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Interpreting
and explaining
the
mass spectrum of
1-chlorobutane
CH3CH2CH2CH2Cl
[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-chlorobutane
[updated
Mar 11th 2026 *]
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mass spectrum of
CH3CH2CH2CH2Cl
LINKS associated
with 1-chlorobutane
The
chemistry of organic halogen compounds
This is a BIG
website, you need to 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 1-chlorobutane
Students and teachers please note
my explanation of the mass spectrum of 1-chlorobutane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
1-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 1-chlorobutane and only the formation of singly charged
positive are considered for the mass spectrum of 1-chlorobutane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
1-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
1-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!
1-chlorobutane C4H9Cl,
 ,
 ,
For more see
Molecular structure, classification and
naming of
halogenoalkanes (haloalkanes)
Interpreting the mass spectrum of 1-chlorobutane
P arent
molecular ion peaks are m/z 92
[CH3CH2CH2CH235Cl]+
and m/z 94
[CH3CH2CH2CH237Cl]+
corresponds to the molecular ion peaks M and M+2 in the
ratio 3 : 1.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of 2-chloro-2-methylpropane.
|
m/z value of
[fragment]+ |
65 |
63 |
57 |
56 |
55 |
51 |
49 |
|
[molecular fragment]+ |
[CH2CH237Cl]+ |
[CH2CH235Cl]+ |
[CH3CH2CH2CH2]+ |
[C4H8]+ |
[C4H7]+ |
[CH237Cl]+ |
[CH235Cl]+ |
|
m/z value of
[fragment]+ |
43 |
42 |
41 |
39 |
29 |
28 |
27 |
|
[molecular fragment]+ |
[CH3CH2CH2]+ |
[C3H6]+ |
[C3H5]+ |
[C3H3]+ |
[CH3CH2]+ |
[C2H4]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 1-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.
[M]+ is the molecular ion peak (M) with an m/z of
92 corresponding to [C4H9Cl]+, the original
1-chlorobutane molecule minus an electron,
[CH3CH2CH2CH2Cl]+
[CH3CH2CH2CH235Cl]+
corresponds to the molecular ion peak M (m/z 92). because of the
greater abundance of the 35Cl isotope.
[CH3CH2CH2CH237Cl]+
corresponds to the molecular ion peak M+2 (m/z 94),
ratio 3 : 1.
The molecular ions are so unstable, there is even less chance of observing the
m/z 94 M+2 ion peak of [CH3CH2CH2CH237Cl]+
(see note below on isotopes).
The base ion peak for the
mass spectrum of 1-chlorobutane is the m/z 56 ion
[C4H8]+
Note the
expected 3:1 ratio of intensities for chlorine containing ions, why?.
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
ions or
fragment ions containing a chlorine atom from the ionisation and fragmentation of
1-chlorobutane.
Two examples of this are quoted above for m/z 63 and
65, 49 and
51, and 35 and 37, although small, you can see their
intensities are
roughly in the ratio 3 : 1.
Note the molecular ion peaks (M and M+2) are very small indicating
the parent molecular ion of 1-chlorobutane fragments very easily.
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
Suggested possible equations to explain some of the most abundant ion peaks
in the mass spectrum of
1-chlorobutane
Small peaks at m/z 49 equate to the [CH235Cl]+
ion and at m/z 63 [CH2CH235Cl]+.
C-C bond scission in
the parent molecular ion.
Tiny peaks at m/z 51 and m/z 65 corresponds to [CH237Cl]+
and [CH2CH237Cl]+.
[CH3CH2CH2CH2Cl]+
===> [CH235Cl]+ or
[CH237Cl]+ + CH3CH2CH2
C-C bond
scission in the parent molecular ion.
[CH3CH2CH2CH2Cl]+
===> [CH2CH235Cl]+
or [CH2CH237Cl]+
+ CH3CH2
Formed by C-C bond scission in parent molecular
ion, low probability due
to strong bond enthalpy, the weaker C-Cl bond is more likely to
be broken.
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.
The most abundant ion, the
base ion peak, m/z 56, is
formed by the elimination of hydrogen chloride from the parent
molecular ion and equates to an ionised butene molecule.
[CH3CH2CH2CH2Cl]+
===> [C4H8]+ +
HCl
Mass changes: 94 - 38 = 56 or 92 -
36 = 56 depending on the chlorine isotope.
Note the m/z peak of 57 could correspond with
the ion [13C12C3H8]+
via the process above, but is most likely to be formed by C-Cl
bond scission of the parent molecular ion and the loss of a
chlorine radical
[CH3CH2CH2CH2Cl]+
===> [C4H9]+ +
Cl
m/z values of 43 and 29 correspond to alkyl fragments of
the original linear chain formed by scission of the C-C bond in the
carbon chain of the 1-chlorobutane parent molecular ion.
