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Interpreting the mass
spectrum of 2-methylpentane
[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 - analysing the
mass spectrum of
2-methylpentane
[spectra
page updated
Mar 13th 2026 *]
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mass spectrum of
CH3CH2CH2CH(CH3)2
Links associated
with 2-methylpentane
The chemistry of ALKANES and the petrochemical
industry
This is a BIG
website, PLEASE take time to explore it
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-methylpentane
Students and teachers please note
my explanation of the mass spectrum of 2-methylpentane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2-methylpentane 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-methylpentane and only the formation of singly charged
positive are considered for the mass spectrum of
2-methylpentane.
I've included a stick diagram and table of m/z ions for the mass spectrum of
2-methylpentane
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-methylpentane.
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,
but the mass spectrometer software does!
2-methylpentane C6H14,
 ,
,
For more
see The molecular structure,
classification and
naming of alkanes
Interpreting the fragmentation pattern of the mass spectrum of
2-methylpentane
[M]+ is the molecular ion peak (M) with an m/z of
86 corresponding to [C6H14]+, the original 2-methylpentane molecule minus an electron,
[(CH3)2CHCH2CH2CH3]+
The very tiny M+1 peak at m/z 87, corresponds to an ionised
2-methylpentane
molecule with one 13C atom in it i.e. an ionised 2-methylpentane 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-methylpentane 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-methylpentane - though
the ratio will be greater.
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2-methylpentane.
The most abundant ion of the molecule under mass
spectrometry investigation (2-methylpentane) 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-methylpentane is the m/z 43 ion
[C3H7]+
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of
2-methylpentane
based on parent molecular ion, m/z 86 [C6H14]+
or
[(CH3)2CHCH2CH2CH3]+
|
m/z value of
[fragment]+ |
71 |
70 |
57
[C4H9]+ |
56 |
55 |
|
[molecular fragment]+ |
[C5H11]+ |
[C5H10]+ |
[(CH3)2CHCH2]+ |
[C4H8]+ |
[C4H7]+ |
|
m/z value of
[fragment]+ |
43 |
42 |
41 |
39 |
29 |
27 |
|
[molecular fragment]+ |
[C3H7]+ |
[C3H6]+ |
[C3H5]+ |
[C3H3]+ |
[CH3CH2]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2-methylpentane
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-methylpentane
Formation of m/z 71 ion:
[(CH3)2CHCH2CH2CH3]+ ===> [C5H11]+
+ CH3
C-C bond scission in the parent molecular ion,
leading to loss of a methyl group from the parent molecular ion,
mass change = 86 - 15 = 71 (M-15 ion
peak)
There are several possible structures of the [C5H11]+
ion.
Formation of m/z 57 ion:
[(CH3)2CHCH2CH2CH3]+ ===> [(CH3)2CHCH2]+
+ CH2CH3
C-C bond scission in the parent molecular ion,
leading to loss of a ethyl group from the parent molecular
ion,
mass change = 86 - 29 = 57 (M-29 ion
peak)
The m/z 57 ion can
lose hydrogen atoms to give the m/z 56 and 55 ions (see data table).
Formation of m/z 43 ion:
[(CH3)2CHCH2CH2CH3]+ ===> [(CH3)2CH]+
+ CH2CH2CH3
C-C bond scission in the parent molecular ion,
mass
change = 86 - 43 = 43 (M-43 ion), this secondary carbocation is more
stable than [ CH2CH2CH3]+,
but this ionisation, although less likely, also forms the less
stable propyl carbocation.
[(CH3)2CHCH2CH2CH3]+ ===> [CH2CH2CH3]+
+ (CH3)2CH
The m/z 43
[C3H7]+ ion is the base peak ion, the most
abundant and 'stable' ion fragment.
The m/z 44 ion is probably formed in the same way, but containing a
13C carbon isotope atom i.e.
[13C12C2H7]+
and not
[C3H8]+
An accurate mass
spectrometer sorts this out, measuring relative fragment ion
masses to four decimal places e.g. using v ery accurate relative isotopic masses,
12C
= 12.0000 13C = 13.0034, 1H = 1.0078
from which you can calculate
(predict) that the accurate relative ion masses are:
For m/z 44: [C3H8]+
= 44.0624 and
[13C12C2H7]+ = 44.058,
a difference of 0.0044 in relative ion mass.
The m/z 43 ion can lose hydrogen atoms to give the
m/z 39, 41 and 42 ions (see data table).
Formation of m/z 29 ion:
[(CH3)2CHCH2CH2CH3]+ ===> [CH2CH3]+
+ (CH3)2CHCH2
C-C bond scission in
the parent molecular ion.
Mass change 86 - 57 =
29 (M-57 ion)
Like the equations above, its another example of
chain scission of the parent molecular ion.
The smaller fragments like the ethyl ion, can also
be formed by chain scission of bigger fragments, but still smaller
than the parent molecular ion of 2-methypentane.
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.
Summary of the mass spectrum of 2-methylpentane
The mass
spectrum of 2-methylpentane features a molecular ion peak at m/z = 86 and
prominent fragment ions from cleavage near the branched methyl group, with
the base peak typically at m/z = 43 due to a stable propyl cation.
