Contents 21 Hydrogen Fluoride

Fluorine cannot be prepared directly by chemical methods. It is prepared in the laboratory and on an industrial scale by electrolysis. Two methods are employed (a) using fused potassium hydrogen-fluoride, KHFj, ill a cell heated electrically to 520-570 K or (b) using fused electrolyte, of composition KF HF = 1 2, in a cell at 340-370 K which can be electrically or steam heated. Moissan, who first isolated fluorine in 1886, used a method very similar to (b) and it is this process which is commonly used in the laboratory and on an industrial scale today. There have been many cell designs but the cell is usually made from steel, or a copper-nickel alloy ( Monel metal). Steel or copper cathodes and specially made amorphous carbon anodes (to minimise attack by fluorine) are used. Hydrogen is formed at the cathode and fluorine at the anode, and the hydrogen fluoride content of the fused electrolyte is maintained by passing in... 

Commercially available hydrogen fluoride usually is not suitable for catalytic hydrogenation because of its sulfur dioxide content An oxidative treatment with manganese dioxide and distillation are needed for the preparation of hydrogena tion-grade hydrogen fluoride [d, 4]... 

As a result of alkylation LAB is obtained with a clearly changed composition in comparison with the use of chloroparaffin. With respect to the dialkyl-tetralin content, values are obtained which are comparable to LAB from the HF alkylation process (same olefin base) (Table 11). Another important difference is the 2-phenylalkane content. The isomer distribution depends on the catalyst. The reaction between straight /z-chloroparaffins or n-olefins with benzene in the presence of aluminum chloride leads to the same isomer distribution. In both cases the 2-phenylalkane content is predominant compared to the 3-, 4-, and 5-phenylalkanes. If hydrogen fluoride is used as catalyst the 2-phenylalkane... 

Sodium LAS synthesized via aluminum chloride catalysis dissolves better than sodium LAS from the hydrogen fluoride route. The main difference is the tetralin content. Dialkyltetralinsulfonates (DATs) function as hydrotropes and this influence can be larger than that of the 2-phenylalkane content. For a homolog (equal alkyl side chains), the higher the DAT content, the lower the... 

The formation of silicon-flvxyride bonds on the surface of silica after treatment with hydrogen fluoride was never proven directly. However, there is a pronounced change in the adsorption and wetting properties. The silica becomes hydrophobic as was mentioned in a patent to Kimberlin (279a). Neimark and collaborators (279b) found a type V isotherm in the methanol adsorption on silica gel which had been treated with a solution of SiF in absolute alcohol. Wilska (280) obtained a water-repellent silica when solutions of HaSiPg were precipitated with ammonia. The Si—F bond is hydrolyzed only slowly. A considerable fluorine content of 7-10% F was reported in an older patent (281) for a silica that had been prepared by hydrolysis of SiF. ... 

Just as in the case of aromatic compounds isoparaffins can be alkylated with sources of alkyl groups other than olefins. Alkyl halides, alcohols, ethers, mercaptans, sulfides, etc., can be used. When olefins are used some alkyl fluorides from a combination of olefin and hydrogen fluoride are always formed. The quantity of this in the product can be greatly reduced by providing conditions under which the alkyl fluoride is used in alkylation. The apparent paradox is provided, in that the fluoride content of the product is lessened by further treatment with hydrogen fluoride. A more thorough treatment of the details of the alkylation of isoparaffins with olefins is found elsewhere in this volume. 

Commercial grade anhydrous hydrogen fluoride contains less than 0.5 wt% water and only small amounts of inorganic impurities. Thus, it can be used without purification in most fluorination reactions. Further purification for special purposes is customarily carried by fractional distillation.2 The water content and the purity are determinated by Karl-Fischer titration1 2 7 or by conductivity measurements.u2,a... 

In such hydrogen fluoride free media, which stabilize aryl cations to a lesser extent than 70% hydrogen fluoride/pyridine mixture, aryl cation deprotonation does not occur very rapidly. On the other hand, the fluoride nucleophilicity increases with the pyridine content thus, very high fluorodediazoniation yields can be reached in hydrogen fluoride free media, provided that the diazotization step is conducted under strictly controlled conditions.40,45 67 Illustrative results, obtained by the following general procedure, are listed in Table 2 and compared to those resulting from other conditions when available.40... 

Pyridinium polyhydrogen fluoride and related polyhydrogen fluorides were developed as a modified liquid complex of anhydrous hydrogen fluoride for fluor-ination reactions.39 These complexes are ionic liquids. It was subsequently found40 that such ionic liquids with higher complexed HF content are advantageous reactions media for alkylation and other reactions. 

For minerals in which the titanium content is high it has been found preferable to attack the ore with potassium hydrogen fluoride, KHF2, or concentrated hydrofluoric acid.13 In one such process the powdered... 

In the same way as intramolecular displacement leads to particularly stable atomic groupings within a molecule, the disproportionation reactions between several molecules are attributable to the tendency for the more stable compound with the higher fluorine content to form. Reactions of this kind are sometimes used to obtain highly fluorinated compounds from products with lower fluorine contents, for example, the catalytic fluorination of chloroalkanes with hydrogen fluoride or with fluorination agents such as antimony(V) fluoride or antimony(III) fluoride. The chlorine compound formed as the second product of the disproportionation process is reused as the starting material for the preparation of the compound to be dispropor-tionated. 

Industrial Procedure,u Into 500 parts of benzotrichloride cooled to 0° in a copper flask equipped with a stirrer, an outlet tube, and an inlet tube reaching to the bottom, is introduced 200 parts of hydrogen fluoride over a period of seventy-two hours. At first, only hydrogen chloride escapes later, a mixture of hydrogen fluoride and benzotrifluoride distils and is condensed in a metal receiver cooled with ice and salt. After completion of the reaction, the contents of the receiver and the reaction flask are united and gently warmed to drive off most of the hydrogen fluoride. The remainder is eliminated by agitation with powdered... 

Exposure Periods, Foliar Burn, and Fluorine Content of Plants Exposed to Hydrogen Fluoride, Agr. Food Chem. (1957) 5, 108. 

The gas escaping at the anode contains about 90 % Fa after the hydrogen fluoride has ]jeen condensed by cooling the remainder consists of oxygen, carbon dioxide, silicon tetrafluoride and carbon fluorides. At an anode current density of 10 A per sq. dm current efficiency equals 95 to 99 per cent. The output of one cell is on the average 1 kg of fluorine per hour. The electrolyte content lasts about one year and then it must be replaced. 

The analysis of chlorine trifluoride has been routinely performed by a gas chromatographic technique using a specifically designed corrosion resistant instrument. Well-defined chromatographic peaks are obtained for chlorine trifluoride and all normally expected components, except hydrogen fluoride. The hydrogen fluoride content is determined by an independent near infrared method. 

One of the oldest methods in the literature for the measurement of organic fluorine compounds is the Wickbold method [125], where organic fluorine is converted to hydrogen fluoride via combustion. The Wickbold method is useful for determining the total organic fluorine content of a sample, but is nonspecihc and does not provide information on individual fluorinated molecules. In addition, for samples containing PFCs, the combustion may lead to incomplete decomposition and subsequent underestimation of the total fluorine content of the sample [125]. 

Hydrogen fluoride/pyridine is generally used as a 70 30w/w mixture (molar ratio HF/ pyridine 9 1). Due to the high hydrogen fluoride content, polyethylene bottles are recommended for reactions with 70% hydrogen fluoride/pyridine. 

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