Reactions on TLC Plates

Glass being an inert material used for TLC-plates renders it ideal for utilization with strong corrosive reagents. 

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  

In actual practice, the resulting Rf value of the original compound together with the chromatographic results of the reaction are usually good enough to identify a compound accurately and precisely. 

Example (t) Citral reacts with 30% H202 in the presence of UV-light for a duration 10 minutes and undergoes catalytic oxidation to yield geranic acid as shown below  

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

---

The use of microwave equipment to promote reactions on TLC plates has also been reported [115]. However, this is not yet widely used in the laboratory, as, apart from the fact that equipment with direct fume extraction is not yet available, aluminum foil has to be ruled out as a support for sorbents ... 

Positive reactions on TLC plates with both ninhydrin and molybdene blue reagents, as well as IR spectra indicated PX to be an amino acid-containing glycerophospholipid [3], Data of H, DEPT, C-H and H-H COSY NMR spectra from... 

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

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

The effect on reactive compounds and potential alteration of mutagenic activity of the acid-base extraction procedure were major concerns. The TLC method was not recommended because of possible mutagen loss due to reactions on the plate. The Florisil and low-pressure chromatographic methods were considered to be research methods. 

The reaction of 2,4-dinitrofluorobenzene (DNFB) (Sanger s reagent [10]) with amino acids is another useful technique which is often employed for the analysis of N-terminal amino acids by TLC and column chromatography after derivatization. The reaction involved in product formation is shown in Fig.4.6. The separated derivatives are determined by measuring the quenching of fluorescence on TLC plates or by UV analysis after column chromatography. The generalized absorption curves of dinitrophenyl (DNP)-amino acids in acidic and alkaline solutions are shown in Fig. 4.7. 

TLC separation of DNS-amines is usually made on layers of silica gel with solvents covering a range of polarity, e.g., chloroform, ethyl acetate, diisopropyl ether and methanol. Seiler and Wiechmann [97] developed 30 solvent systems for the separation of DNS derivatives of over 100 biogenic amines on TLC plates of silica gel. The selective reaction of DNS-C1 with the amino group of catecholamines has been examined [98]. The drugs dopamine, norepinephrine and epinephrine are adsorbed on alumina which protects their hydroxyl groups from dansylation. The N-dansylated compounds are separated with benzene-dioxane-acetic acid (90 25 4) on layers of silica gel. 

A similar method was described for the quantitative determination of Melphalan on TLC plates [63b]. The plate is sprayed with an acidic solution of 4-pyridinecarboxaldehyde 2-benzothiazolylhydrazone and heated over an acetophenone bath at 200°. After cooling, the plate is sprayed with triethylamine, and the red spots are measured at 530 nm with a densitometer. The detection limit is 0.2 pg. The reactions are shown in scheme II B. 

After development on TLC plates, amino-substituted 1,3,5-triazines can be revealed by use of the chloro-toluidine reaction <83JC97>. This is a sensitive method developed for the detection of amino and imino compounds on paper chromatograms. The method depends on the formation of chloramines which are then detected by liberation of iodine from a starch-potassium iodide reagent. 

A systematic study has been performed on the protonation reactions of the hydrides HRu3(CO)9 C2R(OH)R (Fig. 21a) in organic solvents the yields of dehydrated products increase either with time or on increasing the acid concentration, thus indicating direct electrophilic attack of H" " on the alcoholic OH in a process comparable with the protonation mechanisms reported for alcohols on surfaces. Very low yields of dehydrated products were, however, obtained on the silica of the TLC plates the same occurred when hydration reactions of the HRu3(CO)9 C2C(=CH2)R derivatives were attempted either on TLC plates or in a slurry (suspension) of silica and water in hydrocarbon solvents. This indicates that surfaee materials play a minor role in these reactions. 

Sulphhydryl groups react with fluorescein isothiocyanate, and phenolic groups (of tyrosine) react to a small extent under the above reaction conditions. Fluorescein thiohydantions in amounts exceeding 1 nmol are visible as yellow spots on TLC plates and show up as intense greenish-yellow fluorescent spots under UV light. 

The chromogenic spray reagents used to visualize sugars on TLC plates can be classified into four main types, according to the kind of reaction responsible for color formation ... 

Should high temperature be necessary for pyrolysis or to accelerate a reaction, the TLC plate is laid on an aluminium plate, itself lying on an asbestos mat. The layer is then heated with an infra red heater at a distance of 10—20 cm. The so-called quartz surface evaporator (30 cm diameter) has proved most useful in our work (Firm 70). The heating spirals reach temperatures up to 800° C and, like the quartz reflector, are not attacked by the usual acid vapours. Heidbbink [284] has recommended in connection with pyrolysis of the substances, that the chromatograms should be previously placed in a atmosphere of gas, e. g., CI2, Brg NOg. 

The final detection method determines the sample preparation procedures. Highly selective detection methods need less thorough cleanup. That is the case when photosynthesis-inhibiting herbicides are detected on TLC plates by Hill reaction inhibition (3,4). If the final detection is less specific for the pesticides, a greater degree of sample preparation is required. 

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. 

---