Radiation-induced homolytic aromatic substitution. 6. The effect of metal ions on the hydroxylation of benzonitrile, anisole, and fluorobenzene
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The radiation-induced hydroxylation of benzonitrile, anisole, and fluorobenzene has been investigated. In absence of oxidizing agents (O/sub 2/, metal salts) only trace amounts of substituted phenols are formed. Some metal salts (Fe(CN)/sub 6//sup 3 -/, Fe/sup 3 +/, Cr/sub 2/O/sub 7//sup 2 -/) oxidize the intermediate hydroxycyclohexadienyl radicals and give a very high conversion of OH radicals to phenols, whose isomer distribution is dependent on the metal salts. With Cu/sup 2 +/ salts (Cu(ClO/sub 4/)/sub 2/, CuSO/sub 4/, CuCl/sub 2/) no oxidation of the hydroxycyanocyclohexadienyl radicals was observed. Oxidation with K/sub 3/Fe(CN)/sub 6/ leads in a quantitative way to the corresponding phenols. It was shown that in the hydroxylation of anisole, fluorobenzene, bromobenzene, toluene, benzonitrile, and nitrobenzene a linear relationship exists between log 2k/sub m//2k/sub 0/ + 2k/sub m/ + k/sub p/ and log k/sub p//k/sub m/ and sigma/sub M/ values. Electron-withdrawing substituents give a high percentage of meta hydroxylation, whereas electron-donating substituents give a low percentage of meta hydroxylation. As the difference in electron density between ortho and meta position increases the electrophilic OH radicals attack preferentially at the ortho and para positions. It was concluded that dehydration from the initially formed hydroxycyclohexadienyl radicals takes place only if themore » substituent is electron donating (+M effect), but not if the substituent is electron withdrawing (NO/sub 2/, CN).« less