Nucleophilic displacement in polyhalogenoaromatic compounds. Part 12.[1] Additivity of fluorine substituent effects in methoxydefluorination

The rates of methoxydefluorination of all twelve polyfluorobenzenes in dimethyl sulphoxide-methanol mixtures (DMSO-MeOH; 9:1, v/v; 298.2 K) have been measured. Three substituent rate factors (f_o, 60; f_m, 180; f_p, 0.75) are sufficient to reproduce the effect of the fluorine substituent in this reaction upon each member of the series. The solvent effect, comparing these results with an earlier and more limited study in methanol is predominantly a simple acceleration. The effects of substituents upon the rate of methoxydefluorination of fluorobenzene itself are slightly greater (?^?, 6.9) than in the corresponding reaction of pentafluorobenzene derivatives (?^?, 5.8), but the change in sensitivity is much less than that found with nitrobenzene derivatives.