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Interpreting
and explaining the mass
spectrum of 2-methylpropanoic acid
(isobutyric acid)
[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
2-methylpropanoic
acid (mass spectra)
[spectra
page updated
April 3rd 2026 *]
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mass
spectrum of (CH3)2CHCOOH
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Introductory note on the mass spectrum of 2-methylpropanoic acid
Students and teachers please note
my explanation of the mass spectrum of 2-methylpropanoic acid is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2-methylpropanoic acid molecule, the initial ionisation to give the molecular ion is:
M(g) +
high KE e- ==> [M•]+(g) + 2e-
and 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-methylpropanoic acid.
I've included a stick diagram and table of m/z ions for the mass spectrum of
2-methylpropanoic acid
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-methylpropanoic acid.
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 and compared the accurate ion
masses if appropriate for 2-methylpropanoic acid. 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-methylpropanoic acid,
but the mass spectrometer software does!
2-methylpropanoic acid
(isobutyric acid), C4H8O2
The molecular structure and naming of carboxylic
acids and derivatives
Interpreting the fragmentation pattern of the mass spectrum of
2-methylpropanoic acid
[M]+ is the molecular ion peak (M) with an m/z of
88 corresponding to [C4H8O2]+, the original 2-methylpropanoic acid molecule minus an electron,
[(CH3)2CHCOOH]+
The very tiny M+1 peak at m/z 89, corresponds to an ionised
2-methylpropanoic acid
molecule with one 13C atom in it i.e. an ionised
2-methylpropanoic acid molecule of
formula [13C12C3H8O2]+
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-methylpropanoic acid has 4 carbon atoms, so on
average, ~1 in 25 molecules will contain a 13C atom.
The most abundant ion of the molecule under mass
spectrometry investigation (2-methylpropanoic acid) 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-methylpropanoic acid is m/z 43
[C3H7]
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2-methylpropanoic acid.
Unless otherwise indicated, assume the carbon atoms in
2-methylpropanoic acid are the 12C isotope.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of 2-methylpropanoic acid.
The parent molecular ion m/z 88 ion corresponds to
[C4H8O2]+
or
[(CH3)2CHCOOH]+
|
m/z value
[fragment]+ |
73 |
71 |
55 |
45 |
44 |
44 |
|
[molecular fragment]+ |
[C3H5O2]+ |
[(CH3)2CHCO]+ |
[C3H3O]+ |
[COOH]+ |
[13C12C2H7]+ |
[CO2]+ |
|
m/z value
[fragment]+ |
43 |
42 |
42 |
41 |
39 |
29 |
27 |
|
[molecular fragment]+ |
[C3H7]+ |
[C3H6]+ |
[CH2CO]+ |
[C3H5]+ |
[C3H3]+ |
[C2H5]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2-methylpropanoic acid
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
(~1% 13); O = 16
Bond enthalpies = kJ/mol: C-C = 348; C-H = 412;
C-O = 360
Possible
equations to explain the most abundant ion peaks of 2-methylpropanoic acid
(tabulated above)
Formation of m/z 73 ion:
[(CH3)2CHCOOH]+ ===> [CH3CHCOOH]+
+ CH3
C-C bond scission of the parent molecular ion, loss
of methyl group,
mass change 88 - 15 = 73,
M-15 peak for
[C3H5O2]+
Formation of m/z 71 ion:
[(CH3)2CHCOOH]+
===> [(CH3)2CHCO]+
+ OH
C-O bond scission of the parent molecular ion, loss
of hydroxyl group,
mass change 88 - 17 = 73,
M-17 a minor ion peak for
[C4H7O]+
Formation of m/z 55 ion:
[C3H5O2]+ ===> [C3H3O]+
+ H2O
Elimination of water from the m/z 73 ion?
Formation of m/z 45 ion:
[(CH3)2CHCOOH]+ ===> [COOH]+
+ (CH3)2CH
C-C bond scission of the parent molecular ion,
mass
change 88 - 43 = 45 (M-43 ion peak)
It could be, though I think less likely, another m/z 45 ion [C2H5O]+
Note that an accurate mass
spectrometer can sort them out, it can measure relative fragment ion
masses to four decimal places e.g. using v ery accurate relative isotopic masses,
12C
= 12.0000, 1H = 1.0078, 16O
= 15.9949, you can then calculate
(predict) that the accurate relative ion masses are:
For m/z 45:
[COOH]+ =
44.9976 and
[C2H5O]+
= 45.0339,
a difference of 0.0363 in relative ion mass.
Formation of m/z 43 and
44 ions:
[(CH3)2CHCOOH]+ ===> [(CH3)2CH]+
+ COOH
C-C bond scission of the parent molecular ion, mass
change 88 - 45 = 43.
The m/z 43 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
The m/z 43 ion is a more stable secondary
carbocation.
[(CH3)2CHCO]+
===> [(CH3)2CH]+
+ CO
It could also be
formed by CO loss from the m/z 71 ion
Further loss of hydrogen atoms/molecules will give
rise to m/z ions of 42, 41 and 39.
