Other websites have discussed the use of TLC stains. My post here shares one example of a reaction that was successfully visualized using KMnO4 dip.
As part of a project to prepare homologues of a drug, 2-Methyltetrahydrofuran was reacted with HBr for a ring opening to give a hydroxyl at one end and bromine at the other. However these products readily dibrominated and any of the free alcohol compounds left recyclized on contact with silica. Therefore acetic anhydride was added to the reaction and the ring opened product thus immediately acylated. The two products obtained (drawn below) were viewable by NMR in what looked like an 80:20 ratio. The mixture had a fruity, apple-like aroma, with a distinct C-Br bond smell that was absent in the 2-MeTHF start material which smelled minty/ethereal.
As part of a project to prepare homologues of a drug, 2-Methyltetrahydrofuran was reacted with HBr for a ring opening to give a hydroxyl at one end and bromine at the other. However these products readily dibrominated and any of the free alcohol compounds left recyclized on contact with silica. Therefore acetic anhydride was added to the reaction and the ring opened product thus immediately acylated. The two products obtained (drawn below) were viewable by NMR in what looked like an 80:20 ratio. The mixture had a fruity, apple-like aroma, with a distinct C-Br bond smell that was absent in the 2-MeTHF start material which smelled minty/ethereal.
Using "Merck TLC Silica gel 60 F254", this TLC was run in 3:1 hexanes/EtOAc. As these compounds had no chromophore, they were not visible in a UV box. A number of stains were tried, including iodine vapours which sometimes gave faint spots. KMnO4 which at first gave no spot was re-tried. When heated gently with a hair dryer (not a heat gun) two spots for the compounds became visible! If the hair dryer was not switched off within 2 seconds of the appearance of the spots, they vanished, presumably due to entering the vapour phase and disappearing into the air. If the hair dryer was stopped at the right moment, the TLC remained visualizable for 2 or more hours (see photo). These spots were in the same location as those seen from the iodine chamber, however these compounds departed the plate before the iodine could effectively stain them.
This project was abandoned to focus on other pursuits, but if I revisit it I'd like to try dry column vacuum chromatography (DCVC) to separate the compounds. While they looked to be separable it must be remembered that each spot is a racemic mixture of two compounds as a chiral centre has been created at the point on the alkyl chain with the methyl group.
This project was abandoned to focus on other pursuits, but if I revisit it I'd like to try dry column vacuum chromatography (DCVC) to separate the compounds. While they looked to be separable it must be remembered that each spot is a racemic mixture of two compounds as a chiral centre has been created at the point on the alkyl chain with the methyl group.
