|
Interpreting the infrared 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 infrared spectra
of pentane
email doc
brown
Use your
mobile phone or ipad etc. in 'landscape' mode
This is a BIG
website, you need to take time to explore it
Infrared spectroscopy - spectra index
See also
comparing the infrared, mass,
1H NMR and 13C NMR
spectra of the 3 alkane isomers of C5H12
Spectra obtained from a liquid film of pentane. The right-hand part of the of the
infrared spectrum of pentane, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of pentane and most organic compounds. It is due to a
unique set of complex overlapping vibrations of the atoms of the molecule
of pentane.
Pentane C5H12,
 ,
,
For more
see The molecular structure and
naming of alkanes
Interpretation of
the spectrum of pentane
The most prominent infrared absorption lines of pentane
Most of these are due to the vibrations of the C-H bonds
e.g. stretching and bending deformations at ~2900 cm-1 and ~1500 cm-1.
There is no characteristic absorption band for a
functional group, as alkanes don't have one.
The absence of a specific functional group band
will show that particular functional group is absent from the
pentane molecular structure.
|
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. |
 |
 |
 |
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. |
 |
 |
 |
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 |
|
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: image and diagram of the infrared spectrum
of pentane, complete infrared absorption spectrum of pentane, comparative spectra of
pentane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of
pentane,
important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum
of pentane, revision of infrared spectroscopy of pentane, fingerprint region analysis of
pentane, how to identify pentane from its infrared spectrum, identifying organic
compounds like pentane from their infrared spectrum Diagram of absorption of wavenumber
peaks in the infrared spectrum of pentane. Characteristic peak wavenumbers in the infrared
spectrum of pentane. Finger print identification pattern using the infrared
spectrum of pentane. Revision notes on the infrared spectrum of pentane. Matching
and deducing the structure of the pentane molecule from its infrared
spectrum. Infrared spectroscopy of alkane, infrared spectra of
pentane, an isomer of molecular formula C5H12
Associated links
The chemistry of ALKANES
revision notes INDEX
The mass spectrum of Pentane
The H-1 NMR spectrum of pentane
The C-13 NMR spectrum of Pentane
Infrared spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
Use My Google search site box
Email doc b:
chem55555@hotmail.com
|
