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Interpreting the mass
spectrum of 2,3-dimethylbutane
[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
2,3-dimethylbutane
[updated
Nov
4th 2025]
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brown
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mass spectrum of (CH3)2CHCH(CH3)2
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Links associated with 2,3-dimethylbutane
Mass spectroscopy - spectra index
See also
comparing infrared, mass, 1H NMR & 13C NMR
spectra of the structural alkane isomers of C6H14
Introductory note on the mass spectrum of 2,3-dimethylbutane
Students and teachers please note
my explanation of the mass spectrum of 2,3-dimethylbutane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2,3-dimethylbutane 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 2,3-dimethylbutane and only the formation of singly charged
positive are considered for the mass spectrum of
2,3-dimethylbutane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
2,3-dimethylbutane
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
2,3-dimethylbutane.
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
2,3-dimethylbutane,
but the mass spectrometer software does!
2,3-dimethylbutane C6H14
 ,
,
For more
see The molecular structure,
classification and
naming of alkanes
Interpreting the fragmentation pattern of the mass spectrum of
2,3-dimethylbutane
[M]+ is the molecular ion peak (M) with an m/z of
86 corresponding to [C6H14]+, the original 2,3-dimethylbutane molecule minus an electron,
[(CH3)2CHCH(CH3)2]+.
The minute M+1 peak at m/z 87, corresponds to an ionised
2,3-dimethylbutane
molecule with one 13C atom in it i.e. an ionised
2,3-dimethylbutane molecule of
formula 13C12C5H14
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.
2,3-dimethylbutane has 6 carbon atoms, so on
average, ~1 in 17 molecules of will contain a 13C atom.
A similar argument applies to fragment ions from the
breakdown of the parent molecular ion of 2,3-dimethylbutane - though
the ratio will be greater e.g. the m/z 44 ion.
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 2,3-dimethylbutane is the m/z ion 43
[C3H7]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2,3-dimethylbutane.
|
m/z value of
[fragment]+ |
71
[C5H11]+ |
57 |
55 |
44 |
43
[C3H7]+ |
|
[molecular fragment]+ |
[(CH3)2CHCHCH3]+ |
[C4H9]+ |
[C4H7]+ |
13C12C2H7 |
[(CH3)2CH]+ |
|
m/z value of
[fragment]+ |
42 |
41 |
39 |
29 |
27 |
|
[molecular fragment]+ |
[C3H6]+ |
[C3H5]+ |
[C3H3]+ |
[C2H5]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2,3-dimethylbutane
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.
Atomic masses: H = 1; C = 12 (13 for ~1
in 100)
Bond enthalpies = kJ/mol: C-C = 348;
C-H = 412
Examples of possible equations to explain some of the most abundant ion peaks
in the mass spectrum of
2,3-dimethylbutane
Formation of m/z 71 ion:
[(CH3)2CHCH(CH3)2]+ ===> [(CH3)2CHCHCH3]+
+ CH3
[C6H14]+
===> [C5H11]+
+ CH3
C-C bond chain scission in the parent molecular ion, methyl fragment lost from
parent molecular ion,
mass change = 86 - 15 = 71
(M-15 ion)
I've quoted a possible structure of the [C5H11]+
ion, but ?.
Many other fragments are formed by hydrogen
atom/molecule loss, so
you get m/z ion sequences like 71 ==>70, 57 ==> 56 ==> 55, 43 ==> 39 and 29 ==>
27 etc. (see examples below).
Formation of m/z 43 ion:
[(CH3)2CHCH(CH3)2]+ ===> [(CH3)2CH]+
+ (CH3)2CH
[C6H14]+ ===> [C3H7]+
+ [C3H7]
C-C bond chain scission in the parent molecular ion,
mass change = 86 - 43 = 43
(M-43 ion peak)
The molecular ion is
split in two forming the M-43 ion.
The m/z 43 ion is the
base peak ion, the most
abundant and 'stable' ion fragment.
m/z sequences like 43, 42, 41 or 29, 28, 27, indicate
successive hydrogen atom loss.
The m/z 44 ion
is probably formed in the same way i.e.
[13C12C2H7]+
with a carbon-13 isotope in it,
rather than the [C3H6]+ 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 44: [C3H8]+
= 44.0624 and
[13C12C2H7]+ = 44.0580,
a difference of 0.0044 in relative ion mass .