Key Fragmentation Points in Mass Spectrum of
2-Methylpentane
2-Methylpentane (C6H14)
undergoes fragmentation via electron ionization (EI), producing a molecular ion
and several alkyl fragment ions. Here's a breakdown of the most significant
peaks:
| m/z |
Ion Formula |
Fragment Origin |
Notes |
| 86 |
C6H14⁺ |
Molecular ion (M⁺) |
Weak peak due to instability of alkane
M⁺ |
| 71 |
C5H11⁺ |
Loss of
CH3⁺
(methyl group) |
Common in branched alkanes |
| 57 |
C4H9⁺ |
Loss of
C2H5⁺
(ethyl group) |
Secondary carbocation |
| 43 |
C3H7⁺ |
Loss of
C3H7⁺
(propyl group) |
Base peak
(most intense) |
| 29 |
C2H5⁺ |
Ethyl cation |
Often present in alkane spectra |
| 15 |
CH3⁺ |
Methyl cation |
Small peak, diagnostic for alkanes |
Sources:
NIST
Chemistry WebBook,
MassBank EU
Common Misconceptions in Mass Spectrometry
- Assuming the molecular ion is
always the base peak: In
alkanes like 2-methylpentane, the molecular ion (m/z = 86) is often weak due
to fragmentation.
- Confusing fragment ions with
parent ions: Students may
misidentify m/z = 43 or 57 as molecular ions. Emphasize calculating
molecular mass first.
- Ignoring branching effects:
Branching alters fragmentation pathways. Cleavage near the methyl branch
stabilizes carbocations, influencing peak intensities.
Exam Revision Tips for Mass Spectrometry
These tips align with AQA, Edexcel, OCR, WJEC,
CCEA, CIE, IB, and US AP Chemistry syllabi:
- Start with molecular formula:
Calculate molar mass to identify the molecular ion peak.
- Recognize base peak:
Often the most stable carbocation (e.g., m/z = 43 for propyl⁺).
- Use fragmentation logic:
- Cleavage near branching points yields
more stable ions.
- Loss of methyl (CH3, 15),
ethyl (C2H5,
29), or propyl (C3H7,
43) groups is common.
- Compare spectra:
Practice with straight-chain versus branched alkanes to see how branching
affects fragmentation.
- Link to structure:
Use skeletal formulas to visualize where bonds break.
- Combine with IR/NMR:
Exams may ask for structural deduction using multiple spectra.
|
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. |
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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. |
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 |
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Infrared spectra above, mass spectra below. |
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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 |
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Mass spectra above, 1H NMR spectra below. |
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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) |
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1H NMR spectra above, 13C NMR spectra below. |
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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 |
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13C NMR spectra above. |
Key words & phrases:: isohexane
image diagram on how to interpret and explain the mass spectrum of
2-methylpentane m/z m/e base peaks, image and diagram of the mass spectrum of
2-methylpentane, details of the mass spectroscopy of 2-methylpentane, low and high resolution mass
spectrum of 2-methylpentane, prominent m/z peaks in the mass spectrum of
2-methylpentane, comparative
mass spectra of 2-methylpentane, the molecular ion peak in the mass spectrum of
2-methylpentane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2-methylpentane, characteristic pattern of peaks in the mass spectrum of
2-methylpentane, relative
abundance of mass ion peaks in the mass spectrum of 2-methylpentane, revising the mass
spectrum of 2-methylpentane, revision of mass spectroscopy of 2-methylpentane, most abundant ions in the
mass spectrum of 2-methylpentane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2-methylpentane, how to analyse the mass
spectrum of 2-methylpentane, how to describe explain the formation of fragmented ions in the
mass spectra of 2-methylpentane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2-methylpentane recognising the
base ion peak of 2-methylpentane interpreting
interpretation the mass spectrum of 2-methylpentane:
isohexane Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
2-methylpentane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 2-methylpentane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 2-methylpentane. The m/e m/z value of the base ion peak in the
mass spectrum of 2-methylpentane. Possible examples of equations showing the formation
of the ionised fragments in 2-methylpentane. Revision notes on the mass spectrum of
2-methylpentane.
Matching and deducing the structure of the 2-methylpentane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic alkanes,
mass spectra of 2-methylpentane, an isomer of molecular formula C6H14
How do you interpret the mass spectrum of 2-methylpentane How to interpret
the mass spectrum of 2-methylpentane Explanatory diagram of the mass spectrum of the
2-methylpentane molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
2-methylpentane. How to explain the mass spectrum of 2-methylpentane. The m/z value of the
molecular ion peak in the mass spectrum of 2-methylpentane. Identifying
2-methylpentane from
its mass spectrum pattern. The m/z m/e peak analysis of the mass
spectrum of the 2-methylpentane molecule. The uses of the mass spectrum of the
2-methylpentane molecule. The distinctive features of the mass spectrum of
the 2-methylpentane molecule explained. explaining the fragmentation pattern of the mass spectrum of
2-methylpentane equations showing the
formation of the ionised fragments in the mass spectrum of
2-methylpentane
what does the mass spectrum tell you about the structure and
properties of the 2-methylpentane molecule? Data table of ionised fragments in
the mass spectrum of 2-methylpentane and equations for their formation in the
fragmentation of 2-methylpentane molecules
Links associated
with
2-methylpentane
The chemistry of ALKANES
revision notes INDEX
The infrared spectrum of
2-methylpentane
The H-1 NMR spectrum of
2-methylpentane
The C-13 NMR spectrum of
2-methylpentane
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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