|
Interpreting the mass
spectrum of 2-chlorobutane
[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-chlorobutane
[updated
Mar 12th 2026 *]
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analysis infrared spectrum of
CH3CH2CHClCH3
Links associated
with 2-chlorobutane
The
chemistry of organic halogen compounds
This is a BIG
chemistry website, please take time to explore it
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 2-chlorobutane
Students and teachers please note
my explanation of the mass spectrum of 2-chlorobutane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2-chlorobutane 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-chlorobutane and only the formation of singly charged
positive are considered for the mass spectrum of 2-chlorobutane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
2-chlorobutane
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-chlorobutane.
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-chlorobutane
For more see
Molecular structure, classification and
naming of
halogenoalkanes (haloalkanes)
Interpreting the fragmentation pattern of the mass spectrum of
2-chlorobutane
[M]+ is the molecular ion peak (M) with an m/z of
92 corresponding to [C4H9Cl]+, the original 2-chlorobutane molecule minus an electron,
[CH3CH35ClCH2CH3]+
Since this ion is so unstable, there is less chance of observing
m/z 94 M+2 ion [CH3CH37ClCH2CH3]+
(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, 65 and
63, and 64 and
62, you can see they are
roughly in the ratio 3 : 1 in the mass spectrum diagram above.
You might, but not here, see a very tiny M+1 peak at m/z 93, corresponds to an ionised
2-chlorobutane
molecule with one 13C atom in it i.e. an ionised 2-chlorobutane 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.
2-chlorobutane 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-chlorobutane) 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 peak ion for
the mass spectrum of 2-chlorobutane is the m/z 57 ion
[C4H9]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2-chlorobutane.
Unless otherwise indicated, assume the carbon atoms in
2-chlorobutane are the 12C isotope.
The parent molecular ions
for the mass spectrum of 2-chlorobutane are the
m/z of
92 and 94 ions corresponding to [
[CH3CH35ClCH2CH3]+
and [CH3CH37ClCH2CH3]+
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of 2-chlorobutane.
|
m/z value
[fragment]+ |
79 |
77 |
65 |
64 |
63 |
62 |
|
[molecular fragment]+ |
[CH3CH37ClCH2]+ |
[CH3CH35ClCH2]+ |
[CH2CH237Cl]+ |
[CH2CH37Cl]+ |
[CH2CH235Cl]+ |
[CH2CH35Cl]+ |
|
m/z value of
[fragment]+ |
58 ? |
57
[C4H9]+ |
56 |
55 |
51 |
50 ? |
49 |
|
[molecular fragment]+ |
[13CC3H8]+ |
[CH3CHCH2CH3]+ |
[C4H8]+ |
[C4H7]+ |
[CH237Cl]+ |
[CH335Cl]+ |
[CH235Cl]+ |
|
m/z value of
[fragment]+ |
43
[C3H7]+ |
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 2-chlorobutane
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
Possible equations to explain some of the most abundant ion peaks
in the mass spectrum of
2-chlorobutane
Note the molecular ion peaks (M and M+2) are very small indicating
the parent molecular ion of 2-chlorobutane fragments very easily.
Formation of m/z 77 and 79 ions:
[CH3CHClCH2CH3]+ ===> [CH3CH37ClCH2]+
or [CH3CH35ClCH2]+
+ CH3
C-C bond scission to free an end methyl group.
Low probability due to strength of C-C bond,
scission of the weaker C-Cl bond more likely.
The ions could also be [CH37ClCH2CH3]+
and [CH35ClCH2CH3]+,
either way the m/z 77 and 79 ions correspond to [C3H6Cl]+.
Mass loss 92 - 15 = 77 and 94 - 15 = 79.
Note the expected 3:1 ratio of intensities expected
for chlorine containing fragment ions.
Formation of m/z 63 and 65 ions:
[CH3CHClCH2CH3]+ ===> [CH3CH37Cl]+
or [CH3CH37Cl]+ +
CH2CH3
C-C bond scission to free ethyl group.
Low probability due to strength of C-C bond,
scission of the weaker C-Cl bond more likely.
Note the expected 3:1 ratio of intensities expected
for chlorine containing fragment ions.
Mass loss 92 - 29 = 63 and 94 - 15 = 65.
Formation of m/z 62 and 64 ions:
[CHClCH2CH3]+ ===> [CH2CH35Cl]+
or [CH2CH37Cl]+ +
CH3
C-C bond scission to free a methyl group from the
m/z 77 and 79 ions.
Mass losses: 77 - 15 = 62 and 79 - 15
= 64 for the fragment ion [C2H3Cl]+.
Low probability due to strength of C-C bond,
scission of the weaker C-Cl bond more likely.
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 57 ion:
[CH3CHClCH2CH3]+ ===> [CH3CHCH2CH3]+
+ Cl
Formed by the scission of the C-Cl bond, the weakest
bond in the 2-chlorobutane molecule.
