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Interpreting the mass
spectrum of 2,2-dimethylpropane
[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-dimethylpropane
[updated
Nov
4th 2025]
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mass
spectrum of
C(CH3)4
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Links associated with 2,2-dimethylpropane
Mass spectrometry - spectra index
See also
comparing the infrared, mass, 1H NMR and
13C NMR
spectra of the 3 alkane isomers of C5H12
Introductory note on the mass spectrum of 2,2-dimethylpropane
Students and teachers please note
my explanation of the mass spectrum of 2,2-dimethylpropane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2,2-dimethylpropane 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-dimethylpropane and only the formation of singly charged
positive are considered for the mass spectrum of
2,2-dimethylpropane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
2,2-dimethylpropane
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-dimethylpropane.
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-dimethylpropane,
but the mass spectrometer software does!
2,2-dimethylpropane C5H12
 ,
,
For more
see The molecular structure and
naming of alkanes
Interpreting the fragmentation pattern of the mass spectrum of
2,2-dimethylpropane
[M]+ is the molecular ion peak (M) with an
m/z of
72 corresponding to [C5H12]+, the original 2,2-dimethylpropane molecule minus an electron,
[C(CH3)4]+.
This molecular ion of 2,2-dimethylpropane is very
unstable and only shows up as a minute peak.
The even more minute M+1 peak at m/z 73, corresponds to an ionised
2,2-dimethylpropane
molecule with one 13C atom in it i.e. an ionised
2,2-dimethylpropane molecule of
formula 13C12C4H12
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-dimethylpropane has 5 carbon atoms, so on
average, ~1 in 20 molecules will contain a 13C atom.
The most abundant ion of the molecule under mass
spectrometry investigation 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
the mass spectrum of 2,2-dimethylpropane is the m/z ion 43
[C3H7]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2,2-dimethylpropane.
The parent molecular ion peak
is m/z of
72 corresponding to [C5H12]+
or
[C(CH3)4]+.
|
m/z value of
[fragment]+ |
57
[C4H9]+ |
41 |
39 |
29 |
27 |
15 |
|
[molecular fragment]+ |
[C(CH3)3]+ |
[C3H5]+ |
[C3H3]+ |
[C2H5]+ |
[C2H3]+ |
[CH3]+ |
Atomic masses: H = 1; C = 12
(~1% are 13)
Bond enthalpies = kJ/mol: C-C = 348;
C-H = 412
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2,2-dimethylpropane
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 possible equations to explain the most abundant ion peaks
in the mass spectrum of
2,2-dimethylpropane
Formation of m/z 57 ion:
[C(CH3)4]+ ===> [C(CH3)3]+
+ CH3
C-C bond scission in the parent molecular ion,
mass
change 72 - 15 = 57, the [C4H9]+
(M-15 ion peak)
The m/z 57 ion is the
base peak ion, the most
abundant and 'stable' ion fragment, formed by loss of a methyl group
from the parent molecular ion.
Note that it is a tertiary carbocation - extra
stability from the +I effect of the three methyl groups.
The m/z 58 ion
is probably formed in the same way i.e.
[13C12C3H9]+
with a carbon-13 isotope in it
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 39
and 41 ions:
[C4H9]+
===> [C3H5]+ + [CH4]
Elimination of methane? from the m/z 57 ion, 57 - 16
= 41
[C3H5]+
===> [C3H3]+ + H2
Elimination of hydrogen from m/z 39 ion?
Formation of m/z 27 and
29 ions:
[C5H12]+.
===>
[C2H5]+.+
[C3H7]
C-C bond scission of parent molecular ion.
mass change 72 - 43 = 29 (M-43 ion peak)
and e.g.
[C2H5]+.
===> [C2H3]+.+
H2
Formation of m/z 15 ion:
The methyl carbocation ion, m/z 15, will result from the
breakdown of larger alkyl ions, but a much lower probability of
formation compared to the m/z 57 ion - carbocation stabilised by the
methyl groups.
e.g.: [C(CH3)3]+ ===> [CH3]+
+ C(CH3)2
mass change 72 - 57 =
15 (M-57 ion)
similarly from the molecular ion/fragment [C(CH3)3]+
or [C(CH3)2]+
Key
points about the mass spectrum of 2,2-dimethylpropane
The mass
spectrum of 2,2-dimethylpropane (neopentane) is characterized by a weak
molecular ion at m/z 72 and a dominant base peak at m/z 57 due to loss of a
methyl group.