[CH3CH2CH2CH2Cl]+
===> [C3H7]+ +
CH2Cl
[CH3CH2CH2CH2Cl]+
===> [C2H5]+ +
CH2CH2Cl
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.
Formation of m/z 42 ion:
[?]+ ===> [C3H6]+
+ ?
Ionised propene molecule formed.
Formation of m/z 41 ion:
[?]+ ===> [C3H5]+
+ ?
Formation of m/z 39 ion:
[C3H5]+ ===> [C3H3]+
+ H2
Formation of m/z 29 ion:
[?]+ ===> [C2H5]+
+ ?
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:
[CH3CH2CH2CH2Cl]+ ===> [CH3]+
+ CH2CH2CH2Cl
C-C bond scission of the parent molecular ion (or
other fragment) to free a positively charged methyl group
Summary of the mass spectrum of 1-chlorobutane and extra
comments and practice questions
A structured breakdown of the mass spectrum of
1-chlorobutane (C4H9Cl), with
emphasis on key fragment ions, common pitfalls, and exam
strategies.
Overview: Mass
Spectrometry of 1-Chlorobutane
- Molecular ion (M⁺): Formed by electron
impact ionization, giving the full molecular mass.
- Fragmentation: Occurs via cleavage of
C–C and C–Cl bonds, producing characteristic ions.
- Isotopic pattern: Chlorine has two
isotopes (35Cl and 37Cl), giving a
3:1 peak ratio at M⁺ and M⁺+2.
Prominent m/z Ions
and their origins
in the mass spectrum of 1-chlorobutane
|
m/z |
Ion Formula |
Fragment Origin |
Notes |
| 92 |
C4H935Cl⁺ |
Molecular ion (M⁺) |
Base peak or moderate intensity |
| 94 |
C4H937Cl⁺ |
M⁺ + 2 due to 37Cl isotope |
~1/3 intensity of m/z 92 |
| 57 |
C4H9⁺ |
Loss of Cl radical |
Common alkyl fragment |
| 56 |
C4H8⁺ |
Loss of HCl from
C4H9Cl⁺ |
Base ion peak |
| 43 |
C3H7⁺ |
Further C–C cleavage |
Propyl cation |
| 41 |
C3H5⁺ |
Allylic-type fragment |
Less intense |
| 39 |
C3H3⁺ |
Smaller hydrocarbon fragment |
Often weak |
| 35 |
35Cl⁺ |
Chlorine isotope ion |
Diagnostic for halogen presence |
| 37 |
37Cl⁺ |
Chlorine isotope ion |
~1/3 intensity of m/z 35 |
Common
Misconceptions about
the
mass spectrum of 1-chlorobutane
(see also below)
- Confusing M⁺ and base peak: The
molecular ion (m/z 92) is not always the tallest peak. The
base peak may be a fragment like m/z 57.
- Ignoring isotopic patterns: Students
often overlook the M⁺/M⁺+2 ratio.
Chlorine’s 3:1 pattern is a key clue for halogenated
compounds.
- Assuming all fragments are charged:
Only ions are detected — neutral fragments
(e.g. Cl•) are invisible to the spectrometer.
- Overinterpreting low m/z peaks: Peaks
below m/z 30 are often noise or common hydrocarbon fragments
— focus on diagnostic ions.
Exam Tips for
questions involving
the mass spectrum of 1-chlorobutane
(see also above)
- Always identify M⁺ and M⁺+2: Mention
the chlorine isotopic signature explicitly.
- Use fragmentation logic: Show how loss
of Cl gives m/z 57, and further cleavage gives m/z 43.
- Compare with isomers: 2-chlorobutane
may show different fragmentation due to branching — useful
for structure deduction.
- Sketch fragmentation pathways: Helps
visualize bond cleavage and ion formation.
- Link to IR/NMR: In synoptic questions,
combine mass spec with other spectra for full structural
analysis.
Practice Question:
Mass Spectrum of 1-Chlorobutane
Compound: 1-chlorobutane (C4H9Cl)
A student obtains the mass spectrum of a compound with molecular ion peaks at
m/z 92 and m/z 94, and a base peak at m/z 57. The
student suspects the compound is a straight-chain chloroalkane.
Question:
- Explain why the molecular ion peak appears as a pair at m/z 92
and m/z 94.
- Predict the relative intensities of the m/z 92 and m/z
94 peaks and justify your answer using isotopic abundances.
- Identify the fragment ion responsible for the base peak at m/z
57 and explain its formation.