U nlikely to be [CH3CO]+
= [C2H3O]+?
Note that an accurate mass
spectrometer can sort them out, it can measure 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, 16O
= 15.9949, you can then calculate
(predict) that the accurate relative ion masses are:
For m/z 43:
[CH3CO]+ = [C2H3O]+ = 43.0183, [C3H7]+ = 43.0546,
a difference of 0.0363 in relative ion mass,
The m/z 44 ion is likely to be the same as m/z 43, but with a
carbon-13 atom in it
[13C12C2H6]+
and less likely to be the
[C3H8]+,
[C2H4O]+
or [CO2]+ ion.
Note that an accurate mass
spectrometer can sort them out, it can measure 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, you can then calculate
(predict) that the accurate relative ion masses are:
For m/z 44: [C3H8]+
= 44.0624,
[13C12C2H7]+ = 44.058,
[CO2]+
= 43.9898 and possibly [C2H4O]+ = 44.0261,
all these relative ion masses are distinguishable in a modern mass
spectrometer.
Formation of m/z 42 ion:
? ===>
[CH2CO]+
+ ?
Theoretically, but less unlikely, to be the
C3H8
or
[13C12C2H5]+
Note that an accurate mass
spectrometer can sort them out, it can measure 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,
you can then calculate
(predict) that the accurate relative ion masses are:
[CH2CO]+
=
42.0105,
[C3H6]+
= 42.0468 and
[13C12C2H5]+
= 42.0424, all sufficiently different relative
ion mass to be separately detected and measured.
Formation of m/z 41 ion:
[?]+ ===>
[C3H5]+ + ?
Formation of m/z ions
<41:
[?]+ ===>
[C3H3]+ + ?
The triangular m/z 39 ion is a very common ion in
mass spectra.
[?]+ ===>
[C2H5]+ + ?
[?]+ ===>
[C2H3]+ + ?
The m/z 27 is quite a prominent ion and can be
formed by proton loss from the m/z 29 ion.
Key words & phrases: C4H8O2 (CH3)2CHCOOH image diagram on how to interpret and explain the mass spectrum of
2-methylpropanoic acid m/z m/e base peaks, image and diagram of the mass spectrum of
2-methylpropanoic acid, details of the mass spectroscopy of 2-methylpropanoic
acid, low and high resolution mass
spectrum of 2-methylpropanoic acid, prominent m/z peaks in the mass spectrum of
2-methylpropanoic acid, comparative
mass spectra of 2-methylpropanoic acid, the molecular ion peak in the mass spectrum of
2-methylpropanoic acid,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2-methylpropanoic acid, characteristic pattern of peaks in the mass spectrum of
2-methylpropanoic acid, relative
abundance of mass ion peaks in the mass spectrum of 2-methylpropanoic acid, revising the mass
spectrum of 2-methylpropanoic acid, revision of mass spectroscopy of
2-methylpropanoic acid, most abundant ions in the
mass spectrum of 2-methylpropanoic acid, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2-methylpropanoic acid, how to analyse the mass
spectrum of 2-methylpropanoic acid, how to describe explain the formation of fragmented ions in the
mass spectra of 2-methylpropanoic acid equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2-methylpropanoic acid
recognising the base ion peak of 2-methylpropanoic acid interpreting
interpretation the mass spectrum of 2-methylpropanoic acid
isobutyric acid carboxylic acid
functional group How do you interpret the mass spectrum of
2-methylpropanoic acid How to interpret
the mass spectrum of 2-methylpropanoic acid Explanatory diagram of the mass spectrum of the
2-methylpropanoic acid molecule in
terms of its molecular structure.
Table listing data of the m/z ion prominent main peaks in the mass spectrum of
2-methylpropanoic acid. How to explain the mass spectrum of
2-methylpropanoic acid. The m/z value of the
molecular ion peak in the mass spectrum of 2-methylpropanoic acid. Identifying
2-methylpropanoic acid from
its mass spectrum pattern. The m/z m/e peak analysis interpretation
diagram of the mass
spectrum of the 2-methylpropanoic acid molecule. The uses of the mass spectrum of the
2-methylpropanoic acid molecule. The distinctive features of the mass spectrum of
the 2-methylpropanoic acid molecule explained. explaining the fragmentation pattern of the mass spectrum of
2-methylpropanoic acid equations showing the
formation of the ionised fragments in the mass spectrum of
2-methylpropanoic acid
what does the mass spectrum tell you about the structure and
properties of the 2-methylpropanoic acid molecule? Data table of ionised fragments in
the mass spectrum of 2-methylpropanoic acid and equations for their formation in the
fragmentation of the ionised 2-methylpropanoic acid molecule.
Links associated
with
2-methylpropanoic acid
The infrared spectrum of 2-methylpropanic
acid (isobutyric acid)
The H-1
NMR spectrum of 2-methylpropanic acid (isobutyric acid)
The C-13
NMR spectrum of 2-methylpropanic acid (isobutyric acid)
The chemistry of CARBOXYLIC ACIDS and DERIVATIVES
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spectroscopy (on the mass spectrum of 2-methylpropanoic acid) are
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