Its high
symmetry leads to fewer, more intense fragment peaks.
Key Features of the
Mass Spectrum
2,2-Dimethylpropane (C5H12)
is a highly branched alkane with a compact, symmetrical structure.
This influences its fragmentation pattern:
- Molecular ion (M⁺) at m/z = 72:
Often weak due to instability of the parent ion.
- Base peak at m/z = 57:
Corresponds to the tert-butyl cation (C4H9⁺), formed
by loss of a methyl radical (–CH3).
- Other minor peaks:
m/z = 43 and 41 from further fragmentation of the tert-butyl ion.
Prominent m/z Peaks
and Their Origins
| m/z |
Ion Formula |
Fragment Origin |
Relative Intensity |
Notes |
| 72 |
C5H12⁺ |
Molecular ion (M⁺) |
Low |
Often weak due to high fragmentation |
| 57 |
C4H9⁺ |
Loss of CH3
(methyl radical) |
Base peak |
Tert-butyl cation, highly stable |
| 43 |
C3H7⁺ |
Further fragmentation of C4H9⁺ |
Medium |
Propyl cation |
| 41 |
C3H5⁺ |
Allylic-type fragment (rearrangement) |
Low |
Less common in alkanes |
| 15 |
CH3⁺ |
Methyl cation |
Low |
Often seen in alkanes |
Common
Misconceptions
- Assuming the molecular ion is
always the base peak: In
alkanes like 2,2-dimethylpropane, the molecular ion is often weak or absent.
- Confusing m/z 57 with a ketone
fragment: m/z 57 is common in
both alkanes (tert-butyl) and acylium ions from ketones—context matters.
- Overlooking symmetry:
Symmetrical molecules fragment in fewer, more predictable ways—this
simplifies the spectrum.
Exam Revision Tips
For A-level (AQA, Edexcel, OCR, WJEC,
CCEA), CIE, IB, and US AP Chemistry:
- Know common alkyl fragment ions:
m/z 15 (CH3⁺),
29 (C2H5⁺),
43 (C3H7⁺),
57 (C4H9⁺)
are frequently tested.
- Use molecular ion to deduce Mr:
Even if weak, M⁺ gives the molecular mass—essential for identifying the
compound.
- Link fragmentation to structure:
Highly branched alkanes like neopentane favour cleavage that forms stable
carbocations (e.g., tert-butyl).
- Practice with isomers:
Compare spectra of pentane, 2-methylbutane, and 2,2-dimethylpropane to see
how branching affects fragmentation.
- Watch for base peak clues:
A base peak at m/z 57 often suggests a branched alkane or a ketone—use IR or
context to distinguish.
|
Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the 3 alkane isomers of C5H12
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 pentane,
2-methylbutane and 2,2-dimethylpropane image sizes. |
 |
 |
 |
Comparing the
infrared
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane
exemplify infrared spectra of the alkane homologous series CnH2n+2
hydrocarbon
molecules, where n = 5 |
|
INFRARED SPECTRA
(above): There are, as expected, differences in the fingerprint region at
wavenumbers 1500 to 400 cm-1, but there is no
specific infrared absorption band for a functional group. The
infrared spectra of pentane and 2-methylbutane seem very
similar, but that of 2,2-dimethylpropane seems much simpler. |
 |
 |
 |
Comparing the
mass
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane
exemplify the mass spectra of the alkane series CnH2n+2
hydrocarbon
molecules, where n = 5 |
|
MASS SPECTRA (above):
All three hydrocarbons show some similarities in their mass
spectra e.g. m/z ions 27 to 29 for [C2Hx]+
(x = 2 and 4). The molecular ion peaks will
be the same for all three isomers (m/z 72),
but it is very tiny for 2,2-dimethypropane. The pattern ratios
for m/z 39 to 43 are similar for pentane and 2-methylbutane, but
m/z 42 and 43 ions are almost absent from the
2,2-dimethylpropane spectrum. The base peak ion for pentane is
m/z 43, but for 2-methylbutane and 2,2-dimethylpropane it is m/z
57. |
 |
 |
 |
Comparing the
1H proton NMR
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane exemplify the 1H proton NMR spectra of the alkane
homologous series CnH2n+2
hydrocarbon
molecules where, n = 5 |
|
1H NMR SPECTRA (above): The 1H NMR spectra of
all three molecules give different proton ratios for the
different 1H chemical environments i.e. pentane's
proton ratio is 3:2:1 (from 6:4:2 H's in the molecule).
