Functional groups, stain reaction

For a natural product mixture that has been separated on a TLC plate, the most common chemical tests are the TLC colorimetric stain reagents that give a visual indication of the presence of various functional groups. These spray reagents are usually applied as an aerosol to form a thin uniform layer on the TLC plate and, most commonly, heated to hasten the reactions which result in color formation. There are numerous stains listed in several standard texts on the subject (11,12). Table 3 of Chapter 7 lists some of the most useful and widely used of these. Whereas these tests are very useful and easy to perform, it is important to remember that very few are very specific, i.e., they rarely detect compounds solely of the given class, and they often will not detect every single compound of that class. 

Specific properties of dyes depend on the functional groups present. A characteristic is the presence of two types of functional groups, chromophores and auxochromes. Chromophore groups are related to the class of dyes (such as azo dyes and nitro dyes) and are responsible for the behaviour of dyes in oxidation and reduction reactions. Auxochrome groups are responsible for the staining properties and behaviour to acids, alkalis, light and heat. The structures of synthetic dyes approved in the EU are given in formulae 11-43 to 11-58. 

Antioxidants (Newton, 1986 Regan, 1986) - Antioxidants inhibit polymer oxidation. They are normally used with UV absorbers since they often they exhibit a synergistic effect. Antioxidants can be divided in primary and secondary types. The primary antioxidants are able to trap free radicals formed by oxidation or UV reaction processes. Aromatic amines and substituted phenols are of this type. Care should be taken when using aromatic amines as these can stain light colored sealants. Secondary antioxidants can break down any hyperoxides that may be formed, preventing carbonyl group formahon and free radical initiahon. Thioesters and organophosphites function as secondary antioxidants. 

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