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 kp = 3.7 × 1010 and kH2O = 1.5 × 108 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 pKa = −20.9 and pKR = −11.6. For a series of arenonium and benzylic cations, correlation of log kp with pKa, taking account of the limit to kp set by the relaxation of water (1011 s−1), leads to extrapolation of kp = 9.0 × 1010 s−1 and pKa = −24.5 for the benzenonium ion and kp = 6.5 × 1010 s−1 and pKa = −22.5 for the 1-naphthalenonium ion. Combining these pKa’s with estimates of equilibrium constants pKH2O for the hydration of benzene and naphthalene, and...