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Interpreting the mass
spectrum of 2,2-dimethylpentane
[Author
©
Dr WP 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: Mass
spectrometry - analysing the
mass spectrum of
2,2-dimethyl pentane
[updated
Nov 4th 2025]
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brown
Re-edit
mass spectrum of CH3CH2CH2C(CH3)3
Links associated with 2,2-dimethylpentane
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website, PLEASE take time to explore it
Mass spectroscopy - 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 2,2-dimethylpentane
Students and teachers please note
my explanation of the mass spectrum of 2,2-dimethylpentane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2,2-dimethylpentane 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 2,2-dimethylpentane and only the formation of singly charged
positive are considered for the mass spectrum of
2,2-dimethylpentane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
2,2-dimethylpentane
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
2,2-dimethylpentane.
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
2,2-dimethylpentane,
but the mass spectrometer software does!
2,2-dimethylpentane C7H16
The molecular structure and
naming of alkanes
Interpreting the fragmentation pattern of the mass spectrum of
2,2-dimethylpentane
[M]+ is the molecular ion peak (M) with an m/z of
100 corresponding to [C7H16]+, the original
2,2-dimethylpentane molecule minus an electron,
[(CH3)3CCH2CH2CH3]+.
The tiny M+1 peak at m/z 101, corresponds to an ionised
2,2-dimethylpentane
molecule with one 13C atom in it i.e. an ionised
2,2-dimethylpentane 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.
2,2-dimethylpentane has 7 carbon atoms, so, on average,
~1 in every 14 molecules 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. the
m/z 58 ion can be [13C12C3H9]+
rather than [C4H10]+
Either way, for identification
purposes, all these peaks add uniqueness to the fragmentation pattern
of the mass spectrum of 2,2-dimethylpentane.
The most abundant ion of the molecule under mass
spectrometry investigation (2,2-dimethylpentane) 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 for
2,2-dimethylpentane is m/z ion 43
[C3H7]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2,2-dimethylpentane.
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 2,2-dimethylpentane.
The parent molecular ion of 2,2-dimethylpentane 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]+ |
99 |
71 |
57 |
56 |
55 |
|
[molecular fragment]+ |
[C7H15]+ |
[C5H11]+ |
[C4H9]+ |
[C4H8]+ |
[C4H7]+ |
|
m/z value of
[fragment]+ |
43 |
41 |
39 |
29 |
27 |
|
[molecular fragment]+ |
[C3H7]+ |
[C3H5]+ |
[C3H3]+ |
[C2H5]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2,2-dimethylpentane
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 2,2-dimethylpentane
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 99 ion:
[(CH3)3CCH2CH2CH3]+
===> [C7H15]+
+ H
Proton loss from the parent molecular ion, but most fragmentation in alkanes arises from C-C bond
scission (C-C bond weaker than C-H bond).
e.g. ethene is often eliminated to give a smaller
fragment e.g. the m/z 71 ion gives the m/z ion 43
[C5H11]+ ===>
[C3H7]+
+ C2H4
Mass change 71 - 28 = 43
(M-28 ion peak)
Many other fragments are formed by
successive loss of hydrogen
atoms, so
you get m/z ion sequences like 71 ==>70, 57 ==> 55, 43 ==> 39 and 29 ==>
27 etc. (see examples below).
Formation of m/z 71 ion:
[(CH3)3CCH2CH2CH3]+
===> [C5H11]+
+ CH2CH3
C-C bond scission in the parent molecular ion of
2,2-dimethylpentane.
Here an end ethyl group is broken off, mass change =
100 - 29 = 71 (M-29 ion)
Formation of m/z 57 ion:
[(CH3)3CCH2CH2CH3]+
===> [(CH3)3C]+
+ CH2CH2CH3
C-C bond scission in the parent molecular ion of
2,2-dimethylpentane.
Loss of the end propyl group, mass change = 100 - 43 = 57
(M-43 ion)
The m/z 57 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
This is a tertiary carbocation, stabilised by the +I
inductive effect of the three methyl groups.
The m/z 58 ion
is probably formed in the same way i.e.
[13C12C3H9]+
rather than the [C4H10]+ ion.
Note that an accurate mass
spectrometer can sort out 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, from which you can 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)3CCH2CH2CH3]+ ===> [CH2CH2CH3]+
+ C4H9
C-C bond scission in the parent molecular ion of
2,2-dimethylpentane.
Loss of a C4H9 group,
mass
change = 100 - 57 = 43 (M-57 ion peak),
a propyl carbocation.
Formation of m/z 29 ion:
[(CH3)3CCH2CH2CH3]+ ===> [CH2CH3]+
+ C5H11
C-C bond scission in the parent molecular ion of
2,2-dimethylpentane.
Here an end ethyl group is broken off,
mass change =
100 - 71 = 29 (M-71 ion)
Sequences including m/z values of
57, 56 55 or 43, 42, 41, 40, 39 or
29, 28, 28, 27, 26, indicate
successive hydrogen atom loss from the m/z 57, 43 or 29 ions.
Key
points about the mass spectrum of 2,2-dimethypentane
2,2-Dimethylpentane’s mass spectrum
shows a weak molecular ion at m/z = 100 and a dominant base peak at m/z =
43, due to stable carbocation fragmentation.
Key ions arise from cleavage at branched
positions, with methyl and ethyl losses.
