Strengths of unsaturated fluoro-acids and -bases

Since fluoroalkyl groups are uniformly acid-strengthening and the order of magnitude of the effect, relative to other haloalkyl groups, is consistent with electronegativities F Cl, etc., it may be expected that the same situation occurs when fluorine is attached to unsaturated carbon. Inspection of the data in Table 4.5 quickly indicates a more complicated situation because, while fluorine substitution increases the acidity relative to the hydrocarbon analogue, the acidities are lower than the corresponding chlorocarbon compounds in the cases of the acrylic acids and phenols. 

It has been concluded from pKa measurements and fluorine NMR data on pentafluoro-biphenyl derivatives, for example C6F5C6H4CO2H, C6F5C6H4F etc., that the penta-fluorophenyl group inductively withdraws electrons more strongly than phenyl but much less strongly than trifluoromethyl, whilst pentafluorophenyl and pentachlorophenyl have a similar capacity for electron withdrawal [40], 

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