Applications of infrared spectroscopy physico-chemical problems

Abstract

During recent years some important developments and improvements have been made in the determination of organic functional groups. The need for accurate methods in functional group analysis is an obvious one, and the most favoured procedures are those which combine the highest accuracy with the criteria of rapidity and reliability. The first functional group analysis -- the alkoxyl determination -- was developed by Zeisel 76 years ago, and has been the subject of considerable attention by analysts ever since. This reflects the continuing importance of the method in the analysis of naturally occurring substances.With very few exceptions, all the methods for the alkoxyl determination are based on the classical Zeisel reaction, involving cleavage of the ether linkage with boiling hydriodic acid. An alkyl iodide is liberated and subsequently absorbed and determined either gravimetrically, or iodometrically by the Viebock-Schwappach procedure.The apparent basic simplicity of the alkoxyl determination is somewhat offset by the rigorous experimental control demanded, but it still remains one of the simplest,. most compact, and rapid of functional group determinations. For attainment of the highest accuracy, it is necessary that a) the alkoxy compound is rendered soluble in the reaction mixture to ensure complete reaction; b) traces of iodine and hydriodic acid vapours are not permitted to reach the absorber; and c) volatile reaction products which may react with alkyl iodides in the absorber are completely removed. Phenol is the solubilising agent most generally employed, and, in order to fulfil cases b) and c), a scrubber is placed between the reaction vessel and the absorber. An inert carrier gas, nitrogen or carbon dioxide, is - 2. - employed to sweep over liberated alkyl iodides. The apparatus, therefore, consists of a reaction vessel with an inlet -tube for the carrier gas, a condenser to minimise volatilisation of hydriodic acid and iodine vapours, a scrubber, and an absorber. Apparatus. Pregl's apparatus5 has for many years been the accepted design in Britain, and a slightly modified apparatus is now accepted as the British standard; this is described fully in B.S. 1428: Part Cl, 1954. The apparatus consists of a combined reaction flask and water condenser connected by suitable ground -glass joints to a spiral gas scrubber and delivery tube which is led into a receiving vessel. Many individual variants on this design have been proposed to comply with the particular conditions desired for the determinations in question. (See, for example, refs. 4 and 5).