SUBSTITUENT EFFECTS OF THE TRIFLUOROMETHYL GROUP ON ELECTROPHILIC ADDITIONS TO ALKENES - SOLVOLYSIS OF THE TRIFLUOROMETHYL GROUP - PROTONATION OF ALKENES LESS BASIC THAN ETHYLENE, P+ VALUES OF DEACTIVATED STYRENES, AND REACTIVITY-SELECTIVITY EFFECTS

Rates of solvent addition to aryl-substituted α-trifluoromethylstyrenes ArC(CF3)=CH2 in aqueous sulfuric acid solutions at 25 °C were measured and correlated with the acidity function H0 of the medium. When aryl is p-tolyl the observed reactivity is almost identical with that of ethylene, and for aryl equal to phenyl, p-chlorophenyl, and w-chlorophenyl the reactivities are distinctly less than that of ethylene. The styrene reactivities are correlated with those of other alkenes by our previously introduced equation log k2 = -10.52∑σp+-8.92, and extend the total range of reactivity covered by this equation to 22 orders of magnitude. Ethylene is less reactive than predicted by this equation but is still proposed to react by the same mechanism as the other alkenes, namely, rate-determining protonation on carbon. The low rate is attributed to the inadequacy of the standard σ+ parameter of hydrogen to fully account for its minimal electron-donating ability in the absence of any other substituent. The reactivity of the styrenes themselves is correlated by the σp+ parameters of the aryl substituents with ρ+ = -4.0. Even in this situation of high electron demand the sensitivity of the reaction to the substituents as measured by the magnitude of ρ+ is not greatly enhanced. These results show the operation of linear free energy relationships over vast differences in reactivities, with minimal influence of reactivity-selectivity effects. © 1979, American Chemical Society. All rights reserved.