Ethers from fluoroaromatics

Diazotization in the presence of boron trifluoride enables diazonium tetrafluoroborates to be isolated from the reaction mixture and purified. Subsequent controlled decomposition produces the required fluoroaromatic. Although explosion hazards and the toxicity of the isolated salts are significant concerns with this process, known as the Balz-Schiemann process, 4,4 -di-fluorobenzophenone (BDF. 6) has been prepared by this route as a monomer for the production of the engineering plastic poly(ether ether ketone) , or PEEK , by condensation with 1,4-dihydroxybenzene in the presence of potassium carbonate. BDF 6 is superior to its chlorine analog because in aromatic systems the nucleophilic displacement of fluorine is more facile than that of chlorine, leading to a shorter polymerization time and a better quality product containing less degradation impurities. 

Another approach is shown in Figure 14. Here, we have an aryl ether cross-link resulting from a p-fluoroaromatic pendant group which undergoes facile nucleophilic substitution with the dlpotasslum salt of hexafluoroacetone blsphenol. Of course, n is much greater than m and can be controlled by altering stoichiometry. 

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