LIFETIMES OF IMINIUM IONS IN AQUEOUS-SOLUTION

Iminium cations have been generated in aqueous solution from the solvo lysis of anilinothioethers ArN(CH3)CH(2)SR at 25 degrees C. Common ion inhibition of the solvolysis of anilinothioethers was observed when t he thiolate anion leaving group was added to the reaction mixture. Thi s inhibition results from trapping of the iminium ion by RS(-) to rege nerate the anilinothioether. Rate constants, k(H2O), for hydration of the iminium ions were determined from common ion inhibition of the sol volysis of anilinothioethers in the presence of added thiolate ions an d a rate constant of 5 x 10(9) M(-1) s(-1) for diffusion-controlled tr apping of the iminium ions by RS(-), which regenerates the starting ma terial. The values of k(H2O) for H2C=N+(CH3)C(6)H(4)X are 1.0 x 10(8), 3.1 x 10(7), 3.0 x 10(7), 5.5 x 10(6), 4.5 x 10(6), and 3.1 x 10(6) s (-1) for X = 4-NO2, 4-CN, 3-NO2, 3-Cl, 4-COO-, and 4-Cl, respectively. Rate constants for the solvolysis of anilinothioethers, k(solv) depen d on the pK(a) of the thiolate leaving group with beta(1g) = -0.93 +/- 0.09, which shows that the dissociation reaction has a very late tran sition state. This result is consistent with rate-limiting diffusion-c ontrolled encounter of RS(-) with H2C=N+(CH3)Ar in the reverse directi on. The value of beta(nuc) = 1.00-0.93 = 0.07 +/- 0.09 approximate to 0 is expected for a diffusion-controlled addition reaction. The solvol ysis reaction is sensitive to the basicity of the electron-donating ni trogen atom of the aniline with a value of beta(dg) = 0.79 +/- 0.06; i .e., there is a large amount of electron donation from the nitrogen at om that provides the driving force for cleavage of the CH2-S bond in t he late transition state. Hammett correlations of the rate constants f or the solvolysis reaction, k(solv), show a better correlation with si gma(-) than with sigma for substituents on the aniline and follow rho( -) = -3.3 +/- 0.3. This also indicates a large amount of electron dona tion and an important contribution of resonance in the transition stat e of the solvolysis reaction. Reaction of the iminium ions with water is accelerated by electron-withdrawing substituents on the aniline rin g and follows rho(-) = 1.5 +/- 0.2 and beta(dg) = -0.35 +/- 0.03; i.e. , the rate of reaction of the iminium cation with water is approximate ly half as sensitive to polar substituents as the solvolysis reaction.