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Interpreting
and explaining the mass
spectrum of 2-iodo-2-methylpropane
(tert-butyl iodide)
[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-iodo-2-methylpropane
(mass spectra)
[spectra
page updated
April 1st 2026 *]
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Re-edit mass spectrum of (CH3)3CI
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Mass spectrometry - spectra index
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See also
comparison of the infrared, mass, 1H NMR and 13C NMR
spectra of the four isomers of C4H9I
Introductory note on the mass spectrum of 2-iodo-2-methylpropane
Students and teachers please note
my explanation of the mass spectrum of 2-iodo-2-methylpropane is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2-iodo-2-methylpropane 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-iodo-2-methylpropane.
I've included a stick diagram and
table of m/z ions for the mass spectrum of 2-iodo-2-methylpropane
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-iodo-2-methylpropane.
2-iodo-2-methylpropane
(tert-butyl iodide), C4H9I,
(CH3)3C-I
The molecular structure and naming of haloalkanes
Interpreting the fragmentation pattern of the mass spectrum of
2-iodo-2-methylpropane
[M]+ is the parent molecular ion peak (M) with an m/z of
184 corresponding to
[C4H9I]+, the original 2-iodo-2-methylpropane molecule minus an electron,
[(CH3)3C-I]+
Iodine consists of 100% of the 127I
isotope, so there are no double peak complexities with
2-iodo-2-methylpropane that you get with the mass spectra of organic
chlorine and bromine compounds, where you get M+2 peaks due to two
isotopes of the halogen of mass difference 2 units and other double
peaks to m/z units apart.
See
Mass spectroscopy index
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-iodo-2-methylpropane has 4 carbon atoms, so on
average, ~1 in 25 molecules will contain a 13C atom.
You may find two peaks one unit apart with a height
ratio of 25:1 e.g. m/z ions 57 and 58.
The most abundant ion of the molecule under mass
spectrometry investigation (2-iodo-2-methylpropane) 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-iodo-2-methylane is the m/z 57 ion
[C4H9]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2-iodo-2-methylpropane.
Note: You don't
get the 'twin peak' complications with organo-iodine compounds that you
do with organo-chlorine and organo-bromine compounds due to them having
two stable isotopes (35Cl, 37Cl, 79Br
and 81Br).
Unless otherwise indicated, assume the carbon atoms in
2-iodo-2-methylpropane are the 12C isotope.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of 2-iodo-2-methylpropane.
The parent molecular ion of
2-iodo-2-methylpropane m/z 184
[C4H9I]+
or
[(CH3)3C-I]+
Data table of some of the ions formed in the
fragmentation pattern of the mass spectrum of 2-iodo-2-methylpropane
|
m/z value of
[fragment]+ |
184 |
169 |
156 |
128 |
127 |
58 |
|
[molecular fragment]+ |
[C4H9I]+ |
[C3H6I]+ |
[C2H5I]+ |
[HI]+ |
[I]+ |
[13C12C3H9]+ |
|
m/z value of
[fragment]+ |
57 |
56 |
55 |
41 |
39 |
29 |
28 |
27 |
|
[molecular fragment]+ |
[C4H9]+ |
[C4H8]+ |
[C4H7]+ |
[C3H5]+ |
[C3H3]+ |
[C2H5]+ |
[C2H4]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2-iodo-2-methylpropane
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); I = 127
Bond enthalpies = kJ/mol: C-C = 348; C-H = 412; C-I
= 238
Possible
equations to explain some of the most abundant ion peaks of
2-iodo-2-methylpropane
(tabulated above)
Formation of m/z 169 ion:
[(CH3)3C-I]+ ===> [C3H6I]+
+ CH3
C-C bond scission in parent molecular ion, methyl
group lost,
mass change 184 - 15 = 169
(M-15 ion peak)
Small peak, low probability due to the high C-C bond
enthalpy compared to the weaker C-I bond.
Formation of m/z 156 ion:
[(CH3)3C-I]+ ===> [C2H5I]+
+ C2H4
C-C bond scission in parent molecular ion,
elimination of ethene molecule,
mass change 184 - 28 =
156 (M-28 ion peak)
Small peak, low probability due to the high C-C bond
enthalpy compared to the C-I bond.
