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Interpreting the mass
spectrum of 2-methylpropan-1-ol
(isobutyl alcohol)
[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: Molecular
spectroscopy - analysing the
mass spectrum of
2-methylpropan-1-ol
[updated
October Nov 4th 2025]
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Re-edit
mass spectrum of
(CH3)2CHCH2OH
Links associated
with 2-methylpropan-1-ol
The
chemistry of
ALCOHOLS
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website, you need to take time to explore it
Mass spectroscopy - spectra index
Introductory note on the mass spectrum of 2-methylpropan-1-ol
Students and teachers please note
my explanation of the mass spectrum of 2-methylpropan-1-ol is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2-methylpropan-1-ol 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-methylpropan-1-ol and only the formation of singly charged
positive are considered for the mass spectrum of
2-methylpropan-1-ol.
I've included a stick diagram and table of m/z ions for the mass spectrum of
2-methylpropan-1-ol
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-methylpropan-1-ol.
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-methylpropan-1-ol C4H10O
(CH3)2CHCH2OH
The molecular structure and naming of aliphatic
alcohols and ethers
Interpreting the fragmentation pattern of the mass spectrum of
2-methylpropan-1-ol
[M]+ is the molecular ion peak (M) with an m/z of
74 corresponding to [C4H10O]+, the original 2-methylpropan-1-ol molecule minus an electron,
[(CH3)2CHCH2OH]+
The very tiny M+1 peak at m/z 75, corresponds to an ionised
2-methylpropan-1-ol
molecule with one 13C atom in it i.e. an ionised
2-methylpropan-1-ol molecule of
formula [13C12C3H10O]+
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-methylpropan-1-ol 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-methylpropan-1-ol) is usually given an arbitrary abundance value of
100, called the base ion peak, and all other abundances
('intensities') are measured against it.
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2-methylpropan-1-ol.
The base ion peak is
for 2-methypropan-1-ol is the m/z
43 ion
[C3H7]+
Unless otherwise indicated, assume the carbon atoms in
2-methylpropan-1-ol are the 12C isotope.
The parent molecular ion for 2-methylpropan-1-ol is the
m/z ion
74 corresponding to
[C4H10O]+
or [(CH3)2CHCH2OH]+
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of 2-methylpropan-1-ol.
|
m/z value of
[fragment]+ |
73 |
59 |
56 |
55 |
45 |
44 |
43 |
42 |
|
[molecular fragment]+ |
[C4H9O]+ |
[C3H7O]+ |
[C4H8]+ |
[C4H7]+ |
[C2H5O]+ |
[C3H8]+ |
[C3H7]+ |
[C3H6]+ |
|
m/z value of
[fragment]+ |
41 |
39 |
33 |
32 |
31 |
29 |
28 |
27 |
17 |
15 |
|
[molecular fragment]+ |
[C3H5]+ |
[C3H3]+ |
[?]+ |
[13CH2OH]+ |
[CH2OH]+ |
[C2H5]+ |
[C2H4]+ |
[C2H3]+ |
[OH]+ |
[CH3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2-methylpropan-1-ol
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; O-H = 463
Possible
equations to explain the most abundant ion peaks of 2-methylpropan-1-ol
(tabulated above)
Formation of m/z 73 ion:
[(CH3)2CHCH2OH]+ ===> [(CH3)2CHCHOH]+
+ H
C-H bond scission in
the parent molecular to lose a proton.
mass change 74 - 1 =
73 (M-1 ion peak)
Formation of m/z 59 ion:
[(CH3)2CHCH2OH]+ ===> [C3H7O]+
+ CH3
C-C bond scission of
the parent molecular ion, loss of methyl group,
mass change
= 74 - 15 = 59 (M-15 ion peak)
Formation of m/z 55 ion:
[(CH3)2CHCHOH]+ ===> [C4H8]+
+ H2O
Elimination of water from the m/z 73 ion to give an
ionised butene molecule, mass change 73 - 18 = 55 (M-18 ion).
Formation of m/z 43 and
44 ions:
[(CH3)2CHCH2OH]+ ===> [C3H7]+
+ CH2OH
C-C bond scission in
the parent molecular ion,
mass change 74 - 31 = 43
(M-31 ion peak)
The m/z 43 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
The m/z 43 ion can lose a proton to give the m/z 42
ion (ionised propene molecule), further proton loss to give m/z 41
and 39 ions.
m/z 41 ion could be
formed by hydrogen molecule elimination from the m/z 43 ion
[C3H7]+
===>
[C3H5]+ + H2
The m/z 44 ion could also be formed in the
same way, but containing one 13C carbon isotope atom i.e.
[13C12C2H7]+
(not on diagram) rather than
[C3H8]+
(on diagram).
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, 16O
= 15.9949, you can then 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,
also the possibilities
of [C2H4O]+ = 44.0261 and
[13C12CH3O]+ =
44.0217
Formation of m/z 31 ion:
[(CH3)2CHCH2OH]+ ===> [CH2OH]+
+ C3H7
C-C bond scission, mass change 74 - 43 = 31
(M-43 ion)
Note this is the same C-C bond scission in the m/z
43 ion formation - quite often either fragment can be ionised, but
only one of the two fragments can carry the positive charge.
Formation of 27, 28 and 29 m/z ions e.g.
m/z 29: [C3H7]+
==>
[C2H5]+
+
CH2
m/z 27: [C2H5]+
==>
[C2H3]+
+ H2
m/z 27: [C2H5]+
==>
[C2H3]+
+ H2
but they can also arise from other fragments with at
least three carbon atoms.
