Stabilities and Partitioning of Arenonium Ions in Aqueous Media
The phenathrenonium ion is formed as a reactive intermediate in the solvolysis of 9-dichloroacetoxy-9,10-dihydrophenanthrene in aqueous acetonitrile and undergoes competing reactions with water acting as a base and nucleophile. Measurements of product ratios in the presence of azide ion as a trap and 'clock' yield rate constants k(p) = 3.7 x 10(10) and k(H2O) = 1.5 x 10(8) s(-1), respectively. Combining these with rate constants for the reverse reactions (protonation of phenanthrene and acid-catalyzed aromatization of its water adduct) gives equilibrium constants pK(a) = -20.9 and pK(R) = -11.6. For a series of arenonium and benzylic cations, correlation of log k(p) with pK(a), taking account of the limit to k(p) set by the relaxation of water (10(11) s(-1)), leads to extrapolation of k(p) = 9.0 x 10(10) s(-1) and pK(a) = -24.5 for the benzenonium ion and k(p) = 6.5 x 10(10) s(-1) and pK(a) = -22.5 for the 1-naphthalenonium ion. Combining these pK(a)'s with estimates of equilibrium constants pK(H2O) for the hydration of benzene and naphthalene, and the relationship pK(R) = pK(a) + pK(H2O) based on Hess's law, gives pK(R) = -2.3 and -8.0 respectively, and highlights the inherent stability of the benzenonium ion. A correlation exists between the partitioning ratio, k(p)/k(H2O), for carbocations reacting in water and K(H2O) the equilibrium constant between the respective reaction products, i.e., log(k(p)/k(H2O)) = 0.46pK(H2O) -3.7. It implies that k(p) exceeds k(H2O) only when K(H2O) > 10(8). This is consistent with the proton transfer (a) possessing a lower intrinsic reactivity than reaction of the carbocation with water as a nucleophile and (b) being rate-determining in the hydration of alkenes (and dehydration of alcohols) except when the double bond of the alkene is unusually stabilized, as in the case of aromatic molecules.
Journal Of The American Chemical Society
American Chemical Society
The Science Foundation Ireland [04/IN3/B581]