2-methylbutane's proton ratio is 6:1:2:3 and
2,2-dimethylpropane's doesn't have a proton ratio, all hydrogen
atoms are equivalent. This means all three isomeric C5H12
hydrocarbons can be distinguished from their 1H NMR spectra. |
 |
 |
 |
Comparing the
carbon-13 NMR
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane exemplify the carbon-13 NMR spectra of
members of the alkane homologous series CnH2n+2
hydrocarbon
molecules, where n = 5 |
|
13C NMR SPECTRA
(above): The
13C NMR spectra of the three molecules show different numbers of
carbon-13 chemical environments i.e different numbers of 13C NMR
resonance lines. So, pentane gives three 13C chemical
shifts,
2-methylbutane four and 2,2-dimethylpropane two. This means all
three isomeric C5H12 hydrocarbons can be
distinguished from their 13C NMR spectra. |
Key words & phrases: neopentane
dimethylpropane
image diagram on how to interpret and explain the mass spectrum of
2,2-dimethylpropane m/z m/e base peaks, image and diagram of the mass spectrum of
2,2-dimethylpropane, details of the mass spectroscopy of 2,2-dimethylpropane, low and high resolution mass
spectrum of 2,2-dimethylpropane, prominent m/z peaks in the mass spectrum of
2,2-dimethylpropane, comparative
mass spectra of 2,2-dimethylpropane, the molecular ion peak in the mass spectrum of
2,2-dimethylpropane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2,2-dimethylpropane, characteristic pattern of peaks in the mass spectrum of
2,2-dimethylpropane, relative
abundance of mass ion peaks in the mass spectrum of 2,2-dimethylpropane, revising the mass
spectrum of 2,2-dimethylpropane, revision of mass spectroscopy of
2,2-dimethylpropane, most abundant ions in the
mass spectrum of 2,2-dimethylpropane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2,2-dimethylpropane, how to analyse the mass
spectrum of 2,2-dimethylpropane, how to describe explain the formation of fragmented ions in the
mass spectra of 2,2-dimethylpropane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2,2-dimethylpropane recognising
the base ion peak of 2,2-dimethylpropane neopentane
dimethylpropane Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
2,2-dimethylpropane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 2,2-dimethylpropane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 2,2-dimethylpropane. The m/e m/z value of the base ion peak in the
mass spectrum of 2,2-dimethylpropane. Possible examples of equations showing the formation
of the ionised fragments in 2,2-dimethylpropane. Revision notes on the mass spectrum of
2,2-dimethylpropane.
Matching and deducing the structure of the 2,2-dimethylpropane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic alkanes,
mass spectra of 2,2-dimethylpropane, an isomer of molecular formula C5H12
How do you interpret the mass spectrum of
2,2-dimethylpropane How to interpret
the mass spectrum of 2,2-dimethylpropane Explanatory diagram of the mass spectrum of the
2,2-dimethylpropane molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
2,2-dimethylpropane. How to explain the mass spectrum of
2,2-dimethylpropane. The m/z value of the
molecular ion peak in the mass spectrum of 2,2-dimethylpropane. Identifying
2,2-dimethylpropane from
its mass spectrum pattern. The m/z m/e peak analysis interpretation
diagram of the mass
spectrum of the 2,2-dimethylpropane molecule. The uses of the mass spectrum of the
2,2-dimethylpropane molecule. The distinctive features of the mass spectrum of
the 2,2-dimethylpropane molecule explained. explaining the fragmentation pattern of the mass spectrum of
2,2-dimethylpropane equations showing the
formation of the ionised fragments in the mass spectrum of
2,2-dimethylpropane
what does the mass spectrum tell you about the structure and
properties of the 2,2-dimethylpropane molecule? Data table of ionised fragments in
the mass spectrum of 2,2-dimethylpropane and equations for their formation in the
fragmentation of 2,2-dimethylpropane molecules
Links associated
with
2,2-dimethylpropane
The chemistry of ALKANES
revision notes INDEX
The infrared spectrum for 2,2-dimethylpropane
The H-1 NMR spectrum for 2,2-dimethylpropane
The C-13 NMR spectrum for 2,2-dimethylpropane
Mass spectroscopy index
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