Effective Charge on the Nucleophile and Leaving Group During the Stepwise Transfer of the Trizazinyl Group Between Pyridines in Aqueous-Solution

Second-order rate constants (k(xpy)) have been measured for the displacement reaction between substituted pyridines (xpy) and 1'-(2,6-diphenoxy-1,3,5-triazin-2-yl)pyridinium in aqueous solution. The rate constants for the reverse reaction (k(-xpy)) have also been measured for substituted pyridine leaving groups. The plots of log k(xpy) and log k(-xpy) against pK(a)(xpy) each consist of two intersecting linear correlations consistent with a two-step mechanism involving a Meisenheimer-like intermediate. The overall transfer of the triazin-2-yl group between substituted pyridines has a beta(eq) value of 1.25. There is negligible coupling between the bonding changes in both steps, and the substituent effects indicate that bond formation is half complete in the addition step. Reaction of substituted pyridines with 2,6-diphenoxy-1,3,5-triazin-2-yl chloride has a similar bonding change in the addition step, The 1'-triazin-2-ylpyridinium ion species exist in aqueous solution in equilibrium with the pseudobase formed by addition of water at the 2-position of the pyridinium ring.