EFFECTIVE CHARGE ON THE NUCLEOPHILE AND LEAVING GROUP DURING THE STEPWISE TRANSFER OF THE TRIAZINYL GROUP BETWEEN PYRIDINES IN AQUEOUS-SOLUTION

Second-order rate constants (k(xpy)) have been measured for the displa cement reaction between substituted pyridines (xpy) and 1'-(2,6-diphen oxy-1,3,5-triazin-2-yl)pyridinium in aqueous solution. The rate consta nts for the reverse reaction (k(-xpy)) have also been measured for sub stituted pyridine leaving groups. The plots of log k(xpy) and log k(-x py) against pK(a)(xpy) each consist of two intersecting linear correla tions consistent with a two-step mechanism involving a Meisenheimer-li ke intermediate. The overall transfer of the triazin-2-yl group betwee n substituted pyridines has a beta(eq) value of 1.25. There is negligi ble coupling between the bonding changes in both steps, and the substi tuent effects indicate that bond formation is half complete in the add ition step. Reaction of substituted pyridines with 2,6-diphenoxy-1,3,5 -triazin-2-yl chloride has a similar bonding change in the addition st ep, The 1'-triazin-2-ylpyridinium ion species exist in aqueous solutio n in equilibrium with the pseudobase formed by addition of water at th e 2-position of the pyridinium ring.