Hydrogen fluoride, from decomposition

Catalysis by protonic acids accounts for the hydrolysis of fluorides, such as trityl fluoride [31], or elimination of hydrogen fluoride from various systems (Chapter 6), frequently being autocatalytic. The hydrolysis of benzyl fluoride is roughly proportional to the Hammett acidity function [32], which is consistent with the scheme indicated in Figure 5.14 [1, 33]. Indeed, the decomposition of benzyl fluoride, on storage, may be violent [34]. 

Other preparative methods include direct synthesis from the elements, reaction between gaseous hydrogen fluoride and titanium tetrachloride, and decomposition of barium hexafluorotitanate [31252-69-6] BaTiF, or ammonium, (NH 2TiFg. 

Solutions of this and the hexalluoroantimonate salt in hydrogen fluoride, kept for extended periods between —50 and +50°C, burst the Kel-F or Teflon FEP containers. This was attributed to excess pressure of hydrogen fluoride and nitrogen arising from decomposition of the salts. The variable rates of decomposition indicated catalysis by trace impurities. The salts also decompose exothermally after a short period at ambient temperature. 

Vesicants produce acidic products including hydrogen chloride (HC1), hydrogen bromide (HBr), or hydrogen fluoride (HF), and ethanolamines, thioglycols, or thioethers when hydrolyzed. Arsenous oxide decomposition products from HL (C03-A010) are toxic and may also have vesicant properties. HL will also produce acetylene at higher pH. 

Conversely, exposure to hydrogen fluoride at 0 °C over short periods of time leads to almost quantitative hydrolysis.[37-39] In aprotic organic solvents, decomposition takes place unless the sulfate hemi-ester is neutralized with strong counterions.[37,381 The results of a detailed study are summarized in Figures 1 and 2t4°l which show the effect of various acids used in peptide chemistry in deprotection and in cleavage from the resin as well as the effect of temperature on the hydrolysis of tyrosine 0-sulfate as a sodium salt. 

It was found that the addition of 0.5% hydrogen fluoride to red fuming nitric acid (RFNA) removed the large pressure increase observed when normal RFNA was stored in the aluminum tanks used in certain rocket engine systems. Apparently this small amount of HF passivated the surface of the metal and prevented it from reacting with RFNA to generate decomposition gases. 

Providing no other acidic species are present and water has been used as the absorption medium in either the 02 flask or pyrohydrolytic tube decomposition, the acidimetric titration of hydrogen fluoride is the simplest. Unfortunately this is often not the case, so one of the other finishes such as colorimetry and titrimetry has to be used. One of the best colorimetric procedures employs Ce-Alizarin complexone reagent5 which changes from wine red to dark blue in the presence of fluoride ion. The color takes a while to develop fully, but it is very reliable. 

As previously discussed, the main problems in fluorodediazoniation result either from the formation of Lewis acids during the decomposition step (in the diazonium tetrafluoroborate method) or from the presence (or formation) of water in the reaction medium (during in situ fluorodediazoniation in aqueous tetrafluoroboric acid or anhydrous hydrogen fluoride). To circumvent these problems, stable covalent 3,3-dialkyl-l-aryltriazenes have been decomposed with anhydrous hydrogen fluoride. In such a technique, a diazonium fluoride is generated in an anhydrous medium from a soluble and nonionic precursor, without formation of water or Lewis acid (Scheme 7). 

Aryltriazenes can also be decomposed by hydrogen fluoride in organic solution after extraction from their aqueous mother phase. In this case, hydrogen fluoride can be used in small excess but the nature of the solvent is crucial for example, tetrahydrofuran gives complex mixtures, dichloromethane promotes radical reactions (dimerizations, reductions) and acetic acid favors triazene decomposition before fluorination. Aromatic and haloaromatic compounds seem to be the best solvents.283 Such a technique, especially suited for the rapid introduction of an 18F atom, has been employed to produce [ 8F]haloperidol (3), the specific receptors of which have been localized in the brain by positron emission transaxial tomography.298... 

Zinc oxide is an odorless, amorphous, white or yellowish-white powder. It is incompatible with chlorinated rubber, linseed oil, magnesium, hydrogen fluoride, aluminum + hexachloroethane, zinc chloride or phosphoric acid water should be avoided. On decomposition, toxic fumes are released from zinc oxide. °... 

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