Formation of m/z 41 ion:
[C3H5]+
===> [C3H3]+
+ H 2
Elimination of hydrogen from the m/z 43 ion
Formation of m/z 29 ion:
Sequences including m/z values of 42, 41, 40, 39 or
28, 28, 27, 26, indicate
successive hydrogen atom/molecule loss from the m/z 43 or 29 ions.
Key points
about the mass spectrum of 2,3-dimethylbutane
The mass
spectrum of 2,3-dimethylbutane (C6H14) features a weak
molecular ion at m/z 86 and intense fragment peaks at m/z 43, 42, and 41 due
to stable alkyl carbocations.
The base
peak is m/z 43, typical of branched alkanes.
Key Features of the
Mass Spectrum of 2,3-dimethylbutane
2,3-dimethylbutane is a highly branched
alkane with symmetrical structure, leading to predictable fragmentation:
- Molecular ion (M⁺) at m/z = 86:
Often weak due to extensive fragmentation.
- Base peak at m/z = 43:
Represents a stable propyl or isopropyl cation.
- Strong peaks at m/z = 42 and 41:
Result from further fragmentation of C₃ and C₄ fragments.
- Low-mass ions (e.g., m/z 15, 29):
Common in alkane spectra, but less diagnostic.
Prominent m/z ions
and their origins in the mass spectrum of 2,3-dimethylbutane
| m/z |
Fragment Ion |
Origin |
Notes |
| 86 |
C6H14⁺
(Molecular ion) |
Entire molecule ionized |
Weak due to high fragmentation
tendency |
| 71 |
C5H11⁺ |
Loss of
CH3 |
Secondary peak, stable alkyl cation |
| 57 |
C4H9⁺ |
Loss of
C2H5 |
Butyl-type fragment, moderate
intensity |
| 43 |
C3H7⁺ |
Loss of
C3H7 |
Base peak, highly stable carbocation |
| 42 |
C3H6⁺ |
Dehydrogenated propyl |
Strong peak, common in branched
alkanes |
| 41 |
C3H5⁺ |
Further fragmentation |
Often overlaps with m/z 42 |
| 29 |
C2H5⁺ |
Ethyl fragment |
Small peak, typical of alkanes |
| 15 |
CH3⁺ |
Methyl fragment |
Low intensity, diagnostic for methyl
groups |
Sources:
MassBank EU,
NIST Chemistry WebBook
Common
Misconceptions in Exams
- Assuming the molecular ion is
always the base peak: In
branched alkanes, it is often weak or absent.
- Confusing neutral losses with
fragment ions: Only charged
fragments appear in the spectrum.
- Ignoring symmetry effects:
Symmetrical molecules like 2,3-dimethylbutane fragment easily, reducing M⁺
intensity.
- Overinterpreting low m/z peaks:
Peaks like m/z 15 are common but not diagnostic alone.
Exam Revision Tips
For AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, and
US AP Chemistry:
- Use molecular ion to determine Mr:
Even if weak, it gives the molar mass (86 for C6H14).
-
Use peak
tables: Learn common
m/z values for alkyl fragments m/z 15 = CH3⁺,
29 = C2H5⁺, 43 = C3H7⁺,
57 = C4H9⁺, 71 = C5H11⁺, but not always of
diagnostic use..
- Link fragmentation to structure:
Branched alkanes favour formation of stable carbocations.
- Compare isomers:
Mass spectra help distinguish between hexane, 2-methylpentane, and
2,2-dimethylbutane.
- Practice spectrum interpretation:
Use peak differences to infer lost groups and deduce structure.
|
Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the five structural alkane isomers of C6H14
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 hexane,
2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and
2,3-dimethylbutane image sizes. These five molecules
are structural isomers of saturated alkanes of molecular formula C6H14
and
exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower
aliphatic alkanes (non-cyclic alkanes). |
|
Infrared spectra below. |
 |
 |
 |
INFRARED SPECTRA:
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.
All the absorption
bands are typical of molecules containing saturated alkyl structure and
there are no characteristic infrared absorptions due to a specific
functional group. |
 |
 |
|
Infrared spectra above, mass spectra below. |
 |
 |
 |
MASS SPECTRA: Base ion
peaks plus m/z comments.
Hexane: m/z 57, 42 and 56 prominent
2-methylpentane: m/z 43, 42 and 71 prominent
3-methylpentane: m/z 57, 41 and 56 prominent
2,2-dimethylbutane: m/z 43, 41, 57 and 71
prominent
2,3-dimethylbutane: m/z 43, 41, 42 and 71
prominent |
 |
|
|
Mass spectra above, 1H NMR spectra below. |
 |
 |
 |
1H NMR SPECTRA: They can
all be distinguished by their different integrated proton ratios -
need very high resolution.
