Potassium permanganate purple benzene

Using dicyclohexyl-18-crown-6 it is possible to dissolve potassium hydroxide in benzene at a concentration which exceeds 0.15 mol dm-3 (Pedersen, 1967). The free OH- has been shown to be an excellent reagent for ester hydrolysis under such conditions. The related solubilization of potassium permanganate in benzene, to yield purple benzene , enables oxidations to be performed in this solvent (Hiraoka, 1982). Thus, it is possible to oxidize a range of alkenes, alcohols, aldehydes, and alkylbenzenes under mild conditions using this solubilized reagent. For example, purple benzene will oxidize many alkenes or alcohols virtually instantaneously at room temperature to yield the corresponding carboxylic acids in near-quantitative yields (Sam Simmons, 1972). 

Potassium permanganate can be readily solubilized in nonpolar media by complex-ing the potassium ion with a crown ether. Sam and Simmons [7] found that dicyclo-hexyl-18-crown-6 could solubilize solid potassium permanganate in benzene to the extent of about 0.06 molar. The resulting purple benzene solution was used to oxidize a number of organic substrates in good to excellent yield. Some of their results are recorded in Table 11.3. Similar results have recently been reported by other workers [8, 9]. 

Suspend in a round-bottomed flask 1 g. of the substance in 75-80 ml. of boihng water to which about 0 -5 g. of sodium carbonate crystals have been added, and introduce slowly 4 g. of finely-powdered potassium permanganate. Heat under reflux until the purple colour of the permanganate has disappeared (1-4 hours). Allow the mixture to cool and carefully acidify with dilute sulphuric acid. Heat the mixture under reflux for a further 30 minutes and then cool. Remove any excess of manganese dioxide by the addition of a little sodium bisulphite. Filter the precipitated acid and recrystallise it from a suitable solvent (e.g., benzene, alcohol, dilute alcohol or water). If the acid does not separate from the solution, extract it with ether, benzene or carbon tetrachloride. 

Potassium Permanganate KMn04/i8-Crown-6 (purple benzene)... 

In the presence of 18-crown-6, potassium permanganate dissolves in benzene to give purple benzene, a useful reagent for oxidizing alkenes in an aprotic environment. Use a drawing of the complex to show why KMn04 dissolves in benzene and why the reactivity of the permanganate ion is enhanced. 

Homogeneous oxidations are accomplished with tetrabutylammonium permanganate prepared by treating an aqueous solution of potassium permanganate with tetrabutylammonium bromide and isolating the crystalline precipitate [845, 599]. Tetrabutylammonium permanganate is soluble in dichloromethane, chloroform, acetone, and pyridine and sparingly soluble in benzene ( purple benzene ). 

Thin-layer chromatography using silica get has been used to isolate and identify thiopental [36,47]. Elution from silica gel was achieved with benzene-glacial acetic acid (1 9). The Rf under these conditions was 0.47, the recovery better than 95% and the sensitivity as low as 0.5 pg [36]. Other solvents have included chloroform-acetone (9 1) and dioxan, benzene and aqueous ammonia (20 75 5) [47]. Using a potassium permanganate spray, thiopental could be identified as a yellow spot on a purple background [47]. 

Crown ethers have found a niche in the organic chemistry laboratory as phase transfer reagents. Thus, although potassium permangante (KMn04) is insoluble in benzene (CeHe), the addition of a trace of dibenzo[18]-crown-6 produces a purple solution ( purple benzene ) and permits transfer of the permanganate ion (Mn04 ) into a medium, for example, for oxidation, in which it would otherwise be insoluble. 

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