps distinguish
1-chloro-2-methylpropane from its isomer 2-chlorobutane.
Model Answer
a) Isotopic Pair at m/z 92 and 94
- Chlorine has two major isotopes: ³⁵Cl and ³⁷Cl.
- 1-chloro-2-methylpropane contains one chlorine atom, so the molecular
ion exists in two forms:
- C4H9³⁵Cl → m/z 92
- C4H9³⁷Cl → m/z 94
- These peaks are two mass units apart due to the isotopic difference.
b) Fragment Ion at m/z 57
- The m/z 57 peak corresponds to the tert-butyl cation (C₄H₉⁺).
- It forms when the Cl atom is lost as a neutral radical:
- C4H9Cl → C4H9⁺ (m/z
57) + Cl•
- This cation is highly stable due to tertiary carbocation stabilization.
c) Relative Intensities of m/z 92 and
94
- Natural abundance of chlorine isotopes:
- ³⁵Cl ≈ 75.8%
- ³⁷Cl ≈ 24.2%
- Therefore, the m/z 92 peak will be about three times more
intense than the m/z 94 peak, forming a 3:1 ratio.
d) Molecular Ion Peak Intensity
- The molecular ion peak is less intense because:
- The C–Cl bond is relatively weak and easily cleaved.
- The tert-butyl cation is highly stable, so fragmentation is
favoured.
- Many molecules fragment before reaching the detector intact.
e) Distinguishing from 2-Chlorobutane
- Both compounds show molecular ion peaks at m/z 92 and 94 due to
Cl isotopes.
- However, their fragmentation differs:
- 1-chloro-2-methylpropane forms a stable tert-butyl cation (m/z
57), which becomes the base peak.
- 2-chlorobutane forms a less stable secondary butyl cation (m/z
57 or m/z 43), often with a different base peak.
- The dominance and stability of the m/z 57 peak in
1-chloro-2-methylpropane helps distinguish it.
|
Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the 4 halogenoalkane isomers of C4H9Cl
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-chlorobutane,
2-chlorobutane, 1-chloro-2-methylpropane and 2-chloro-2-methylpropane
image sizes. These four molecules
are structural isomers of molecular formula C4H9Cl
and
exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower
aliphatic halogenoalkanes (haloalkanes, alkyl halides,
chloroalkanes, alkyl chlorides). |
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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. The infrared spectrum of
2-chloro-2-methylpropane is noticeably simpler in the
fingerprint region, perhaps due to
the greater symmetry of the molecule. |
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MASS SPECTRA (above):
Theoretically, all four can give the parent molecular ions of
m/z 92 and 94, but they are all relatively tiny peaks.
2-chlorobutane and 2-chloro-2-methylpropane give a base ion peak
of m/z 57. The base ion peak for 1-chlorobutane is m/z 56 and
that of 1-chloro-2-methylpropane is m/z 43. Each gives different
patterns of pairs of m/z values two mass units apart, in the
peak height ratio of 3:1, if the positive fragment contains a
chlorine atom (35Cl or 37Cl) e.g look for
m/z pairs 49/51, 63/65 and 77/79 in their mass spectra. |
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1H NMR SPECTRA
(above): The 1H NMR spectra of all four molecules give different
integrated proton ratios i.e.1-chlorobutane
four peaks of ratio 3:2:2:2; 2-chlorobutane four peaks of
ratio 3:3:2:1,
1-chloro-2-methylpropane three peaks of ratio 6:2:1 and
2-chloro-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-chlorobutane and
2-chlorobutane show four 13C NMR resonances,
1-chloro-2-methylpropane three 13C NMR resonances and
2-chloro-2-methylpropane only two 13C resonances (3 and 2
chemical environments respectively. Therefore
1-chloro-2-methylpropane and 2-chloro-2-methylpropane can be
distinguished from the other three by their number of resonances
in their 13C NMR spectra, but 1-chlorobutane and 2-chlorobutane
cannot be distinguished from each other from their number of 13C
NMR resonance lines - other data would be required. |
Key words & phrases: C4H9Cl (CH3)2CH2Cl image diagram on how to interpret and explain the mass spectrum of
1-chloro-2-methylpropane m/z m/e base peaks, image and diagram of the mass spectrum of
1-chloro-2-methylpropane, details of the mass spectroscopy of
1-chloro-2-methylpropane, low and high resolution mass
spectrum of 1-chloro-2-methylpropane, prominent m/z peaks in the mass spectrum of
1-chloro-2-methylpropane, comparative
mass spectra of 1-chloro-2-methylpropane, the molecular ion peak in the mass spectrum of
1-chloro-2-methylpropane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 1-chloro-2-methylpropane, characteristic pattern of peaks in the mass spectrum of
1-chloro-2-methylpropane, relative
abundance of mass ion peaks in the mass spectrum of 1-chloro-2-methylpropane, revising the mass
spectrum of 1-chloro-2-methylpropane, revision of mass spectroscopy of
1-chloro-2-methylpropane, most abundant ions in the
mass spectrum of 1-chloro-2-methylpropane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 1-chloro-2-methylpropane, how to analyse the mass
spectrum of 1-chloro-2-methylpropane, how to describe explain the formation of fragmented ions in the
mass spectra of 1-chloro-2-methylpropane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 1-chloro-2-methylpropane
recognising the base ion peak of 1-chloro-2-methylpropane
interpreting interpretation the mass spectrum of 1-chloro-2-methylpropane
isobutyl chloride
Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
1-chloro-2-methylpropane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 1-chloro-2-methylpropane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 1-chloro-2-methylpropane. The m/e m/z value of the base ion peak in the
mass spectrum of 1-chloro-2-methylpropane. Possible examples of equations showing the formation
of the ionised fragments in 1-chloro-2-methylpropane. Revision notes on the mass spectrum of
1-chloro-2-methylpropane.
Matching and deducing the structure of the 1-chloro-2-methylpropane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic halogenoalkanes
haloalkanes alkyl halides alkyl chlorides chloroalkanes,
mass spectra of 1-chloro-2-methylpropane, an isomer of molecular formula C4H9Cl
How do you interpret the mass spectrum of
1-chloro-2-methylpropane How to interpret
the mass spectrum of 1-chloro-2-methylpropane Explanatory diagram of the mass spectrum of the
1-chloro-2-methylpropane molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
1-chloro-2-methylpropane. How to explain the mass spectrum of
1-chloro-2-methylpropane. The m/z value of the
molecular ion peak in the mass spectrum of 1-chloro-2-methylpropane. Identifying
1-chloro-2-methylpropane from
its mass spectrum pattern. The m/z m/e peak analysis of the mass
spectrum of the 1-chloro-2-methylpropane molecule. The uses of the mass spectrum of the
1-chloro-2-methylpropane molecule. The distinctive features of the mass spectrum of
the 1-chloro-2-methylpropane molecule explained. explaining the fragmentation pattern of the mass spectrum of
1-chloro-2-methylpropane equations showing the
formation of the ionised fragments in the mass spectrum of
1-chloro-2-methylpropane
what does the mass spectrum tell you about the structure and
properties of the 1-chloro-2-methylpropane molecule?
Links associated
with
1-chloro-2-methylpropane
The chemistry of HALOGENOALKANES (haloalkanes)
revision notes INDEX
The infrared
spectrum of 1-chloro-2-methylpropane (isobutyl
chloride)
The H-1 NMR
spectrum of 1-chloro-2-methylpropane (isobutyl
chloride)
The C-13
NMR spectrum of 1-chloro-2-methylpropane (isobutyl
chloride)
Mass spectrometry - introduction and spectra index
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