Chemical reactions on TLC plates

Functional group analyses can be performed by chemical reactions on TLC plates (75). 

Miller and Kirchner in 1953, were the pioneer in originating and developing the novel ideal of performing chemical unit-process reactions directly on TLC-plates. The two major steps involved in achieving this objective are, namely ... 

Exactly in the same manner, a number of other chemical unit-process reactions may be accomplished on TLC plates as stated here briefly ... 

Enumerate the various chemical reactions that can be carried out on TLC plates. Support your answer with typical examples, reactions and procedure involved. 

Narang and Michniewicz (87) reported successful application of Avicel-cellulose plates for the rapid and efficient fractionation of complex chemically polymerized thymidine 5 -phosphate reaction mixtures, in situ quantitative analysis of the oligonucleotides on TLC plate by reflectant spectrophotometry, in situ identiflcation of the common mononucleosides and mononucleotides by double-scanning technique and fractionation of oligonucleotides on thick-layer Avicel-cellulose plates on a preparative scale. 

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. 

With the DART ion source with the desorption at an angle option, the scanning of surfaces by DART-MS was possible, and a simple gas beam focus visualization technique was suggested for any surface analysis using DART-MS [74], This visualization is based upon the use of different chemical reactions which lead to color formation upon local heating of the surface (e.g.,derivatiza-tion on a TLC/HPTLC plate). Such a visualization is important for selecting the optimal coordinates of the DART ion source with regard to the sample for an optimal detectability. 

So-called automatic spreaders have been developed to rationalise the coating of hundreds of plates daily (p. 58). The requirements of apparatus in such a laboratory are otherwise the same as described under 2. the apparatus is merely used more rationally. Centralisation, like with spectroscopy, is not however a desirable goal because TLC is a method carried out on the actual work bench of the research and works chemist. For him, it is the simplest, quickest and cheapest expedient for following chemical reactions and for testing purity and for identification. 

After development, TLC plates are dried at or above room temperature to completely remove the mobile phase. The resolved ions on the plate are detected by their original color, natural fluorescence, quenching of fluorescence, or as colored zones produced after chemical reaction with an appropriate reagent. 

Analogous to the on-column derivatization used in gas chromatography, an oxidative chemical transformation was carried out on a TLC plate by m-chloroperbenzoic acid. The oxygen analogs of fenthion, disulfoton, and phorate that were produced were separated after this derivatization (57) (Table 3). Another example of on-plate derivatization involves a reduction reaction carried out in order to determine the residues of methylparathion, parathion, and fenitrothion. Parent compounds and corresponding amino derivatives were also separated (58) (Table 3). 

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