Hexane:
3 1H
δ shifts, H ratio 3:2:2 (6:4:4 in formula)
2-methylpentane:
5 1H
δ shifts, H ratio 6:3:2:2:1
3-methylpentane:
4 1H
δ shifts, H ratio 6:4:3:1
2,2-dimethylbutane: 3 1H
δ shifts, H ratio 9:3:2
2,3-dimethylbutane: 2 1H
δ shifts, H ratio 6:1 (12:2 in formula) |
 |
 |
|
1H NMR spectra above, 13C NMR spectra below. |
 |
 |
 |
13C NMR SPECTRA: From the
number of shifts, you can't distinguish (iii) and (iv) but you can
distinguish them from (i), (ii) and (v). (i) Hexane: 3 13C
δ shifts
(ii) 2-methylpentane: 5 13C
δ shifts
(iii) 3-methylpentane: 4 13C
δ shifts
(iv) 2,2-dimethylbutane: 4 13C
δ shifts
(v) 2,3-dimethylbutane: 2 13C
δ shifts |
 |
 |
|
13C NMR spectra above. |
Key words & phrases: image diagram on how to interpret and explain the mass spectrum of
2,3-dimethylbutane m/z m/e base peaks, image and diagram of the mass spectrum of
2,3-dimethylbutane, details of the mass spectroscopy of 2,3-dimethylbutane, low and high resolution mass
spectrum of 2,3-dimethylbutane, prominent m/z peaks in the mass spectrum of
2,3-dimethylbutane, comparative
mass spectra of 2,3-dimethylbutane, the molecular ion peak in the mass spectrum of
2,3-dimethylbutane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2,3-dimethylbutane, characteristic pattern of peaks in the mass spectrum of
2,3-dimethylbutane, relative
abundance of mass ion peaks in the mass spectrum of 2,3-dimethylbutane, revising the mass
spectrum of 2,3-dimethylbutane, revision of mass spectroscopy of
2,3-dimethylbutane, most abundant ions in the
mass spectrum of 2,3-dimethylbutane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2,3-dimethylbutane, how to analyse the mass
spectrum of 2,3-dimethylbutane, how to describe explain the formation of fragmented ions in the
mass spectra of 2,3-dimethylbutane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2,3-dimethylbutane recognising
the base ion peak of 2,3-dimethylbutane interpreting
interpretation the mass spectrum of 2,3-dimethylbutane Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
2,3-dimethylbutane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 2,3-dimethylbutane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 2,3-dimethylbutane. The m/e m/z value of the base ion peak in the
mass spectrum of 2,3-dimethylbutane. Possible examples of equations showing the formation
of the ionised fragments in 2,3-dimethylbutane. Revision notes on the mass spectrum of
2,3-dimethylbutane.
Matching and deducing the structure of the 2,3-dimethylbutane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic alkanes,
mass spectra of 2,3-dimethylbutane, a structural isomer of molecular formula C6H14
How do you interpret the mass spectrum of
2,3-dimethylbutane How to interpret
the mass spectrum of 2,3-dimethylbutane Explanatory diagram of the mass spectrum of the
2,3-dimethylbutane molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
2,3-dimethylbutane. How to explain the mass spectrum of 2,3-dimethylbutane. The m/z value of the
molecular ion peak in the mass spectrum of 2,3-dimethylbutane. Identifying
2,3-dimethylbutane from
its mass spectrum pattern. The m/z m/e peak analysis interpretation
diagram of the mass
spectrum of the 2,3-dimethylbutane molecule. The uses of the mass spectrum of the
2,3-dimethylbutane molecule. The distinctive features of the mass spectrum of
the 2,3-dimethylbutane molecule explained. explaining the fragmentation pattern of the mass spectrum of
2,3-dimethylbutane equations showing the
formation of the ionised fragments in the mass spectrum of
2,3-dimethylbutane
what does the mass spectrum tell you about the structure and
properties of the 2,3-dimethylbutane molecule? Data table of ionised fragments in
the mass spectrum of 2,3-dimethylbutane and equations for their formation in the
fragmentation of 2,3-dimethylbutane molecules
Links associated
with
2,3-dimethylbutane
The chemistry of ALKANES
revision notes INDEX
The infrared spectrum of
2,3-dimethylbutane
The H-1 NMR spectrum of
2,3-dimethylbutane
The C-13 NMR spectrum of
2,3-dimethylbutane
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
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Chemistry Notes
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