Fluorine Compounds

SECTION 4 Fluorine Compounds w. KWASNIK Genera l Remark s In view of the special position occupied by fluorine among the halogens in the periodic system, the preparation of its compounds is so different from that of the other halogen compounds that it is fitting to consider the fluorine compounds in a separate section. Inorganic fluorine compounds are prepared chiefly by the follow• ing methods: 1. Treatment of the oxides, hydroxides or carbonates with aqueous hydrogen fluoride. Most binary fluorides that do not under• go hydrolysis may be prepared in this way (e.g., alkali fluorides, alkali hydrogen fluorides, alkaline earth fluorides, A1F3, SbF3, ZnF2, PbF2, HgF, AgF). 2. Treatment of the appropriate anhydrous chlorides with anhy• drous hydrogen fluoride (e.g., TiF4, ZrF4, NbFB, TaFB, VF4, SnF4, SbFs, POF3, SOF2). This reaction is capable of much wider applica• tion than that given in the literature until now. This method has become increasingly important since anhydrous hydrogen fluoride became commercially available. 3. Treatment of elements, oxides or halides with elemental fluorine. This method is used chiefly for the preparation of those binary fluorides in which elements reach their highest valence (e.g., IF7, ReF6, UF6, SF6, BiF5, CF4, CoF3, AgF2). The halogen fluorides may often be used instead of elemental fluorine. However, these fluorides can in turn be prepared only from elemental fluorine. There is a drawback in the use of halogen fluorides instead of elemental fluorine in that it is often difficult to separate the free halogen evolved in the reaction from the reaction product. On the other hand, most halogen fluorides are easier to handle in the laboratory (storage, measuring out) than elemental fluorine. Halogen fluorides are definitely to be preferred when, in addition to fluorine, one intends to add a second halogen to an unsaturated substance (e.g., COC1F, COBrF, COIF, SQ,BrF). In general, halogen fluorides may be either more active (C1F3) or less active (IF5) fluorinating agents than the element itself. 150 4. FLUORINE COMPOUNDS 151 4. Special reactions. These are so different from each other and occur so sporadically that they cannot be classified in any sys tematic way. For example, NF3 may be prepared by electrolysis of molten ammonium hydrogen fluoride; OF2 is produced by attack of F2 on 2% sodium hydroxide solution; and C^Fg is formed from F 2 + Ο 2 in a glow discharge tube cooled with liquid nitrogen. Some fluorine compounds may be prepared by any one of several methods (e.g., NOF from NO + F2 or from NOBF4 + NaF), so that the choice of a method of preparation may be based on the availability of starting materials or apparatus. The preparation of organic fluorine compounds also requires methods different from those used for the other organic halogen compounds. Because of the high heat of formation of CF4 (231 kcal.) and HF (64 kcal.), the treatment of organic substances with fluorine results mainly in the formation of CF4 and HF, in addition to charred and tarry substances, while no appreciable quantities of normal substitution products are obtained. This method, which is used in industry for the production of perfluorocarbons, is not suitable for work on a laboratory scale. Organic fluorine compounds are prepared in reasonable yields chiefly by the following procedures. 1. Addition of HF to olefins (e.g., ethyl fluoride). 2. Treatment of a chlorine compound with anhydrous hydrogen fluoride (e.g., benzotrifluoride). This method is limited to com pounds with three fluorine atoms on the carbon atom. Acid fluo rides can also be produced from acid chlorides, using anhydrous hydrogen fluoride. 3. Treatment of chlorine compounds with antimony(III) fluoride (F. Swarts). This method is especially suited for compounds with less than three F atoms on the carbon atom (e.g., 2,2-difluoropropane). 4. Treatment of chlorine compounds with anhydrous hydrogen fluoride in the presence of antimony catalysts. This method is intermediate between the two mentioned above and is very broadly applicable (e.g., dichlorodifluoromethane). 5. Treatment of a halogen compound with a metal fluoride, such as AgF, HgF, HgF2 (e.g., fluoroform). 6. Diazotization with nitrite in hydrofluoric acid medium. This method is used for the preparation of aromatic fluorine compounds (e.g., fluorobenzene). 7. Thermal decomposition of diazonium borofluorides (G. Balz and G. Schiemann). This procedure is also applicable to aromatic fluorine compounds, particularly on the laboratory scale (e.g., p-fluorotoluene). The laboratory equipment of the fluorine chemist is unusual in that ordinary chemical glassware cannot be used in most cases. Apparatus made of nickel, iron, copper, lead, silver, platinum, fluorspar or sintered alumina is used instead. Where transparency is indispensable, quartz equipment is utilized. 