. Can. J. Chem. 53,3408 (1975). Second-order rate constants at'25 "C have been measured for the reaction of triphenylmethyl carbonium ion with pyridine and substituted pyridines in nitromethane and 1,2-dichloroethane solvent. The activation parameters are in the range 1 to 3 kcal mol-I for the enthalpies of activation and -20 to -45 cal mol-I deg-I for the entropies of activation. The results are compared to the analogous measurements for Menschutkin reactions and it is concluded that the enthalpy of activation of a Menschutkin reaction results mainly from the bond breaking process and solvation changes associated with it, since the carbon-nitrogen bond making process seems to be entropy controlled.Rate measurements have also been made for 2-methylpyridine reacting with a series of parasubstituted triphenylmethyl carbonium ion substrates in 1,2-dichloroethane solvent. A plot of log k2 us. Ea+ is non-linear and the curvature is interpreted as a saturation effect. On a aussi mesure, dans le dichloro-1,2 ethane, des vitesses de rhction de la methyl-2 pyridine avec une serie de carbocations triphCnylmethyles substitues en para. La relation log k2 us.