Formation of m/z 127 and 128 ions:
[(CH3)3C-I]+ ===> [I]+
+ C4H9
C-I bond scission, iodine atom freed and ionised,
mass change 184 - 57 = 127
(M-57 ion peak)
this is the weakest bond in the molecule, but
the alkyl fragment is much likely to retain the positive charge,
especially as it has a tertiary carbocation structure.
The m/z of 127 is indicative of an iodine compound.
[(CH3)3C-I]+ ===> [HI]+
+ C4H8
Elimination of a hydrogen iodide molecule, mass
change 184 - 56 = 128.
Both reactions have a low probability judging from
the small abundances - small intensity peaks.
Formation of m/z 57 ion:
[(CH3)3C-I]+ ===> [C4H9]+
+ I
Scission of the weakest bond in the molecule,
breakage of the C-I bond releases an iodine atom and forms the base
peak ion.
The most probable bond scission (see bond enthalpies
above for 2-bromobutane) and the mass change is 184 - 127 = 57.
The alkyl carbocation is much more likely to carry
the positive charge than the iodine atom, especially as it has a
tertiary carbocation structure [(CH3)3C]+.
The m/z 57 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
The m/z 58 ion is probably, [13C12C3H9]+,
i.e. the base peak ion containing a 13C isotope.
Formation of m/z 56 ion:
[(CH3)3C-I]+ ===> [C4H8]+
+ HI
Elimination of hydrogen bromide molecule from the
parent molecular ion, mass change 184 - 128 = 56.
Formation of m/z 55 ion:
[C4H9]+ ===> [C4H7]+
+ H2
Elimination of a hydrogen molecule from the m/z 57 ion.
Loss of a H atom from the same ion can also form the
m/z 55 ion.
Low probability for these fragmentation reactions
due to the high C-H bond enthalpy.
Formation of m/z 41 and 39 ions:
Possible reactions include:
m/z 41: [C4H8]+ ===> [C3H5]+
+ CH3
C-C bond scission of fragment ion.
m/z 39: [C3H5]+ ===> [C3H3]+
+ H2
Formation of m/z 29 and 27 ions:
Possible reactions include:
m/z 29: [(CH3)2CHCH2I]+
===> [C2H5]+ + C2H4I
From bond scission in the parent molecular ion
(above) or C-C bond scission of the fragmentation ions.
m/z 27: [C4H8]+ ===> [C2H3]+
+ C2H5
m/z 29: [C4H8]+ ===> [C2H5]+
+ C2H3
m/z 29: [C4H9]+ ===> [C2H5]+
+ C2H4
There are many
possibilities including proton loss from the m/z 29 ion to give m/z ions 26,
27 and 28
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Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the 4 halogenoalkane isomers of C4H9I
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 1-iodobutane,
2-iodobutane, 1-iodo-2-methylpropane and 2-iodo-2-methylpropane
image sizes. These four molecules
are structural isomers of molecular formula C4H9I
and
exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower
aliphatic halogenoalkanes (haloalkanes, alkyl halides,
iodoalkanes, alkyl iodides). |
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INFRARED SPECTRA
(above):
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. |
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MASS SPECTRA (above):
All four give the parent molecular ion of m/z 184, but it is
only a relatively tiny peak for 2-iodo-2-methylpropane. All four
give the base ion peak of m/z 57. All four give prominent peaks
for m/z ions 29 and 41 and all give a tiny peak from an ionised
iodine atom at m/z 127. They look quite similar to me and lack a
clear fingerprint fragmentation pattern. |
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1H NMR SPECTRA (above): The 1H NMR spectra of
all three molecules give different proton ratios i.e.1-iodobutane
four peaks 3:2:2:2, 2-iodobutane four peaks 3:3:2:1,
1-iodo-2-methylpropane three peaks 6:2:1 and
2-iodo-2-methylpropane one peak '1' (effectively no ratio
involved), so all four molecular structures can be distinguished from each other by their
1H NMR spectra proton ratios, numbers of peaks and (n+1)
rule splitting patterns. |
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13C NMR SPECTRA
(above): The
13C NMR spectra of the four molecules show various numbers of
carbon-13 chemical environments i.e 1-iodobutane and
2-iodobutane show four 13C NMR resonances,
1-iodo-2-methylpropane three 13C NMR resonances and
2-iodo-2-methylpropane only two 13C resonances. Therefore
1-iodo-2-methylpropane and 2-iodo-2-methylpropane can be
