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The mass
spectrum of heptane
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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 heptane
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Mass spectroscopy - spectra index
See also
comparing the
1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16
Heptane,
C7H16 , CH3(CH2)5CH3
,
,
For more
see The molecular structure,
classification and
naming of alkanes
Interpreting the fragmentation pattern of the mass spectrum of heptane
[M]+ is the molecular ion peak (M) with an m/z of
100 corresponding to [C7H16]+, the original
heptane molecule minus an electron,
[CH3(CH2)5CH3]+.
The small M+1 peak at m/z 101, corresponds to an ionised
heptane
molecule with one 13C atom in it i.e. an ionised heptane molecule of
formula 13C12C6H16
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.
Heptane has 7 carbon atoms, so on
average, ~1 in 14 molecules of will contain a 13C atom.
This sort of argument also applies to fragment ions
from the parent molecular ion of heptane - though the ratio will be
greater:
e.g. m/z 44 ion could be [13C12C2H7]+,
m/z 58 ion [13C12C3H9]+,
and m/z 72 ion [13C12C4H11]+
These ions might be more likely than those containing
only 12C isotope atoms
i.e. [C3H8]+,
[C4H10]+ and
[C5H12]+
Either way, for identification
purposes, all these peaks add uniqueness to the
fragmentation pattern of the mass spectrum of heptane
The most abundant ion of the molecule under mass
spectrometry investigation (heptane) 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 heptane.
Unless otherwise indicated, assume the carbon atoms in
heptane are the 12C isotope.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of heptane.
The parent molecular ion of heptane m/z 100:
[C7H16]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of heptane.
| m/z value of
[fragment]+ |
85 |
71 |
70 |
57 |
56 |
55 |
| [molecular fragment]+ |
[C6H13]+ |
[C5H11]+ |
[C5H10]+ |
[C4H9]+ |
[C4H8]+ |
[C4H7]+ |
| m/z value of
[fragment]+ |
43 |
42 |
41 |
39 |
29 |
27 |
| [molecular fragment]+ |
[C3H7]+ |
[C3H6]+ |
[C3H5]+ |
[C3H3]+ |
[C2H5]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of heptane
Examples of equations to explain some of the most abundant ion peaks
in the mass spectrum of heptane
Atomic masses: H = 1; C = 12 (13 for ~1
in 100)
Bond enthalpies = kJ/mol: C-C = 348;
C-H = 412
Formation of m/z 85 ion:
[CH3(CH2)5CH3]+ ===> [CH3(CH2)5]+
+ CH3
Most fragmentation in alkanes arises from C-C bond
scission (C-C bond weaker than C-H).
In this case and end methyl group is broken off by
C-C bond scission in the parent molecular ion,
mass change = 100 - 15 = 85.
Note in the equations below that both fragments are
capable of being ionised, but only one at a time.
Many other fragments are formed by hydrogen
atom/molecule loss, so
get m/z ion sequences for heptane (and its isomers) like 71 ==>70, 57 ==> 56 ==> 55, 43 ==> 39 and 29 ==>
27 etc. (see examples below).
Ethene is often eliminated to give a smaller
fragment e.g. the m/z ion 85 gives the m/z ion 57
[CH3(CH2)5]+ ===> [CH3(CH2)3]+
+ C2H4
Mass change 85 - 28 = 57.
Formation of m/z 71 ion:
[CH3(CH2)5CH3]+ ===> [CH3(CH2)4]+
+ CH2CH3
C-C bond scission in the parent molecular ion of
3-methylhexane.
Here an end ethyl group is broken off, mass change =
100 - 29 = 71.
Formation of m/z 57 ion:
[CH3(CH2)5CH3]+ ===> [CH3(CH2)3]+
+ CH2CH2CH3
C-C bond scission in the parent molecular ion.
Loss of propyl group, mass change = 100 - 43 = 57
Formation of m/z 43 ion:
[CH3(CH2)5CH3]+ ===> [CH2CH2CH3]+
+ CH3(CH2)3
C-C bond scission in the parent molecular ion.
Loss of butyl group, mass change = 100 - 57 = 43
The m/z 43 ion is the base ion peak, the most
abundant and 'stable' ion fragment.
Formation of m/z 29 ion:
[CH3(CH2)5CH3]+ ===> [CH2CH3]+
+ CH3(CH2)4
C-C bond scission in the parent molecular ion.
Here an end ethyl group is broken off and becomes
ionised, mass change =
100 - 71 = 29.
