Alkyl fluorides, preparation

Alkyl fluorides may be prepared in moderate yield by interaction of an alkyl bromide with anhydrous potassium fluoride in the presence of dry ethylene glycol as a solvent for the inorganic fluoride, for example ... 

The order of reactivity of the hydrogen halides parallels their acidity HI > HBr > HCl >> HF Hydrogen iodide is used infrequently however and the reaction of alco hols with hydrogen fluoride is not a useful method for the preparation of alkyl fluorides Among the various classes of alcohols tertiary alcohols are observed to be the most reactive and primary alcohols the least reactive... 

In media such as water and alcohols fluoride ion is strongly solvated by hydro gen bonding and is neither very basic nor very nucleophilic On the other hand the poorly solvated or naked fluoride 10ns that are present when potassium fluoride dis solves m benzene m the presence of a crown ether are better able to express their anionic reactivity Thus alkyl halides react with potassium fluoride m benzene containing 18 crown 6 thereby providing a method for the preparation of otherwise difficultly acces sible alkyl fluorides... 

Alkyl fluorides have been prepared by reaction between elementary fluorine and the paraffins, by the addition of hydrogen fluoride to olefins, by the reaction of alkyl halides with mercurous fluoride, with mercuric fluoride, with silver fluoride, or with potassium fluoride under pressure. The procedure used is based on that of Hoffmann involving interaction at atmospheric pressure of anhydrous potassium fluoride with an alkyl halide in the presence of ethylene glycol as a solvent for the inorganic fluoride a small amount of olefin accompanies the alkyl fluoride produced and is readily removed by treatment with bromine-potassium bromide solution. Methods for the preparation of alkyl monofluorides have been reviewed. ... 

Alkyl fluorides are more reactive than other alkyl halides under Friedel-Crafts conditions, whereas trifluoromethyl groups are less reactive than other trihalomethyl groups Thus, a bromoindane is prepared from 1-bromo-l-fluoro-2,2,3,3-tetramethylcyclopropane and benzene [5] (equation 8) whereas 3-tnfluo romethylphenyldiphenylchloromethane is obtained from 3-trifluoromethyl-benzotrichloride and benzene [9] (equation 9)... 

Cyclohexyl bromide, for exfflnple, is converted to cyclohexene by sodium ethoxide in ethanol over 60 times faster than cyclohexyl chloride. Iodide is the best leaving group in a dehydrohalogenation reaction, fluoride the poorest. Fluoride is such a poor leaving group that alkyl fluorides are rarely used as starting materials in the preparation of alkenes. 

Alkyl fluorides can be prepared by the Finkelstein reaction. The fluoride anion is a bad leaving group the reverse reaction thus does not take place easily, and the equilibrium lies far to the right. As reagents potassium fluoride, silver fluoride or gaseous hydrogen fluoride may be used. 

For reviews of the use of halogen exchange to prepare alkyl fluorides, see Sharts, C.M. ... 

Perdeuteriated isopropyl, t-butyl, and t-pentyl fluorides were prepared from the corresponding deuteriated alkyl chlorides (bromides) by halogen exchange. The perdeuteriated alkyl fluorides were then used in the formation of the deuteriated carbonium ions under similar conditions as those for the protium complexes. The resonance spectra were obtained at 9 2 Mc/s and the data are summarized in Table 7... 

The HF-pyridine reagent is an effective complement to di-methylaminosulfur trifluoride (DAST) reagent6 in the preparation of alkyl fluorides from alcohols. DAST is also useful for the conversion of carbonyl groups to difluoromethylene functions. The... 

Preparation of Tertiary and Secondary-Alkyl Fluorides from Alcohols with Hydrogen Fluoride-Pyridine Reagent... 

Furthermore, 41 % hydrogen fluoride/ polyvinylpyridine combined with jV-bromosuccinimide as coreagent has been used in the preparation of alkyl fluorides from alcohols (Table 18) in good yields. 

Table 1. Preparation of Alkyl Fluorides by the Decomposition of Alkyl Fluoroformates...

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