distinguished from the other three by their number of resonances
in their 13C NMR spectra, but 1-iodobutane and 2-iodobutane
cannot be distinguished from each other from their number of 13C
NMR resonance lines - other data would be required. |
Key words & phrases: isomer of molecular formula
(CH3)3CI image diagram on how to interpret and explain the mass spectrum of
2-iodo-2-methylpropane m/z m/e base peaks, image and diagram of the mass spectrum of
2-iodo-2-methylpropane, details of the mass spectroscopy of
2-iodo-2-methylpropane, low and high resolution mass
spectrum of 2-iodo-2-methylpropane, prominent m/z peaks in the mass spectrum of
2-iodo-2-methylpropane, comparative
mass spectra of 2-iodo-2-methylpropane, the molecular ion peak in the mass spectrum of
2-iodo-2-methylpropane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2-iodo-2-methylpropane, characteristic pattern of peaks in the mass spectrum of
2-iodo-2-methylpropane, relative
abundance of mass ion peaks in the mass spectrum of 2-iodo-2-methylpropane, revising the mass
spectrum of 2-iodo-2-methylpropane, revision of mass spectroscopy of
2-iodo-2-methylpropane, most abundant ions in the
mass spectrum of 2-iodo-2-methylpropane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2-iodo-2-methylpropane, how to analyse the mass
spectrum of 2-iodo-2-methylpropane, how to describe explain the formation of fragmented ions in the
mass spectra of 2-iodo-2-methylpropane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2-iodo-2-methylpropane recognising the base ion
peak of 2-iodo-2-methylpropane interpreting interpretation the mass spectrum of
2-iodo-2-methylpropane
alkyl iodide haloalkane
halogenoalkane tert-butyl iodide
functional group Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
2-iodo-2-methylpropane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 2-iodo-2-methylpropane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 2-iodo-2-methylpropane. The m/e m/z value of the base ion peak in the
mass spectrum of 2-iodo-2-methylpropane. Possible examples of equations showing the formation
of the ionised fragments in 2-iodo-2-methylpropane. Revision notes on the mass spectrum of
2-iodo-2-methylpropane.
Matching and deducing the structure of the 2-iodo-2-methylpropane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic halogenoalkanes iodoalkanes,
mass spectra of 2-iodo-2-methylpropane, an isomer of molecular formula
C4H9I
How do you interpret the mass spectrum of
2-iodo-2-methylpropane How to interpret
the mass spectrum of 2-iodo-2-methylpropane Explanatory diagram of the mass spectrum of the
2-iodo-2-methylpropane molecule in
terms of its molecular structure.
Table listing data of the m/z ion prominent main peaks in the mass spectrum of
2-iodo-2-methylpropane. How to explain the mass spectrum of
2-iodo-2-methylpropane. The m/z value of the
molecular ion peak in the mass spectrum of 2-iodo-2-methylpropane. Identifying
2-iodo-2-methylpropane from
its mass spectrum pattern. The m/z m/e peak analysis interpretation
diagram of the mass
spectrum of the 2-iodo-2-methylpropane molecule. The uses of the mass spectrum of the
2-iodo-2-methylpropane molecule. The distinctive features of the mass spectrum of
the 2-iodo-2-methylpropane molecule explained. explaining the fragmentation pattern of the mass spectrum of
2-iodo-2-methylpropane equations showing the
formation of the ionised fragments in the mass spectrum of
2-iodo-2-methylpropane
what does the mass spectrum tell you about the structure and
properties of the 2-iodo-2-methylpropane molecule? Data table of ionised fragments in
the mass spectrum of 2-iodo-2-methylpropane and equations for their formation in the
fragmentation of the ionised 2-iodo-2-methylpropane molecule.
Links associated
with
2-iodo-2-methylpropane
The chemistry of HALOGENOALKANES (haloalkanes)
revision notes INDEX
The
infrared spectrum of 2-iodo-2-methylpropane (tert-butyl
iodide)
The H-1 NMR spectrum of
2-iodo-2-methylpropane
(tert-butyl iodide)
The C-13 NMR spectrum of
2-iodo-2-methylpropane
(tert-butyl iodide)
Mass spectrometry index
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