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The mass
spectrum of 2-chlorobutane
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Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US
K12 grade 11 grade 12 organic chemistry courses involving molecular
spectroscopy analysing mass spectra of 2-chlorobutane
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Mass spectroscopy - spectra index
See also
Comparing infrared, mass, 1H NMR & 13C NMR
spectra of the 4 structural isomers of C4H9Cl
 ,
 ,
 ,
 ,
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 even less chance of observing
the 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.
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.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of 2-chlorobutane.
| m/z value of
[fragment]+ |
79 |
77 |
65 |
64 |
63 |
62 |
| [molecular fragment]+ |
[CH3CH37ClCH2]+ |
[CH3CH35ClCH2]+ |
[CH2CH237Cl]+ |
[CH2CH37Cl]+ |
[CH2CH235Cl]+ |
[CH2CH35Cl]+ |
| m/z value of
[fragment]+ |
58 ? |
57 |
56 |
55 |
51 |
50 ? |
49 |
| [molecular fragment]+ |
[13CC3H8]+ |
[CH3CHCH2CH3]+ |
[C4H8]+ |
[C4H7]+ |
[CH237Cl]+ |
[CH335Cl]+ |
[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 2-chlorobutane
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.
Note the expected 3:1 ratio of intensities expected
for chlorine containing fragment ions.
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]+.
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.
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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 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
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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