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Interpreting & explaining the mass
spectrum of heptane
CH3(CH2)5CH3
[ 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
spectroscopy analysis of
heptane
[spectra page updated
Mar 24th 2026 *]
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mass spectrum
CH3(CH2)5CH3
Links associated with heptane
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Mass spectrometry - spectra index
See also
comparing the
1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16
Introductory note on the mass spectrum of heptane
Students and teachers please note
my explanation of the mass spectrum of heptane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
heptane 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 heptane and only the formation of singly charged
positive are considered for the mass spectrum of heptane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
heptane
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
heptane.
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 for
heptane,
but the mass spectrometer software does!
Heptane,
C7H16 , CH3(CH2)5CH3
,
,
an alkane
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]+,
the m/z 58 ion [13C12C3H9]+,
and the m/z 72 ion can be [13C12C4H11]+
These ions might be more likely than those containing
only 12C isotope atoms
i.e.
[C3H8]+,
[C4H10]+ and
[C5H12]+
respectively.
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.
The base ion peak of heptane
is m/z 43:
[C3H7]+
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
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.
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 (M-15 ion peak)
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
(M-28 ion peak)
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 (M-29 ion peak)
The m/z 72 ion
is probably formed in the same way but contains a 13C
atom i.e. it has the formula
[13C12C4H11]+
rather than
[C5H12]+
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 72:
[C5H12]+
= 72.0936 and
[13C12C4H11]+
= 72.0892,
a difference of 0.0044 in relative ion mass.
Formation of m/z 57 ion:
[CH3(CH2)5CH3]+ ===> [CH3(CH2)3]+
+ CH2CH2CH3
C-C bond scission in the parent molecular ion
and loss of propyl group,
mass change = 100 - 43 = 57
(M-43 ion peak)
The m/z 58 ion
is probably formed in the same way but contains a 13C
atom i.e. it has the formula [13C12C3H9]+
rather than [C4H10]+
So again,
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 58:
[13C12C3H9]+
= 58.0736,
[C4H10]+
= 58.0780,
a difference of 0.0044 in relative ion mass.
Formation of m/z 43 ion:
[CH3(CH2)5CH3]+ ===> [CH2CH2CH3]+
+ CH3(CH2)3
C-C bond scission in the parent molecular ion
and loss of butyl group,
mass change = 100 - 57 = 43
(M-57 ion peak)
The m/z 43 ion is the base ion peak, the most
abundant and 'stable' ion fragment.
The m/z 44 ion
is probably formed in the same way but contains a 13C
atom i.e. it has the formula [13C12C2H7]+
rather than [C3H8]+
So again,
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]+ = 44.0580,
a difference of 0.0044 in relative ion mass .
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 (M-71 ion peak)
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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. |
|
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 |
 |
 |
6 δ: proton ratio :
6:3:2:2:2:1 |
6 δ shifts |
|
3-methylhexane |
 |
 |
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 |
 |
 |
3 δ: proton ratio:
12:2:2 |
3 δ
shifts |
|
3,3-dimethylpentane |
 |
 |
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
How do you interpret the mass spectrum of heptane How to interpret
the mass spectrum of heptane Explanatory diagram of the mass spectrum of the
heptane molecule in
terms of its molecular structure.
Table listing data of the m/z ion prominent main peaks in the mass spectrum of
heptane. How to explain the mass spectrum of heptane. The m/z value of the
molecular ion peak in the mass spectrum of heptane. Identifying
heptane from
its mass spectrum pattern. The m/z m/e peak analysis interpretation
diagram of the mass
spectrum of the heptane molecule. The uses of the mass spectrum of the
heptane molecule. The distinctive features of the mass spectrum of
the heptane molecule explained. explaining the fragmentation pattern of the mass spectrum of
heptane equations showing the
formation of the ionised fragments in the mass spectrum of heptane
what does the mass spectrum tell you about the structure and
properties of the heptane molecule? Data table of ionised fragments in
the mass spectrum of heptane and equations for their formation in the
fragmentation of heptane molecules.
Links associated
with
heptane
The infrared
spectrum of heptane
The H-1 NMR
spectrum of heptane
The C-13 NMR
spectrum of heptane
The chemistry of ALKANES
revision notes INDEX
Mass spectroscopy index
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TOP OF PAGE
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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
|
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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