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The H-1 hydrogen-1
(proton) NMR 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 H-1 NMR spectra of
pentane
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H-1 proton NMR spectroscopy -
spectra index
See also
comparing the infrared, mass,
1H NMR and 13C NMR
spectra of the 3 alkane isomers of C5H12 
TMS is the acronym for tetramethylsilane, formula Si(CH3)4,
whose protons are arbitrarily given a chemical shift of 0.0 ppm. This is the
'standard' in 1H NMR spectroscopy and all other proton shifts,
called chemical shifts, depend on the individual (electronic)
chemical environment of the hydrogen atoms in an organic molecule - pentane
here.
The chemical shifts quoted in ppm on the diagram of the
H-1 NMR spectrum of pentane represent the peaks of the intensity of the
chemical shifts of pentane (which are often groups of split lines at
high resolution) AND the relative integrated areas under the peaks gives
you the ratio of protons in the different chemical environments of the
pentane molecule.
Pentane C5H12,
 ,
,
For more
see The molecular structure and
naming of alkanes
Interpreting the H-1 NMR spectrum of
pentane
For relatively simple molecules, the low resolution H-1
NMR spectrum of pentane is a good starting point.
The hydrogen atoms (protons) of pentane occupy 3 different
chemical environments so that the H-1 proton low resolution NMR spectra
should show 3 chemical shift peaks (diagram above).
CH3CH2CH2CH2CH3
(note the ratio of the three colours of the
protons in pentane)
Although there are 12 hydrogen atoms in the molecule,
pentane is a symmetrical molecule with only three possible chemical
environments for the 12 hydrogen atoms.
The peak area ratio of different chemical environments
is the same ratio as the number of protons that occupy these different situations.
The molecular proton ratios is 6 : 4 : 2, so the low resolution spectrum
should show 3 peaks in the ratio
3 : 2 : 1, from the integrated proton ratio from the
spectrum.
The high resolution spectrum of pentane is even more
complex than shown in the diagram.
CH3CH2CH2CH2CH3
From the n+1
rule the end CH3 group of protons (H3)
resonance lines are split into a
triplet by the neighbouring CH2 groups (H2),
at a chemical shift of
0.884 ppm).
The chemical shift resonance of the central CH2 group
of protons (H2) is also split into a
quintet (1:4:6:4:1) by four protons from equivalent proton CH2 groups either side of it
(H2)
at a
chemical shift of 1.26 ppm).
The two 'end' CH2 groups of protons (H2)
chemical shift is
split into a sextet (1:5:10:10:5:1) by CH2 (H2)
and CH3 (H3) groups
of protons (total of 5 protons),
centred around a chemical shift ~1.30 ppm.
The last two sets of chemical shift lines are bunched together - you need
very high resolution to sort out a septet of chemical shifts, all very
close together.
This sextet of resonances is characteristic of a
propyl group (CH3CH2CH2).
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Number of protons 1H
causing splitting |
Splitting pattern produced from the
n+1 rule and the theoretical ratio of line intensities |
| 0
means no splitting |
|
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1 |
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| 1
creates a doublet |
|
|
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|
|
1 |
|
1 |
|
|
|
|
|
| 2
creates a triplet |
|
|
|
|
1 |
|
2 |
|
1 |
|
|
|
|
| 3
creates a quartet |
|
|
|
1 |
|
3 |
|
3 |
|
1 |
|
|
|
| 4
creates a quintet |
|
|
1 |
|
4 |
|
6 |
|
4 |
|
1 |
|
|
| 5
creates a sextet |
|
1 |
|
5 |
|
10 |
|
10 |
|
5 |
|
1 |
|
| 6
creates a septet |
1 |
|
6 |
|
15 |
|
20 |
|
15 |
|
6 |
|
1 |
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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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 |
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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 |
|
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. |
 |
 |
 |
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 |
|
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. |
 |
 |
 |
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 |
|
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: Interpreting the proton H-1 NMR spectra of
pentane, low resolution & high resolution proton
nmr spectra of pentane, H-1 nmr spectrum of pentane, understanding the
hydrogen-1 nmr spectrum of pentane, explaining the line splitting patterns in the
high resolution H-1 nmr spectra of pentane, revising the H-1 nmr spectrum of
pentane,
proton nmr of pentane, ppm chemical shifts of the H-1 nmr spectrum of pentane,
explaining and analyzing spin-spin line splitting in the H-1 nmr spectrum, how
to construct the diagram of the H-1 nmr spectrum of pentane, how to work out the
number of chemically different protons in the structure of the pentane organic
molecule Molecular structure diagram of the
proton NMR diagram for the 1H NMR spectrum of pentane. The proton ratio in the
1H NMR spectrum of pentane. Deducing the number of different chemical
environments of the protons in the pentane molecule from the 1H chemical shifts
in the hydrogen-1 NMR spectrum of pentane. Analysing the high resolution 1H NMR
spectrum of pentane. Analysing the low resolution 1H NMR spectrum of pentane. You
may need to know the relative molecular mass of pentane to deduce the molecular
formula from the proton ratio of the 1H NMR spectrum of pentane. Revision notes
on the proton NMR spectrum of pentane. Matching and deducing the structure of
the pentane molecule from its hydrogen-1 NMR spectrum.
Proton NMR spectroscopy of alkane,
1H NMR spectra of pentane, an isomer of molecular formula
C5H12
Associated links
The chemistry of ALKANES
revision notes INDEX
The infrared spectrum of pentane
The mass spectrum of Pentane
The C-13 NMR spectrum of Pentane
H-1 proton NMR spectroscopy index
(Please
read 8 points at the top of the 1H NMR index page)
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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