Chemical Reactions on Thin Layer Chromatography (TLC) Plates

CHEMICAL 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 per-forming chemical unit-process reactions directly on TLC-plates. The two major steps involved in achieving this objective are, namely :

(a) Sample is spotted on a TLC plate in the usual manner and subsequently covered with a specific reagent, and

(b) Soonafter the reaction is completed, the TLC plate is developed using an appropriate solvent thereby separating the products of the reaction.

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 : (i) Citral reacts with 30% H₂O₂ in the presence of UV-light for a duration 10 minutes and undergoes catalytic oxidation to yield geranic acid as shown below :

(ii) Citral undergoes reduction in the presence of 10% w/v solution of LiAlH₄ in ether to produce geraniol as represented in the following reaction :

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

(a) Dehydration : 

Sample spot of terpene alcohols e.g., linalol, be converted to hydrocarbons by adding a drop of conc. H₂SO₄ as shown below :

Consequently, the TLC plate is developed with hexane and since oxygenated compounds, do not move in hexane (i.e., stay-back), only the hydrocarbons thus generated move away from the specific-reaction zone.

(b) Bromination : 

Cargill* in 1962, separated cholestanol from cholesterol by TLC. The mixture is spotted on a TLC plate and reacted with a soln. of Br₂ (0.1% w/v in CHCl₃), taking care that its quantity must be 2 to 3 times the weight of the sample mixture. Development in a solvent system consisting of benzene and ethyl acetate (2 : 1) would result in a clear distinction of cholestanol and reaction products of cholesterol with Br₂.

(c) Enzymatic Reaction : 

Randerath and Randerath** in 1964, demonstrated an enzymatic reaction directly on an anion-exchange layer of cellulose impregnated with polyethylene imine. A buffered solution of phosphodiesterase is applied to the sample spot of cytidine dipohosphate glucose, which is subsequently covered with paraffin and allowed to stand for 45-60 minutes at 23°C. Chromatography of the resulting degradation products gives rise to cytidine 5-monophosphate and glucose 1-phosphate.

(d) Esterification : 

Benneth and Heftmann*** in 1962, showed that it was feasible to esterify the C-3, hydroxy steroids directly on TLC plates by means of tri-fluoroacetic anhydride. After treating the compounds with the anhydride, it is absolutely necessary to dry the pate in the hood for several minutes so as to get rid of the trifluoroacetic acid that is produced as a by-product.