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Interpretation of the Mass spectrum of
Pentane
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 pentane
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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
Pentane C5H12,
 ,
,
For more
see The molecular structure and
naming of alkanes
Interpreting the mass spectrum of pentane
[M]+ is the molecular ion peak (M), with an m/z
of 72 corresponding to [C5H12]+, the
original molecule minus an electron,
[CH3CH2CH2CH2CH3]+.
The small M+1 peak at m/z 73, corresponds to an ionised
pentane molecule with one 13C atom in it i.e. an ionised 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.
Pentane 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 pentane.
| m/z value of
[fragment]+ |
57 |
43 |
42 |
41 |
39 |
29 |
27 |
| molecular fragment |
CH3CH2CH2CH2 |
CH3CH2CH2 |
C3H6 |
C3H5 |
C3H3 |
CH3CH2 |
C2H3 |
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 pentane
Possible equations to explain the most abundant ion peaks
in the mass spectrum of pentane
C-C bond scission to split the linear carbon chain of
the molecular ion of pentane
Formation of m/z ion 57
[CH3CH2CH2CH2CH3]+
===> [CH3CH2CH2CH2]+
+ CH3
Mass change 72 - 15 = 57.
Formation of m/z ion 43
[CH3CH2CH2CH2CH3]+
===> [CH3CH2CH2]+
+ CH3CH2
Mass change 72 - 29 = 43.
The m/z 43 ion is the base peak ion, the most
abundant and 'stable' ion fragment, formed by loss of an ethyl group
from the parent molecular ion.
The m/z 43 ion can lose a hydrogen atoms/molecule to
give the m/z 42, 41 and 39 ions.
Formation of m/z ion 29
[CH3CH2CH2CH2CH3]+
===> [CH3CH2]+ + CH3CH2CH2
Mass change 72 - 43 = 29.
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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: how to interpret and explain the mass spectrum of
pentane, image and diagram of the mass spectrum of
pentane, details of the mass spectroscopy of pentane, low and high resolution mass
spectrum of pentane, prominent m/z peaks in the mass spectrum of pentane, comparative
mass spectra of pentane, the molecular ion peak in the mass spectrum of pentane,
analysing and understanding the fragmentation pattern of the mass spectrum
of pentane, characteristic pattern of peaks in the mass spectrum of pentane, relative
abundance of mass ion peaks in the mass spectrum of pentane, revising the mass
spectrum of pentane, revision of mass spectroscopy of pentane, most abundant
ions in the mass spectrum of pentane, how to construct the
mass spectrum diagram for abundance of fragmentation ions in the mass
spectrum of pentane Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
pentane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of pentane. The m/e m/z value of the molecular ion peak in the
mass spectrum of pentane. The m/e m/z value of the base ion peak in the
mass spectrum of pentane. Possible examples of equations showing the formation
of the ionised fragments in pentane. Revision notes on the mass spectrum of
pentane.
Matching and deducing the structure of the pentane molecule from its mass
spectrum. Mass spectroscopy of
alkane,
mass spectra of pentane, an isomer of molecular formula C5H12
Associated links
The chemistry of ALKANES
revision notes INDEX
The infrared spectrum of pentane
The H-1 NMR spectrum of pentane
The C-13 NMR spectrum of Pentane
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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