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Comparing the
1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16
You can distinguish all 9 isomers from a data combination of their number of
1H NMR
chemical shifts,
and their resulting integrated 1H proton ratios, plus, their number of
13C
chemical shifts. |
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Name of the alkane structural isomer of molecular
formula C7H16 |
Abbreviated structural formulae
of the nine isomers of molecular formula C7H16 (interpretation complications with 3-methylhexane and
2,3-dimethylpentane because they exhibit R/S isomerism due to a
chiral carbon) |
Skeletal formula of the
nine
alkane isomers of
molecular formula C7H16 |
Number of 1H NMR chemical shifts (δ) and
proton ratio (links
to spectrum) |
Number of 13C chemical shifts (δ)
(links
to spectrum) |
| heptane |
|
|
4 δ: proton ratio: 3:2:2:1 (6:4:4:2 in the molecule) |
4 δ shifts |
| 2-methylhexane |
 |
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6 δ: proton ratio :
6:3:2:2:2:1 |
6 δ shifts |
| 3-methylhexane |
 |
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7 δ: proton ratio:
3:3:3:2:2:2:1 (simplification) !!! |
7
δ shifts |
| 3-ethylpentane |
 |
 |
3 δ: proton ratio:
9:6:1 |
3 δ
shifts |
| 2,2-dimethylpentane |
 |
 |
4 δ: proton ratio:
9:3:2:2 |
5 δ shifts |
|
2,3-dimethylpentane |
 |
 |
6 δ: proton ratio:
6:3:3:2:1:1 (simplification) !!! |
6 δ
shifts (simplification) !!! |
|
2,4-dimethylpentane |
 |
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3 δ: proton ratio:
12:2:2 |
3 δ
shifts |
| 3,3-dimethylpentane |
 |
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3 δ: proton ratio:
3:3:2 (6:4:4 in the molecule) |
4 δ
shifts |
| 2,2,3-trimethylbutane |
 |
 |
3 δ: proton ratio:
9:6:1 |
4 δ shifts |
Key words & phrases: C7H16 image diagram on how to interpret and explain the mass spectrum of
heptane m/z m/e base peaks, image and diagram of the mass spectrum of
heptane, details of the mass spectroscopy of heptane, low and high resolution mass
spectrum of heptane, prominent m/z peaks in the mass spectrum of heptane, comparative
mass spectra of heptane, the molecular ion peak in the mass spectrum of heptane,
analysing and understanding the fragmentation pattern of the mass spectrum
of heptane, characteristic pattern of peaks in the mass spectrum of heptane, relative
abundance of mass ion peaks in the mass spectrum of heptane, revising the mass
spectrum of heptane, revision of mass spectroscopy of heptane, most abundant ions in the
mass spectrum of heptane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of heptane, how to analyse the mass
spectrum of heptane, how to describe explain the formation of fragmented ions in the
mass spectra of heptane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of heptane recognising the base ion
peak of heptane interpreting interpretation the mass
spectrum of heptane Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
heptane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of heptane. The m/e m/z value of the molecular ion peak in the
mass spectrum of heptane. The m/e m/z value of the base ion peak in the
mass spectrum of heptane. Possible examples of equations showing the formation
of the ionised fragments in heptane. Revision notes on the mass spectrum of
heptane.
Matching and deducing the structure of the heptane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic alkanes,
mass spectra of heptane, a structural isomer of molecular formula
C7H16
Links associated
with heptane
The chemistry of ALKANES
revision notes INDEX
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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Infrared spectra of the isomers of C7H16
The infrared
spectrum of heptane
The
infrared spectrum of 2-methylhexane
The
infrared spectrum of 3-methylhexane
The
infrared spectrum of 3-ethylpentane
The infrared spectrum of
2,2-dimethylpentane
The infrared spectrum of
2,3-dimethylpentane
The infrared spectrum of
2,4-dimethylpentane
The infrared spectrum of
3,3-dimethylpentane
The infrared spectrum
of 2,2,3-trimethylbutane
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Mass spectra of the isomers of C7H16
The mass
spectrum of heptane
The mass
spectrum of 2-methylhexane
The mass
spectrum of 3-methylhexane
The
mass spectrum of 3-ethylpentane
The mass spectrum of
2,2-dimethylpentane
The mass spectrum of
2,3-dimethylpentane
The mass spectrum of
2,4-dimethylpentane
The mass spectrum of
3,3-dimethylpentane
The mass
spectrum of 2,2,3-trimethylbutane
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H-1 proton NMR spectra of ALKANES
1H NMR spectra of the isomers of C7H16
The H-1 NMR
spectrum of heptane
The
H-1 NMR spectrum of 2-methylhexane
The
H-1 NMR spectrum of 3-methylhexane
The
H-1 NMR spectrum of 3-ethylpentane
The H-1 NMR spectrum of
2,2-dimethylpentane
The H-1 NMR spectrum of
2,3-dimethylpentane
The H-1 NMR spectrum of
2,4-dimethylpentane
The H-1 NMR spectrum of
3,3-dimethylpentane
The H-1
NMR spectrum of 2,2,3-trimethylbutane
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C-13 carbon-13 NMR spectra
of ALKANES
13C NMR spectra of the isomers of C7H16
The C-13 NMR
spectrum of heptane
The
C-13 NMR spectrum of 2-methylhexane
The
C-13 NMR spectrum of 3-methylhexane
The
C-13 NMR spectrum of 3-ethylpentane
The C-13 NMR spectrum of
2,2-dimethylpentane
The C-13 NMR spectrum of
2,3-dimethylpentane
The C-13 NMR spectrum of
2,4-dimethylpentane
The C-13 NMR spectrum of
3,3-dimethylpentane
The
C-13 NMR spectrum of 2,2,3-trimethylbutane
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