The m/z 57 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
The m/z 57 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 the m/z peak of 58 could correspond with the
ion [13C12C3H9]+
rather than the ion
[C4H10]+,
and formed in the same way as the m/z 57 ion.
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, you can then calculate
(predict) that the accurate relative ion masses are:
For m/z 58:
[13C12C3H9]+
= 58.0736,
[C4H10]+
= 58.0780,
a difference of 0.0044 in relative ion mass.
Formation of m/z 56 ion:
[CH3CHClCH2CH3]+ ===> [C4H8]+
+ HCl
Elimination of hydrogen chloride from the parent
molecular ion.
A favourable reaction, since the m/z 56 ion
intensity is almost the same as the m/z 57 base ion peak.
Formation of m/z 41 ion:
[C4H8]+ ===> [C3H5]+
+ CH3
Formation of m/z 39 ion:
[C3H5]+ ===> [C3H3]+
+ H2
Formation of m/z 29 ion:
[CH3CHClCH2CH3]+ ===> [C2H5]+
+ CH3CHCl
C-C bond scission in the parent molecular ion.
Formation of m/z 28 ion:
[C2H5]+ ===> [C2H4]+
+ H
Ionised ethene molecule formed.
Formation of m/z 27 ion:
[C2H5]+ ===> [C2H3]+
+ H2
Formation of m/z 15 ion:
[(CH3)3C35Cl]+ ===> [CH3]+
+ (CH3)2CCl
C-C bond scission of the parent molecular ion
(or other fragment) to free a positively charged methyl group.
Summary of key points for
the infrared spectrum of 2-chlorobutane plus extra exam revision
comments and practice questions
The mass spectrum of 2-chlorobutane with m/z ion table
Key Fragment Ions
in
the infrared spectrum of
2-chlorobutane
|
m/z |
Ion |
Origin /
Fragmentation |
| 92 |
Molecular ion [M]⁺• |
C4H9Cl intact
molecule - electron |
| 57 |
C4H9⁺
base ion peak |
Loss of Cl radical (M⁺• – Cl•) |
| 56 |
C4H8⁺ |
Loss of HCl (M⁺• – HCl) |
| 41 |
C3H5⁺ |
Further fragmentation of alkyl chain |
| 29 |
C2H5⁺ |
Ethyl cation from C-C chain cleavage |
| 27 |
C2H3⁺ |
Vinyl cation from deeper fragmentation |
The base peak (most intense) is typically m/z 57,
representing the sec-butyl cation, a stable fragment due to
carbocation stability.
Common
Misconceptions about
the infrared spectrum of
2-chlorobutane
(see also below)
- Confusing molecular ion with base peak: The molecular
ion (m/z 92) is often weak or absent due to fragmentation; students may
wrongly assume it’s always the tallest peak.
- Ignoring isotopic patterns: Chlorine has two
isotopes—³⁵Cl and ³⁷Cl—so expect a M+2 peak at m/z 94 with
~1/3 intensity of m/z 92.
- Assuming all fragments are simple alkyl ions: Some
peaks arise from rearrangements or loss of neutral molecules like HCl.
Exam Tips for
questions involving
the infrared spectrum of
2-chlorobutane
(see also above)
- Mention isotopic signature: Chlorine-containing
compounds show a distinct M and M+2 pattern—highlighting this earns marks.
- Use fragmentation logic: Explain how loss of Cl or HCl
leads to key ions like m/z 57 or 56.
- Compare with IR or NMR: If asked about limitations,
suggest complementary techniques for full structural elucidation.
- Don’t overinterpret minor peaks: Focus on major ions
and their logical origins unless asked for detailed analysis.
Practice
questions based on the mass spectrum of 2-chlorobutane
Two technically sound and curriculum-aligned
multiple-choice questions on the mass spectrum of 2-chlorobutane
(C4H9Cl).
These are designed to test deeper understanding of
fragmentation patterns, isotopic effects, and structural implications — not just
ion identification — and are suitable for AQA, Edexcel, OCR, WJEC, CCEA, CIE,
IB, and US AP/Honors chemistry students.
Question 1:
Isotopic Signature and Structural Insight
In the mass spectrum of 2-chlorobutane, two molecular ion peaks
are observed at m/z 92 and m/z 94 in a 3:1 intensity ratio.
What does this pattern reveal about the molecule?
- The molecule contains one chlorine atom, which exists as two isotopes
with a 3:1 natural abundance ratio.
- The molecule contains two chlorine atoms, each contributing to the
isotopic pattern.
- The molecule contains a bromine atom, which has a 3:1 isotope ratio.
- The molecule contains a mixture of chlorine and bromine atoms, producing
overlapping isotope peaks.
Correct Answer: A
Explanation:
- Chlorine has two major isotopes:
- A molecule with one chlorine atom will show a
molecular ion (M⁺) peak and an M+2 peak in a
3:1 ratio.
