Fluorinated acids, strength

Fluorinated Acids. This class of compounds is characterized by the strength of the fluorocarbon acids, eg, CF COOH, approaching that of mineral acids. This property results from the strong inductive effect of fluorine and is markedly less when the fluorocarbon group is moved away from the carbonyl group. Generally, their reactions are similar to organic acids and they find apphcations, particularly trifluoroacetic acid [76-05-1] and its anhydride [407-25-0] as promotors in the preparation of esters and ketones and in nitration reactions. 

Effect of acid strength of co-precipitated chroMc aluMna catalyst on the conversion and selectivity in the fluorination of 2-chloro-1,1,1-trifluoroeth-ane to 1,1,1,2-tetrafluoroethane (Rao et al., 1999)... 

The most important use of /3-sultones is for the preparation of fluorinated polymers such as Nafion 64. These solid acid catalysts containing perfluorinated sulfonic acid groups have been known for many years and the presence of the electron-withdrawing F atoms increases the acid strength of the terminal sulfonic acid groups, which become comparable to that of pure sulfuric acid. Prior to the last decade, Nafion had been in use as a superacid, a fuel cell electrolyte and as a membrane-ion separator <1996CHEC-II(1B)1083>. 

Superficially, we would expect the high electronegativity of fluorine to stabilise a carbanion centre, but measurements of acid strengths and exchange rates for a variety... 

Addition of flourine to H-mordenite enhanced considerably the acid strength of this catalyst but decreased the ratio of Brpnsted to Lewis acidity (167). Using IR spectroscopy of adsorbed pyridine, X-Ray diffraction, catalytic activity tests for cumene cracking, and microcalorimetric measurements of ammonia adsorption, it was shown that some of the acidic hydroxyl groups were substituted with fluorine and that the inductive effect of fluorine increased the acid strength of the remaining hydroxyl groups. 

Contact times of over 5 min do not affect the yield, and the octane number is at a slight maximum at a contact time of 15 min. An acid strength of 87.5 per cent gives the maximum octane alkylate, but the balance of the acid phase should be organic. A water content of less than 2 per cent is not very harmful, but octanes are replaced by heavier and lower hydrocarbons as the water content builds up. Fluorine content and olefin content in the alkylate build up in the presence of excess water. 

Partially fluorinated surfactants have several advantages with respect to their per-fluorinated counterparts, due to the low solubility of fuUy fluorinated chains in hydrocarbons. The hydrocarbon segment of a partially fluorinated surfactant provides solubility in commonly used organic solvents, lowers the melting point of the surfactant, reduces volatility and decreases the acid strength of perfluorinated acids [51]. 

The acid strength of carboxylic acids also increases as the number of electronegative atoms in the acid increases. For example, trifluoroacetic acid (CF3COOH) has K = 5.0 X 10 the replacement of three hydrogen atoms of acetic acid with more electronegative fluorine atoms leads to a large increase in acid strength. 

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