There are numerous applications of infrared spectroscopy in organic chemistry. Some of them are described below:
(1) STRUCTURE DETERMINATION
The infrared region is used primarily for analyses of organic compounds because they are readily soluble in a desirable solvent and because they have unique’ and complex spectra. However, work has been done on the infrared Specetra of inorganic compounds in the form of KBr disks or Nujol mulls.
(2) DETECTION OF IMPURITIES The presence of absorption bands at positions where the compound is not expected to absorb indicates the presence of impurities. For example, cyclohexanone is readily detected in cyclohexanol by the intense carbonyl band. When the impure substance is placed in the sample beam and the same thickness of pure sample placed in the reference beam, the spectrum of the impurity is recorded.
(3) USED IN ANALYTICAL WORK
The qualitative analytical use of infrared region is based on the fact. Because of their individuality, infrared spectra of organic compounds are considered equivalent to, or superior to. the preparation of chemical derivatives for the identification of species in organic chemistry. For this reason the infrared portion of the spectrum is often called the fingerprint region.
(4) PERCENTAGE COMPOSITION OF A MIXTURE
If Beer’S law is obeyed, the spectrum of a mixture is that resulting from the superimposition of the spectra of pure components. A . characteristic band is chosen for each component and its intensity measured in the pure state or in mixtures of known. composition. For example, commercial xylene contains the ortho, meta and para isomers which show characteristic absorptions at 740, 690 and 795 c/m respectively, due to the out-of-plane CH bending vibrations.
(5) PROGRESS OF A REACTION
The progress of most reactions can be followed readily by examining the infrared spectra of aliquots withdrawn from the reaction mixture. For example. the oxidation of a secondary alcohol to ketone is accompanied by the disappearance of the O-H band near 3600 c/m and the appearance of the C=O band near 1715 c/m.