2. Mechanico-Chemical Hexafluorobenzene Obtaining Method
One of the ways to improve effectiveness of aromatic compounds fluorodechlorination process is its accompanying by physical impact. Thus, using an example of reaction of nucleophilic chlorine replacement for fluorine we demonstrated a fundamental opportunity of carrying out a solid phase synthesis of fluoroaromatic compounds under conditions of mechanical and chemical activation.
Besides that, liquid phase fluorination processes can also be intensified by using fluorinating agent activated beforehand. An important role here belongs to the preliminary preparation of potassium fluoride, which microdispersion and effective surface are particularly essential [70,73-75].
It was demonstrated, that when carrying out a reaction of hexachlorobenzene, octachloronaphtalene, pentachloropyridine and potassium fluoride or other fluorides of alkali and alkaline-earth metals and also composites of a mixture containing (KF-CaF2, K2CO3-CaF2 [76]) made on their base in planet-friction activator (planet mill APF-1M), which forwards mechanical and chemical activation of potassium fluoride (acceleration is 40 g, activativation period 20 min) at 350-500 oC we can obtain a mixture of chlorofluorobenzenes, octafluoronaphtalene and pentafluoropyridine respectively.
KF mechanical activation leads in acceleration of fluorination process and allows to increase yields of poly-fluorinated benzenes or to shorten the period of reaction [70, 73]. Octafluoronaphtalene can be fluorinated most easily, and hexachlorobenzene is a most difficult to be fluorinated. Thus, in 20 minutes activation period the rate of transformation of octachloronaphtalene into fluorine derivatives amounted to 75%, while in the same period the rate of hexachlorobenzene amounted only to 22%. As the period of treatment was being increased we observed a grow of conversion and substitution rate. The application of other metals fluorides in this reaction allowed to make a conclusion, that activity of fluorinating agent was growing from lithium fluoride to cesium fluoride and from calcium fluoride to barium fluoride. However in all the cases we observe a complicated composition of partly substituted products.
The mixture was accumulated and rectified in an ordinary way to isolate individual products. This approach allows decreasing the process period, the excess of applied potassium fluoride and amount of waste products. It is found, that in potassium fluoride-calcium fluoride systems and also potassium carbonate - calcium fluoride the formation of mixed fluoride of KCaF3, occurs, which is a new fluorinating agent of a high activity. Thus, using a composite mixture containing KF + CaF2 the reaction of pentachloropyridine fluorination actually goes only in 30 minutes, while when using KF the products' yield is 35,8% in 120 min, and CaF2 - 14.8%. Compare to potassium fluoride the mixed fluoride shows higher fluorinating ability not only during mechanical and chemical, but particularly during liquid phase process of fluorination.
Using of fluorinating reagent (KF and KCaF3) allowed to obtain 3-chlorine-4-fluoronitrobenzene out of 3,4-dichloronitrobenzene at 150 oC in the sulfolane medium [73].
To be continued