Advanced Organic Chemistry: PART 15.2 Infrared Spectroscopy
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Theory of infrared
absorption spectra and how
an infrared spectrometer works
infrared spectra (on this page, and added links to
relevant organic section indexes)
Some simple NMR-IR
problem solving questions
15.2.2 Examples of the applications and
uses of infrared spectroscopy
Sub-index for this page
Identification of organic molecules (from the
a polymerisation reaction
alcohol levels in blood samples
Identification of organic molecules
(from the fingerprint pattern)
The complex infrared spectra of most
organic molecules produces a unique fingerprint pattern,
particularly in the 1500 to 400 cm-1 region. Also
specific functional group stretching vibrations can help too.
Infrared fingerprinting is used in
Some infrared spectrometers can
immediately compare the spectrum with a computer database of
thousands of compounds.
In the example below you can see
significant differences in the fingerprint region and also the
lack of the strong O-H absorption in the ether.
Comparing the infrared
spectra of the three isomers of C3H8O
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 propan-1-ol,
propan-2-ol and methoxyethane image sizes.
I wasn't able to obtain an infrared
spectrum for methoxyethane, so I've added the infrared spectrum
of ethoxyethane to enable a few comparisons with two aliphatic
propan-2-ol and methoxyethane
are structural isomers of molecular formula C3H8O
propan-2-ol and methoxyethane
exemplify infrared spectra of the lower members of the homologous series
of aliphatic alcohols and ethers
(above): There are, as expected, differences in the fingerprint region at
wavenumbers 1500 to 400 cm-1, but most absorptions
for all three molecules are the various C-O and the many C-H
vibrational modes. However, there is one characteristic distinguishing
absorption only present in the infrared spectra of alcohols, but
not in ethers, that is the broad O-H stretching vibration
peaking at ~3350 cm-1. There is also another broad
absorption band (origin?) peaking at ~650 cm-1 in the
alcohol spectra, but not in the ether spectra.
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Following a polymerisation reaction
By measuring at a specific frequency over
time, you can monitor and measure changes in the character or
quantity of a particular bond in an organic molecule.
This is very useful for measuring the
degree of polymerisation in polymer manufacture.
The progress of formation of an epoxy resin being hardened by an
amine cross linking agent can be monitored by observing the
appearance of a hydroxy group in the spectrum of a polymerising
sample (or by the disappearance of an epoxy group).
The C-O stretching vibration in the
triangular C-O-C epoxy group decreases, at wavenumbers,
The O-H group in the resin increases as it
'cures', O-H stretching vibration measured at ~3600 cm-1.
Both absorptions can be measured relative
to each other from the intensity of the two peaks.
Measuring alcohol levels in blood samples
An infrared spectroscopy technique can be
used to measure the concentration of ethanol ('alcohol') in
blood e.g. in a suspected drunk driver.
A portable 'handy' instrument to do this
is called a breathalyzer.
A beam of infrared radiation is passed
through a sample of the suspect's breath and the absorption at a
particular frequency is measured - a series of filters is used
to select the intensity (transmittance) of specific analytical wavenumbers
used for the alcohol vapour analysis
C-H and C-O stretching vibrations are
C-H absorption at 2940 cm-1
and the C-O bond gives a double peak at 1102 and 1055 cm-1
The concentration of alcohol in the breath
can be related to the concentration of alcohol in the
All Advanced Organic
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