- Deduce the structure of the compound from the fragmentation pattern and
justify your answer.
- Explain how the mass spectrum could help distinguish 1-chlorobutane from
its isomer 2-chlorobutane.
Model Answer
a) Isotopic Pair at m/z 92 and 94
- Chlorine has two major isotopes: ³⁵Cl and ³⁷Cl.
- 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) 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.
c) Fragment Ion at m/z 57
- The m/z 57 peak corresponds to the butyl cation (C4H9⁺).
- It forms when the Cl atom is lost as a neutral radical:
-
C4H9Cl →
C4H9⁺ (m/z 57) + Cl•
- This is a common fragmentation pathway due to the relatively weak C–Cl
bond.
d) Structural Deduction
- The presence of a strong m/z 57 peak suggests a straight-chain
butyl cation.
- The molecular ion peaks at m/z 92 and 94 confirm the presence
of one chlorine atom.
- The fragmentation pattern supports a primary chloroalkane with the Cl at
the end of a straight chain.
- Therefore, the structure is CH3CH2CH2CH2Cl
— 1-chlorobutane.
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-chlorobutane forms a straight-chain butene cation by HCl loss (m/z
56, little m/z 57).
- 2-chlorobutane forms a straight-chain butyl cation by C-Cl
fission (m/z 56).
- 2-chlorobutane also forms a secondary butyl cation (m/z 57,
little m/z 56), which may become the base peak.
- The dominance and identity of the base peak helps distinguish the
isomers.
- See comparisons below.
|
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: how to interpret and explain the mass spectrum of
1-chlorobutane, image and diagram of the mass spectrum of
1-chlorobutane, details of the mass spectroscopy of 1-chlorobutane, low and high resolution mass
spectrum of 1-chlorobutane, prominent m/z peaks in the mass spectrum of
1-chlorobutane, comparative
mass spectra of 1-chlorobutane, the molecular ion peak in the mass spectrum of
1-chlorobutane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 1-chlorobutane, characteristic pattern of peaks in the mass spectrum of
1-chlorobutane, relative
abundance of mass ion peaks in the mass spectrum of 1-chlorobutane, revising the mass
spectrum of 1-chlorobutane, revision of mass spectroscopy of 1-chlorobutane, most abundant ions in the
mass spectrum of 1-chlorobutane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 1-chlorobutane, how to analyse the mass
spectrum of 1-chlorobutane, how to describe explain the formation of fragmented
ions in the mass spectra of 1-chlorobutane
n-butyl chloride Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
1-chlorobutane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 1-chlorobutane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 1-chlorobutane. The m/e m/z value of the base ion peak in the
mass spectrum of 1-chlorobutane. Possible examples of equations showing the formation
of the ionised fragments in 1-chlorobutane. Revision notes on the mass spectrum of
1-chlorobutane.
Matching and deducing the structure of the 1-chlorobutane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic halogenoalkanes
haloalkanes alkyl halides alkyl chlorides chloroalkanes,
mass spectra of 1-chlorobutane, an isomer of molecular formula
C4H9Cl explaining the m/z ion
fragmentation pattern in the mass spectrum of 1-chlorobutane How do you interpret the mass spectrum of
1-chlorobutane How to interpret
the mass spectrum of 1-chlorobutane Explanatory diagram of the mass spectrum of the
1-chlorobutane molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
1-chlorobutane. How to explain the mass spectrum of 1-chlorobutane. The m/z value of the
molecular ion peak in the mass spectrum of 1-chlorobutane. Identifying
1-chlorobutane from
its mass spectrum pattern. The m/z m/e peak analysis of the mass
spectrum of the 1-chlorobutane molecule. The uses of the mass spectrum of the
1-chlorobutane molecule. The distinctive features of the mass spectrum of
the 1-chlorobutane molecule explained. explaining the fragmentation pattern of the mass spectrum of
1-chlorobutane equations showing the
formation of the ionised fragments in the mass spectrum of
1-chlorobutane
what does the mass spectrum tell you about the structure and
properties of the 1-chlorobutane molecule? Data table of ionised
fragments in the mass spectrum of 1-chlorobutane and equations for their
formation in the fragmentation of 1-chlorobutane molecules
Links associated with 1-chlorobutane
The chemistry of HALOGENOALKANES (haloalkanes)
revision notes INDEX
The infrared spectrum of 1-chlorobutane
(n-butyl chloride)
The H-1 NMR spectrum of 1-chlorobutane
(n-butyl chloride)
The C-13 NMR
spectrum of 1-chlorobutane
(n-butyl chloride)
Mass spectrometry - introduction and spectra index
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