Formation of m/z ion 17
[R-OH]+
===> [OH]+ + R (R = rest of
fragment or molecular ion)
Formation of m/z ion 15
[R-CH3]+
===> [CH3]+ + R
(R = rest of fragment)
Summary of the mass spectrum of 2-methylpropan-1-ol (isobutyl alcohol)
The mass spectrum of
2-methylpropan-1-ol (isobutyl alcohol) shows a molecular ion
peak at m/z = 74 and a base peak at m/z = 43, with prominent
fragments arising from cleavage near the hydroxyl group and
branching point.
These peaks help confirm the
structure and functional group.
Key Fragmentation
Peaks in Mass Spectrum of 2-Methylpropan-1-ol
| m/z |
Ion Formula |
Fragment Origin |
Notes |
| 74 |
C4H10O⁺ |
Molecular ion (M⁺) |
Weak peak due to instability of
alcohol M⁺ |
| 59 |
C3H7O⁺ |
Loss of CH₃ (methyl group) |
Common alcohol fragment |
| 43 |
C3H7⁺ |
Propyl cation from α-cleavage |
Base peak ion
(most intense) |
| 31 |
CH2OH⁺ |
Hydroxymethyl cation |
Indicates presence of –OH group |
| 29 |
C2H5⁺ |
Ethyl cation |
Typical alkyl fragment |
| 15 |
CH3⁺ |
Methyl cation |
Small peak, diagnostic for alkanes |
Sources:
NIST Chemistry WebBook,
MassBank EU
Common
Misconceptions about
mass spectra like that of
2-methylpropan-1-ol
- Assuming the molecular ion is always the base peak:
Alcohols often fragment easily, so M⁺ at m/z = 74 is weak.
- Confusing fragment ions with parent ions: m/z = 43 is
the base peak but not the molecular ion.
- Ignoring the hydroxyl group’s role: The –OH group
promotes α-cleavage, forming CH2OH⁺ at m/z = 31.
Exam Revision Tips
for questions involving mass spectra like that of 2-methylpropan-1-ol
These tips align with AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, and US AP
Chemistry syllabi:
- Start with molecular formula: C4H10O
gives M⁺ at m/z = 74.
- Use fragmentation logic:
- α-cleavage near the –OH group yields CH₂OH⁺ (m/z = 31).
- Loss of CH3
gives m/z = 59.
- Stable carbocations like C3H7⁺
dominate (m/z = 43).
- Compare with isomers: 1-butanol and tert-butanol show
different fragmentation due to structure.
- Link to structure: Draw the molecule and predict where
bonds break.
- Combine with IR and NMR: Exams often require
multi-spectral interpretation.
Key words & phrases: C4H10O (CH3)2CHCH2OH image diagram on how to interpret and explain the mass spectrum of
2-methylpropan-1-ol m/z m/e base peaks, image and diagram of the mass spectrum of
2-methylpropan-1-ol, details of the mass spectroscopy of 2-methylpropan-1-ol, low and high resolution mass
spectrum of 2-methylpropan-1-ol, prominent m/z peaks in the mass spectrum of
2-methylpropan-1-ol, comparative
mass spectra of 2-methylpropan-1-ol, the molecular ion peak in the mass spectrum of
2-methylpropan-1-ol,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2-methylpropan-1-ol, characteristic pattern of peaks in the mass spectrum of
2-methylpropan-1-ol, relative
abundance of mass ion peaks in the mass spectrum of 2-methylpropan-1-ol, revising the mass
spectrum of 2-methylpropan-1-ol, revision of mass spectroscopy of
2-methylpropan-1-ol, most abundant ions in the
mass spectrum of 2-methylpropan-1-ol, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2-methylpropan-1-ol, how to analyse the mass
spectrum of 2-methylpropan-1-ol, how to describe explain the formation of fragmented ions in the
mass spectra of 2-methylpropan-1-ol equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2-methylpropan-1-ol recognising
the base ion peak of 2-methylpropan-1-ol
interpreting interpretation the mass spectrum of 2-methylpropan-1-ol
isobutyl alcohol How do you interpret the mass spectrum of
2-methylpropan-1-ol How to interpret
the mass spectrum of 2-methylpropan-1-ol Explanatory diagram of the mass spectrum of the
2-methylpropan-1-ol molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
2-methylpropan-1-ol. How to explain the mass spectrum of
2-methylpropan-1-ol. The m/z value of the
molecular ion peak in the mass spectrum of 2-methylpropan-1-ol. Identifying
2-methylpropan-1-ol from
its mass spectrum pattern. The m/z m/e peak analysis of the mass
spectrum of the 2-methylpropan-1-ol molecule. The uses of the mass spectrum of the
2-methylpropan-1-ol molecule. The distinctive features of the mass spectrum of
the 2-methylpropan-1-ol molecule explained. explaining the fragmentation pattern of the mass spectrum of
2-methylpropan-1-ol equations showing the
formation of the ionised fragments in the mass spectrum of
2-methylpropan-1-ol
what does the mass spectrum tell you about the structure and
properties of the 2-methylpropan-1-ol molecule? Data table of ionised fragments in
the mass spectrum of 2-methylpropan-1-ol and equations for their formation in the
fragmentation of 2-methylpropan-1-ol molecules
Links associated
with
2-methylpropan-1-ol
The infrared
spectrum of 2-methylpropan-1-ol
The H-1 NMR
spectrum of 2-methylpropan-1-ol
The C-13 NMR
spectrum of 2-methylpropan-1-ol
The chemistry of ALCOHOLS
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