Doc Brown's Advanced Organic Chemistry - spectroscopy revision notes

PART 15.2 Uses and applications of infrared spectroscopy

15.2.1 Theory of infrared absorption spectra and how an infrared spectrometer works

15.2.3 Index of 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

(a) Identification of organic molecules (from the fingerprint pattern)

(b) Following a polymerisation reaction

(c) Measuring alcohol levels in blood samples

(a) 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 forensic science.

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 alcohols Comparing the infrared spectra of propan-1-ol, propan-2-ol and methoxyethane Propan-1-ol, propan-2-ol and methoxyethane are structural isomers of molecular formula C3H8O Propan-1-ol, propan-2-ol and methoxyethane exemplify infrared spectra of the lower members of the homologous series of aliphatic alcohols and ethers
INFRARED SPECTRA (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.

TOP OF PAGE and sub-indexes

(b) 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, 1260-1240 cm^-1.

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.

(c) 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 used.

C-H absorption at 2940 cm^-1 and the C-O bond gives a double peak at 1102 and 1055 cm^-1 (spectrum below).

The concentration of alcohol in the breath can be related to the concentration of alcohol in the bloodstream.