- 2-chlorobutane contains one Cl atom, so the observed
pattern at m/z 92 (³⁵Cl) and m/z 94 (³⁷Cl)
confirms this.
- It also shows that it is a C4 chloro-alkane based on the
molecular ion values.
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 |
The molecule contains only
chlorine; no bromine is present. |
Question 2:
Fragmentation and Carbocation Stability in the mass spectrum of 2-chlorobutane
In the mass spectrum of 2-chlorobutane, a prominent fragment ion
appears at m/z 57.
Which of the following best explains the formation of this peak?
- It results from cleavage of the C–Cl bond, forming a secondary
carbocation.
- It is the molecular ion peak, representing the intact molecule.
- It arises from the loss of a methyl radical from the molecular ion.
- It is formed by rearrangement to a more stable tertiary carbocation.
Correct Answer: A
Explanation:
- 2-chlorobutane undergoes fragmentation by losing the chlorine atom
(Cl•), forming a secondary carbocation: CH3CH⁺CH2CH3
- This ion has m/z 57 and is relatively stable due to the
secondary carbon center.
- This fragment is often the base peak due to its
abundance and stability.
Distractor Analysis:
| Option |
Why It’s
Incorrect |
| B |
The molecular ion is at m/z 92/94,
not 57. |
| C |
Loss of a methyl radical would
give m/z 77, not 57. |
| D |
No rearrangement to a tertiary
carbocation occurs in this structure; 2-chlorobutane lacks a
tertiary center. |
|
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). |
 |
 |
 |
 |
|
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. |
 |
|
 |
 |
|
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. |
 |
 |
 |
 |
|
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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 |
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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 CH3CHClCH2CH3 image diagram on how to interpret and explain the mass spectrum of
2-chlorobutane m/z m/e base peaks, image and diagram of the mass spectrum of
2-chlorobutane, details of the mass spectroscopy of 2-chlorobutane, low and high resolution mass
spectrum of 2-chlorobutane, prominent m/z peaks in the mass spectrum of
2-chlorobutane, comparative
mass spectra of 2-chlorobutane, the molecular ion peak in the mass spectrum of
2-chlorobutane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2-chlorobutane, characteristic pattern of peaks in the mass spectrum of
2-chlorobutane, relative
abundance of mass ion peaks in the mass spectrum of 2-chlorobutane, revising the mass
spectrum of 2-chlorobutane, revision of mass spectroscopy of 2-chlorobutane, most abundant ions in the
mass spectrum of 2-chlorobutane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2-chlorobutane, how to analyse the mass
spectrum of 2-chlorobutane, how to describe explain the formation of fragmented ions in the
mass spectra of 2-chlorobutane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2-chlorobutane recognising the
base ion peak of 2-chlorobutane interpreting interpretation the mass spectrum of
2-chlorobutane sec-butyl chloride
Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
2-chlorobutane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 2-chlorobutane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 2-chlorobutane. The m/e m/z value of the base ion peak in the
mass spectrum of 2-chlorobutane. Possible examples of equations showing the formation
of the ionised fragments in 2-chlorobutane. Revision notes on the mass spectrum of
2-chlorobutane.
Matching and deducing the structure of the 2-chlorobutane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic halogenoalkanes
haloalkanes alkyl halides alkyl chlorides chloroalkanes,
mass spectra of 2-chlorobutane, an isomer of molecular formula
C4H9Cl explaining the m/z ion
fragmentation pattern of 2-chlorobutane How do you interpret the mass spectrum of
2-chlorobutane How to interpret
the mass spectrum of 2-chlorobutane Explanatory diagram of the mass spectrum of the
2-chlorobutane molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
2-chlorobutane. How to explain the mass spectrum of 2-chlorobutane. The m/z value of the
molecular ion peak in the mass spectrum of 2-chlorobutane. Identifying
2-chlorobutane from
its mass spectrum pattern. The m/z m/e peak analysis of the mass
spectrum of the 2-chlorobutane molecule. The uses of the mass spectrum of the
2-chlorobutane molecule. The distinctive features of the mass spectrum of
the 2-chlorobutane molecule explained. explaining the fragmentation pattern of the mass spectrum of
2-chlorobutane equations showing the
formation of the ionised fragments in the mass spectrum of
2-chlorobutane
what does the mass spectrum tell you about the structure and
properties of the 2-chlorobutane molecule? Data table of ionised
fragments in the mass spectrum of 2-chlorobutane and equations for their
formation in the fragmentation of 2-chlorobutane molecules
Links associated
with
2-chlorobutane
The chemistry of HALOGENOALKANES (haloalkanes)
revision notes INDEX
The
infrared spectrum of 2-chlorobutane (sec-butyl
chloride)
The
H-1 NMR spectrum of 2-chlorobutane (sec-butyl
chloride)
The
C-13 NMR spectrum of 2-chlorobutane (sec-butyl
chloride)
Mass spectroscopy index
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