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The mass
spectrum of 2,2-dimethylpropane
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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 2,2-dimethylpropane
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Mass spectroscopy - spectra index
See also
comparing the infrared, mass, 1H NMR and
13C NMR
spectra of the 3 alkane isomers of C5H12
2,2-dimethylpropane C5H12
 ,
,
For more
see The molecular structure and
naming of alkanes
Interpreting the fragmentation pattern of the mass spectrum of
2,2-dimethylpropane
[M]+ is the molecular ion peak (M) with an m/z of
72 corresponding to [C5H12]+, the original 2,2-dimethylpropane molecule minus an electron,
[C(CH3)4]+.
This molecular ion of 2,2-dimethylpropane is very
unstable and only shows up as a minute peak.
The even more minute M+1 peak at m/z 73, corresponds to an ionised
2,2-dimethylpropane
molecule with one 13C atom in it i.e. an ionised
2,2-dimethylpropane molecule of
formula 13C12C4H12
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.
2,2-dimethylpropane has 5 carbon atoms, so on
average, ~1 in 20 molecules will contain a 13C atom.
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2,2-dimethylpropane.
| m/z value of
[fragment]+ |
57 |
41 |
39 |
29 |
27 |
15 |
| [molecular fragment]+ |
[C(CH3)3]+ |
[C3H5]+ |
[C3H3]+ |
[C2H5]+ |
[C2H3]+ |
[CH3]+ |
Atomic masses: H = 1; C = 12
Bond enthalpies = kJ/mol: C-C = 348;
C-H = 412
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2,2-dimethylpropane
Possible equations to explain the most abundant ion peaks
in the mass spectrum of
2,2-dimethylpropane
Formation of m/z 57 ion:
[C(CH3)4]+ ===> [C(CH3)3]+
+ CH3
C-C bond scission in the parent molecular ion, mass
change 72 - 15 = 57.
The m/z 57 ion is the base peak ion, the most
abundant and 'stable' ion fragment, formed by loss of a methyl group
from the parent molecular ion.
Note that it is a tertiary carbocation - extra
stability from the +I effect of the three methyl groups.
Formation of m/z 15 ion:
The methyl carbocation ion, m/z 15, will result from the
breakdown of larger alkyl ions.
e.g.: [C(CH3)3]+ ===> [CH3]+
+ C(CH3)2
similarly from the molecular ion/fragment [C(CH3)4]+
or [C(CH3)2]+
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Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the 3 alkane isomers of C5H12
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 pentane,
2-methylbutane and 2,2-dimethylpropane image sizes. |
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Comparing the
infrared
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane
exemplify infrared spectra of the alkane homologous series CnH2n+2
hydrocarbon
molecules, where n = 5 |
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INFRARED SPECTRA
(above): There are, as expected, differences in the fingerprint region at
wavenumbers 1500 to 400 cm-1, but there is no
specific infrared absorption band for a functional group. The
infrared spectra of pentane and 2-methylbutane seem very
similar, but that of 2,2-dimethylpropane seems much simpler. |
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Comparing the
mass
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane
exemplify the mass spectra of the alkane series CnH2n+2
hydrocarbon
molecules, where n = 5 |
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MASS SPECTRA (above):
All three hydrocarbons show some similarities in their mass
spectra e.g. m/z ions 27 to 29 for [C2Hx]+
(x = 2 and 4). The molecular ion peaks will
be the same for all three isomers (m/z 72),
but it is very tiny for 2,2-dimethypropane. The pattern ratios
for m/z 39 to 43 are similar for pentane and 2-methylbutane, but
m/z 42 and 43 ions are almost absent from the
2,2-dimethylpropane spectrum. The base peak ion for pentane is
m/z 43, but for 2-methylbutane and 2,2-dimethylpropane it is m/z
57. |
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Comparing the
1H proton NMR
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane exemplify the 1H proton NMR spectra of the alkane
homologous series CnH2n+2
hydrocarbon
molecules where, n = 5 |
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1H NMR SPECTRA (above): The 1H NMR spectra of
all three molecules give different proton ratios for the
different 1H chemical environments i.e. pentane's
proton ratio is 3:2:1 (from 6:4:2 H's in the molecule).
2-methylbutane's proton ratio is 6:1:2:3 and
2,2-dimethylpropane's doesn't have a proton ratio, all hydrogen
atoms are equivalent. This means all three isomeric C5H12
hydrocarbons can be distinguished from their 1H NMR spectra. |
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Comparing the
carbon-13 NMR
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane exemplify the carbon-13 NMR spectra of
members of the alkane homologous series CnH2n+2
hydrocarbon
molecules, where n = 5 |
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13C NMR SPECTRA
(above): The
13C NMR spectra of the three molecules show different numbers of
carbon-13 chemical environments i.e different numbers of 13C NMR
resonance lines. So, pentane gives three 13C chemical
shifts,
2-methylbutane four and 2,2-dimethylpropane two. This means all
three isomeric C5H12 hydrocarbons can be
distinguished from their 13C NMR spectra. |
Key words & phrases: neopentane
dimethylpropane
image diagram on how to interpret and explain the mass spectrum of
2,2-dimethylpropane m/z m/e base peaks, image and diagram of the mass spectrum of
2,2-dimethylpropane, details of the mass spectroscopy of 2,2-dimethylpropane, low and high resolution mass
spectrum of 2,2-dimethylpropane, prominent m/z peaks in the mass spectrum of
2,2-dimethylpropane, comparative
mass spectra of 2,2-dimethylpropane, the molecular ion peak in the mass spectrum of
2,2-dimethylpropane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2,2-dimethylpropane, characteristic pattern of peaks in the mass spectrum of
2,2-dimethylpropane, relative
abundance of mass ion peaks in the mass spectrum of 2,2-dimethylpropane, revising the mass
spectrum of 2,2-dimethylpropane, revision of mass spectroscopy of
2,2-dimethylpropane, most abundant ions in the
mass spectrum of 2,2-dimethylpropane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2,2-dimethylpropane, how to analyse the mass
spectrum of 2,2-dimethylpropane, how to describe explain the formation of fragmented ions in the
mass spectra of 2,2-dimethylpropane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2,2-dimethylpropane recognising
the base ion peak of 2,2-dimethylpropane neopentane
dimethylpropane Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
2,2-dimethylpropane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 2,2-dimethylpropane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 2,2-dimethylpropane. The m/e m/z value of the base ion peak in the
mass spectrum of 2,2-dimethylpropane. Possible examples of equations showing the formation
of the ionised fragments in 2,2-dimethylpropane. Revision notes on the mass spectrum of
2,2-dimethylpropane.
Matching and deducing the structure of the 2,2-dimethylpropane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic alkanes,
mass spectra of 2,2-dimethylpropane, an isomer of molecular formula C5H12
Links associated
with 2,2-dimethylpropane
The chemistry of ALKANES
revision notes INDEX
The infrared spectrum for 2,2-dimethylpropane
The H-1 NMR spectrum for 2,2-dimethylpropane
The C-13 NMR spectrum for 2,2-dimethylpropane
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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