Hydrogen difluoride

Two well-known salts of ammonia (qv) are the normal ammonium fluoride [12125-01 -8] NH F, and ammonium bifluoride [1341 -49-7] NH4HF2 the latter is sometimes named ammonium acid, or hydrogen difluoride. Much of the commercial interest in the ammonium fluorides stems from their chemical reactivity as less ha2ardous substitutes for hydrofluoric acid. 

Properties. Other names for potassium bifluoride are potassium hydrogen difluoride and potassium acid fluoride. This white crystalline salt is a soft, waxy soHd. The crystal forms of potassium bifluoride are tetragonal and cubic (21). The bifluoride ion in KHF2 averages 0.2292 nm between fluoride... 

Tetrabutylphosphonium fluoride, tetrabutylphosphonium hydrogen difluoride and tetrabutylphosphonium dihydrogen trifluonde can be prepared from tetrabutylphosphonium hydroxide and hydrogen fluoride (equation 19)... 

An extreme example of the fluorine-hydrogen bond is found in the hydrogen difluoride ion, HFS. This ion exists in acidic solutions of fluorides,... 

Minkwitz, R. et al., Z. Naturforsch. B, Chem. Sci., 1988, 43B, 1478-1480 The impure material is very explosive [1]. The pine material, prepared by a ther-molytic method, is stable as a solid at -103°C, but on melting it decomposes to ammonium hydrogen difluoride and nitrogen [2],... 

In contrast, chemical etching of the inner wall of the fused-silica capillaries was used to increase the surface area. This enables achievement of a higher phase ratio since more alkyl functionalities can be attached to the surface, thus improving both the separation process and loadability of the column. The surface morphology of the etched capillary depends on the time the methanol solution of ammonium hydrogen difluoride is left in contact with the capillary and temperature at which the reaction is carried out (Fig. 10) [77]. The surface features have been described by Pesek to range from spikes of silica material extending... 

Fig. 11A-C. Scanning electron micrographs of fused silica capillary surfaces etched with methanolic ammonium hydrogen difluoride solution. (Reprinted with permission from [78], Copyright 2000 Elsevier). Etching process was carried out for A 3 h at 300 °C B, 2 h at 300 °C and 2 h at 400 °C C 2 h at 300 °C and 1 h at 400 °C...

Quaternary ammonium hydrogen difluorides and dihydrogen trifluorides... 

Quaternary ammonium hydrogen difluorides KHF2 (7.8 g, O.l mol) is added to the quaternary ammonium hydrogen sulphate (O.l mol), neutralized with KHCOj (10 g), in CHClj (500 ml) and H20 (25 ml) and the mixture is stirred at room temperature for ca. 

I h. The organic phase is separated and evaporated to yield the quaternary ammonium hydrogen difluoride. Any dihydrogen trifluoride salt formed at the same time can be removed by dissolution of the product in MeCN and neutralization with anhydrous... 

The ability of quaternary ammonium halides to form weakly H-bonded complex ion-pairs with acids is well established, as illustrated by the stability of quaternary ammonium hydrogen difluoride and dihydrogen trifluorides [e.g. 60] and the extractability of halogen acids [61]. It has also been shown that weaker acids, such as hypochlorous acid, carboxylic acids, phenols, alcohols and hydrogen peroxide [61-64] also form complex ion-pairs. Such ion-pairs can often be beneficial in phase-transfer reactions, but the lipophilic nature of H-bonded complex ion-pairs with oxy acids, e.g. [Q+X HOAr] or [Q+X HO.CO.R], inhibits O-alkylation reactions necessitating the maintenance of the aqueous phase at pH > 7.0 with sodium or potassium carbonate to ensure effective formation of ethers or esterification [49,64]. 

Carbanions, generated by the reaction of benzylsilanes with tetra-n-butylammo-nium fluoride react with non-enolizable aldehydes to produce the alcohol [67], When a stoichiometric amount of the ammonium fluoride is used, the methylarene corresponding to the benzylsilane is frequently a by-product and arises from formation of the hydrogen difluoride salt during the reaction. When only catalytic amounts of the ammonium fluoride initiate the reaction, the formation of the methylarene is suppressed. In a similar type of reaction (although the mechanism is not known) between aldehydes and ketones, allyl bromide, and tin in the presence of trimethylsilyl chloride the yield of the but-l-en-4-ol is raised significantly by the addition of tetra-n-butylammonium bromide, particularly in the reactions with... 

Oxygen difluoride can be produced by electrolysis of an aqueous solution of HF or, alternatively, electroylzing molten potassium hydrogen difluoride, KHF2, in the presence of water. 

Potassium fluoride-poly(hydrogen fluoride) salts are useful fluorine sources for bromo- and iodofluorinations of various alkenes (Method F).49 The best yields arc obtained with KH3F4 and KH4F5. The salts are prepared by mixing the calculated amounts of potassium hydrogen difluoride and anhydrous hydrogen fluoride in a reaction vessel made of polyethylene or Hastel-loy B. The mixture is stirred and heated at 60-70 C to form a homogeneous solution of the salt which solidifies at room temperature. 

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