152 W. KWASN1K To be able to produce fluorides at any time and avoid lengthy preparations, it is advisable to have on hand the following commonly used pieces of equipment: several nickel, fluorspar or sintered alumina boats; a nickel or Monel reaction tube (30 cm. long, 2,5 cm, diameter) with ground joints at both ends; an iron reaction tube; several cylindrical iron vessels; three to five quartz traps with attached ground joint seals; three Pyrex glass traps with attached ground joint seals; several quartz drying tubes; several quartz U tubes; two iron condensers; an iron trap; several steel cylinders, 0.5- to 5-liter capacity (for storage of gaseous fluorides). For work with low-boiling fluorides it is useful to have on hand a part-glass, part-quartz vacuum system provided with a spiral quartz manometer. Connection of the various parts of a quartz apparatus is best accomplished with normal, ungreased ground joints, which are made airtight by an exterior layer of cement (picein). Metal-to-quartz ground joint connections may also be made airtight with picein, but to ensure better adhesion of the cement the metal surface should be very hot. Stopcocks should be greased only lightly, preferably with viscous fluorocarbons; in difficult cases they may be replaced by copper diaphragm valves. Metal pieces may be connected with one another by means of threaded joints, flanges or ground joints. Lead rings with asbestos inserts or soft iron or copper washers may all be used as gaskets where flange connec• tions are used. Needle valves with a steel stem, brass seat and lead-asbestos packing have proven suitable for use with steel cylinders and autoclaves. Fluorspar apparatus is made by the following method: freshly precipitated CaF2 is mixed with water to a thick paste. To obtain plasticity, hydrochloric acid is added until the acidity of the paste is about 0.02N. It is then poured into plaster molds. Two-part molds are used for boats, three-part molds for tubing. After removal from the molds, the pieces are scraped smooth with a spatula if necessary and then air dried for a few days. Since the strength is low, all subsequent handling must be very careful. If the fluor• spar apparatus cannot be fired together with ordinary ceramic ware in a tunnel kiln at about 1250°C, then it should be fired in a Globar furnace. The pieces, embedded in ZrC^, are placed in a porcelain tube and the oven is slowly heated to 1250°C. During the firing, dry nitrogen is passed through the procelain tube to remove water vapor and carbon dioxide. Fluorspar equipment prepared in 4. FLUORINE COMPOUNDS 153 this way is nonporous and smooth. It can be worked on a wet emery wheel. In general, however, it is brittle and must be handled with great care. As a result of drying and firing the pieces shrink by about 1/3 and this should be taken into account in the design. Boats can always be made without difficulty, whereas tubes (say, of 15-cm. length, 1.5-cm, I.D., 2-mm. wall thickness) some• times undergo deformation if the furnace temperature is some• what too high. [O. Ruff and A. Riebeth, Z. anorg. allg. Chem. 173, 373 (1928); O. Ruff and J. Fischer, Z. anorg. allg. Chem. 179, 166 (1929); O. Ruff and W. Kwasnik, Z. anorg. allg. Chem. 209, 113 (1932).] When quartz or glass equipment is used, one must always bear in mind that the reaction products may be contaminated with fluro- silicates or I^SiF6. Under these conditions, gaseous fluorides often contain SiF4. Gaseous fluorides condensed by means of Dry Ice or liquid oxygen* dissolve air in appreciable quantities, as do almost all low-boiling substances. Care must therefore be taken to remove the dissolved air by repeated distillation of the product under vacuum. The air dissolved in these low-boiling compounds may lower their melting point by as much as 20°. General equipment of a fluorine laboratory should include large, high-suction hoods, rubber gloves, protective goggles, a gas mask, and an H2-02 torch for work with quartz apparatus. Dry Ice and liquid oxygen are practically indispensable for the preparation of low-boiling fluorides. One should have a bottle of 10% ammonium carbonate solution ready in case of accident. Skin injuries caused by HF or fluorides should be bathed immediately with this solu• tion or treated with compresses containing this solution. This treatment should be administered even before medical assistance and should be continued for half an hour. Chlorin e Monofluorid e C1F CI, + F2 = 2 C1F 70.92 38 108.92 A vertical nickel or Monel cylinder encased in a furnace (Fig. 107) serves as reaction vessel. A metered stream of chlorine gas is introduced through a nozzle-type tube. Fluorine gas is fed *Caution: liquid oxygen is dangerous in contact with oxidizable substances. 154 W. KWASNIK through a side tube. The lower part of the cylinder serves as a separator for the solid fluorides (NiF3, FeF3) formed by corrosion of the container walls, so that plugging is avoided.

Load more