Stabilities and Partitioning of Arenonium Ions in Aqueous Media

Overview

Title: 

Stabilities and Partitioning of Arenonium Ions in Aqueous Media

Abstract: 

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.

Authors: 

D. A. Lawlor, R. A. More O’Ferrall, S. N. Rao

DOI: 

Journal: 

Journal Of The American Chemical Society

Year: 

2008

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