Key Mass Spectral
Features of 2,2-Dimethylpentane
2,2-Dimethylpentane (C7H16) is a branched alkane
with a molecular weight of 100. Its mass spectrum reflects typical alkane
fragmentation, favouring stable carbocations.
| m/z |
Ion Formula |
Fragment Origin |
Notes |
| 100 |
C7H16⁺• |
Molecular ion (M⁺•) |
Often weak or absent due to
instability |
| 85 |
C6H13⁺ |
Loss of CH3
(methyl) |
Common alkane cleavage |
| 71 |
C5H11⁺ |
Loss of C2H5
(ethyl) |
Secondary fragmentation |
| 57 |
C4H9⁺ |
Butyl fragment |
Seen in branched alkanes |
| 43 |
C3H7⁺ |
Propyl or isopropyl carbocation |
Base peak
due to high stability |
| 29 |
C2H5⁺ |
Ethyl carbocation |
Often present in alkane spectra |
| 15 |
CH3⁺ |
Methyl carbocation |
Sometimes weak or absent |
Sources:
NIST Chemistry WebBook, AskFilo Chemistry Forum
Common
Misconceptions in Mass Spectrometry
- Expecting a strong molecular ion: Alkanes often show
weak or absent M⁺• due to fragmentation before detection.
- Assuming base peak is always M⁺•: In alkanes, the most
stable carbocation (e.g. m/z = 43) dominates.
- Confusing fragment origins: Students may misattribute
m/z = 43 to methyl loss, when it’s actually a propyl/isopropyl ion.
- Overlooking rearrangements: Branched alkanes can
undergo hydride shifts or complex cleavages not seen in straight chains.
Exam Revision Tips
(AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, AP)
- Know common alkyl fragment ions
:
m/z 15 (CH3⁺),
29 (C2H5⁺),
43 (C3H7⁺),
57 (C4H9⁺)
are frequently tested, but not always diagnostic.
- Use molecular formula: Calculate M⁺• from C7H16
= 100 to confirm identity.
- Link structure to fragmentation: Branching favours
stable carbocations—expect intense peaks from tertiary centers.
- Practice with spectra: Compare linear versus branched
alkane spectra to spot differences in fragmentation.
- Watch for distractors: Some questions include spectra
with overlapping peaks—focus on diagnostic ions.
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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
2,2-dimethylpentane m/z m/e base peaks, image and diagram of the mass spectrum of
2,2-dimethylpentane, details of the mass spectroscopy of 2,2-dimethylpentane, low and high resolution mass
spectrum of 2,2-dimethylpentane, prominent m/z peaks in the mass spectrum of
2,2-dimethylpentane, comparative
mass spectra of 2,2-dimethylpentane, the molecular ion peak in the mass spectrum of
2,2-dimethylpentane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2,2-dimethylpentane, characteristic pattern of peaks in the mass spectrum of
2,2-dimethylpentane, relative
abundance of mass ion peaks in the mass spectrum of 2,2-dimethylpentane, revising the mass
spectrum of 2,2-dimethylpentane, revision of mass spectroscopy of
2,2-dimethylpentane, most abundant ions in the
mass spectrum of 2,2-dimethylpentane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2,2-dimethylpentane, how to analyse the mass
spectrum of 2,2-dimethylpentane, how to describe explain the formation of fragmented ions in the
mass spectra of 2,2-dimethylpentane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2,2-dimethylpentane recognising
the base ion peak of 2,2-dimethylpentane
interpreting interpretation the mass spectrum of 2,2-dimethylpentane
(CH3)3CCH2CH2CH3 Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
2,2-dimethylpentane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 2,2-dimethylpentane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 2,2-dimethylpentane. The m/e m/z value of the base ion peak in the
mass spectrum of 2,2-dimethylpentane. Possible examples of equations showing the formation
of the ionised fragments in 2,2-dimethylpentane. Revision notes on the mass spectrum of
2,2-dimethylpentane.
Matching and deducing the structure of the 2,2-dimethylpentane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic alkanes,
mass spectra of 2,2-dimethylpentane, a structural isomer of molecular formula
C7H16
How do you interpret the mass spectrum of
2,2-dimethylpentane How to interpret
the mass spectrum of 2,2-dimethylpentane Explanatory diagram of the mass spectrum of the
2,2-dimethylpentane molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
2,2-dimethylpentane. How to explain the mass spectrum of
2,2-dimethylpentane. The m/z value of the
molecular ion peak in the mass spectrum of 2,2-dimethylpentane. Identifying
2,2-dimethylpentane from
its mass spectrum pattern. The m/z m/e peak analysis interpretation
diagram of the mass
spectrum of the 2,2-dimethylpentane molecule. The uses of the mass spectrum of the
2,2-dimethylpentane molecule. The distinctive features of the mass spectrum of
the 2,2-dimethylpentane molecule explained. explaining the fragmentation pattern of the mass spectrum of
2,2-dimethylpentane equations showing the
formation of the ionised fragments in the mass spectrum of
2,2-dimethylpentane
what does the mass spectrum tell you about the structure and
properties of the 2,2-dimethylpentane molecule? Data table of ionised fragments in
the mass spectrum of 2,2-dimethylpentane and equations for their formation in the
fragmentation of 2,2-dimethylpentane molecules
Links associated
with
2,2-dimethylpentane
The infrared spectrum of
2,2-dimethylpentane
The H-1 NMR spectrum of
2,2-dimethylpentane
The C-13 NMR spectrum of
2,2-dimethylpentane
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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