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Interpreting the mass
spectrum of 2-iodobutane
Doc
Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US
K12 grade 11 grade 12 organic chemistry courses involving molecular
spectroscopy analysing mass spectra of 2-iodobutane
See also
comparison of the infrared, mass, 1H NMR and 13C NMR
spectra of the four isomers of C4H9I
2-iodobutane
(sec-butyl iodide), C4H9I,
CH3-CHI-CH2-CH3
Interpreting the fragmentation pattern of the mass spectrum of 2-iodobutane
[M]+ is the parent molecular ion peak (M) with an m/z of
184 corresponding to [C4H9I]+, the original 2-iodobutane molecule minus an electron,
[CH3CHICH2CH3]+.
Iodine consists of 100% of the 127I isotope, so
there are no double peak complexities with 2-iodobutane 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 and other double peaks two m/z
units apart.
See
Mass spectroscopy index
The small M+1 peak at m/z 185, corresponds to an ionised
2-iodobutane
molecule with one 13C atom in it i.e. an ionised 2-iodobutane molecule of
formula [13C12C3H9I]+
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-iodobutane 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 57/58 and 184/185.
The most abundant ion of the molecule under mass
spectrometry investigation (2-iodobutane) 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-iodobutane.
Unless otherwise indicated, assume the carbon atoms in
2-iodobutane are the 12C isotope.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of 2-iodobutane.
The parent molecular ion of 2-iodobutane m/z
184:
[CH3CHICH2CH3]+
Data table of some of the ions formed in the
fragmentation pattern of the mass spectrum of 2-iodobutane
| m/z value of
[fragment]+ |
185 |
184 |
155 |
128 |
127 |
58 |
| [molecular fragment]+ |
[13C12C3H9I]+ |
[C4H9I]+ |
[C2H4I]+ |
[HI]+ |
[I]+ |
[13C12C3H9]+ |
| m/z value of
[fragment]+ |
57 |
55 |
41 |
39 |
29 |
28 |
27 |
| [molecular fragment]+ |
[C4H9]+ |
[C4H7]+ |
[C3H5]+ |
[C3H3]+ |
[C2H5]+ |
[C2H4]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2-iodobutane
Atomic masses: H = 1; C = 12; 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-iodobutane
(tabulated above)
Formation of m/z 155 ion:
[CH3CHICH2CH3]+ ===> [C2H4I]+
+ C2H5
C-C bond scission in the parent molecular ion, mass
change 184 - 29 = 155.
Very low probability of formation due to large C-C bond
enthalpy (compared to C-I bond enthalpy).
Formation of m/z 127 and 128 ions:
[CH3CHICH2CH3]+ ===> [I]+
+ C4H9
C-I bond scission, iodine atom freed and ionised, mass change 184 - 57 = 127,
this is the weakest bond in the molecule, but
the alkyl fragment is much likely to retain the positive charge.
The m/z of 127 is indicative of an iodine compound.
[CH3CHICH2CH3]+ ===> [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 peaks.
Formation of m/z 57 ion:
[CH3CHICH2CH3]+ ===> [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.
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:
[CH3CHICH2CH3]+ ===> [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, 28 and 27 ions:
Possible reactions include:
m/z 29: [CH3CHICH2CH3I]+
===> [CH3CH2]+
+ CH2CH2I
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 28: [C4H8]+ ===> [C2H4]+
+ C2H4
m/z 28: [C4H9]+ ===> [C2H4]+
+ C2H5
m/z 29: [C4H8]+ ===> [C2H5]+
+ C2H3
m/z 29: [C4H9]+ ===> [C2H5]+
+ C2H4
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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 C4H9I CH3CHICH2CH3 CH3CH2CHICH3 image diagram on how to interpret and explain the mass spectrum of
2-iodobutane m/z m/e base peaks, image and diagram of the mass spectrum of
2-iodobutane, details of the mass spectroscopy of 2-iodobutane, low and high resolution mass
spectrum of 2-iodobutane, prominent m/z peaks in the mass spectrum of
2-iodobutane, comparative
mass spectra of 2-iodobutane, the molecular ion peak in the mass spectrum of
2-iodobutane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2-iodobutane, characteristic pattern of peaks in the mass spectrum of
2-iodobutane, relative
abundance of mass ion peaks in the mass spectrum of 2-iodobutane, revising the mass
spectrum of 2-iodobutane, revision of mass spectroscopy of 2-iodobutane, most abundant ions in the
mass spectrum of 2-iodobutane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2-iodobutane, how to analyse the mass
spectrum of 2-iodobutane, how to describe explain the formation of fragmented ions in the
mass spectra of 2-iodobutane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2-iodobutane recognising the base ion
peak of 2-iodobutane interpreting interpretation the mass spectrum of
2-iodobutane
haloalkane halogenoalkane alkyl iodide sec-butyl iodide
functional group
Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
2-iodobutane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 2-iodobutane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 2-iodobutane. The m/e m/z value of the base ion peak in the
mass spectrum of 2-iodobutane. Possible examples of equations showing the formation
of the ionised fragments in 2-iodobutane. Revision notes on the mass spectrum of
2-iodobutane.
Matching and deducing the structure of the 2-iodobutane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic halogenoalkanes iodoalkanes,
mass spectra of 2-iodobutane, an isomer of molecular formula
C4H9I
Links associated
with 2-iodobutane
The chemistry of HALOGENOALKANES (haloalkanes)
revision notes INDEX
The infrared spectrum of 2-iodobutane
(sec-butyl iodide)
The H-1
NMR spectrum of 2-iodobutane (sec-butyl iodide)
The
C-13 NMR spectrum of 2-iodobutane (sec-butyl iodide)
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
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