Reactions during fluorination

Other side reactions during fluorinations with DAST are skeletal rearrangements, which arc frequently observed in steroid chemistry. For example, treatment of 19-hydroxycholest-4-en-3-one (12) with DAST in refluxing acetonitrile gives the rearranged product 13. Further examples of similar rearrangements are cited in ref 38. 

An attempt is also being made to preserve metal-metal bonds during direct fluorination. It has been found that the reaction of fluorine with hexamethyldigermane leads primarily to tris(trisfluoromethyl)-germanium fluoride (34). 

A residue remains undecomposed above 893 K. The mass fraction of this residue is 0.656, whereas the mass fraction of Mg contained within the pyrolant is Mg(0-600). This indicates that the residue is produced by oxidation of the Mg by the fluorine produced upon thermal decomposition of the Tf An X-ray analysis revealed that the remaining residue above 893 K in the TG experiments consisted of Mg and Mgp2. The oxidation reaction during the decomposition of Mg-Tf pyrolants is thus ... 

Subsequent papers in this series have reported the electrochemical fluorination of di- and tri-fluorobenzenes, (Part II) [17] trifluoromethyl-benzenes, (Part III) [18] chlorobenzene, (Part IV) [19] as well as side reactions during the fluorination of halobenzenes, (Part V) [20]. 

Expln reaction during reduction of fluorinated compounds with LiAlH4 6 E384... 

If this trend is extrapolated to lower temperatures it is clear that at temperatures varying between -78 and -150°C for carbon—hydrogen bonds on various types of hydrocarbons, AF becomes zero and then endothermic. Under these conditions (AG < 0) the reaction with elemental fluorine will not and should not proceed at all Wfe have observed this in many cases experimentally. At temperatures as low as -150°C the reaction of fluorine with all studied organic compounds either does not occur or is extremely slow. We have occasionally reported reactions in the cryogenic reactor as low as -120°C. It should be explained that the — 120°C reported is the temperature of a single trap in a multizone reactor rather than that for the entire reactor. It is possible that fluorination occurs during the volatilization process at such low temperatures. 

Chlorine is much the most abundant halogen. It is six times more abundant than fluorine and 450 times more abundantthan bromine. This is understood, because both stable Cl isotopes are produced in the main line ofnuclear reactions during oxygen burning in stars. Cl is the 20th most abundant element in the universe, being almost identical in abundance to potassium and 1.5 times more abundant than titanium. But measurements of Cl abundance in stars are few, because it is rare and its emission lines are unfavorable. 

Heat treatment of the S-type fluoride in a fluorine atmosphere Based on the results above mentioned, Fujimoto et al. developed a new fluorination procedure in order to prepare the perfluorinated pitch, and obtained two types of other fluorinated pitches [23,24], The new process is by the heat treatment at 200-400°C of S-type of fluorinated pitch prepared at relatively low temperature in a fluorine atmosphere. They firstly fluorinated the mesophase pitch at 70°C for 10 h (first step for the preparation of S-type fluorinated pitch) and then heated up to a selected temperature between 200°C and 400°C, and maintained this temperature for 12 h (second step for the heat-treatment of fluorinated pitch). Thus, they obtained two kinds of fluorocarbons, a transparent resin (R-type) and a liquid (L-type). L-type is a viscous fluid containing some volatile materials and the viscosity gradually becomes higher when it is kept for a few weeks in an air atmosphere even at ambient temperature. They reported that the R-type was obtained in the nickel boat in the heating zone and L-type at the bottom of the vertical reaction vessel which was cooled down by the water. Therefore, it is likely that the liquid fluorocarbon is formed by the vaporization of some component contained in the S-type fluoride or decomposition reaction during the heat treatment of the S-type fluoride. The yields of these compounds depends on the heat treatment temperature. In Fig. 3, the yields of the R-type and L-type fluorocarbons are plotted as a function of the heat treatment temperature of the S-type fluoride. The yield of the former decreases with increase of the heat treatment temperature and finally, at 400 C, it can not be obtained at all. On the other hand, the yield of the latter increases with increase of temperature and it is selectively obtained at 400°C. 

These compounds may be prepared directly from the elements, although this route is not always the most suitable. The reactivity of fluorine is such that the pentafluoride is formed as a secondary product with phosphorus and arsenic and also to a smaller extent with antimony. Bismuth becomes coated during fluorination with a solid layer of fluoride which hinders further reaction. 

The reaction for fluorine (Rl) is highly exothermic, while the reactions for chlorine (R2), bromine (R3), and iodine (R4) are endothermic. The heats of these reactions are 30.8, — 1.2, — 16.7, and — 32.7 kcal/mol for reactions (Rl), (R2), (R3), and (R4), respectively. According to Hammond s postulate, reaction (Rl) should have an early TS, and reactions (R2) and (R3) should have late TSs. What the electronic states are during these reactions, and how the CASVB method describes the electronic structure, are our interests in this section. 

Poly(tetrafluoroethylene) (FIFE) is used at a very low level (0.01 to 0.5 wt%) in combination with other flame retardants to suppress flaming drips. The flame retardant action of PTFE is not related to any chemical reaction of fluorine or halogen fluoride. During polymer processing at 200 to 300°C, PTFE particles soften, the shear force of extrusion elongates the particles up to 500%, and microfibrils are formed. Upon combustion the microfibrils shrink back when the polymer melts and a network that prevents dripping is formed. This flame retardant action of PTFE is a physical phenomenon. 

---