THE EFFECT OF SOLVENT ON REACTIONS OF P-NITROPHENYL ACETATE WITH ALICYCLIC SECONDARY-AMINES AND WITH ANIONIC NUCLEOPHILES IN MECN-H2O MIXTURES OF VARYING COMPOSITIONS

Second-order rate constants have been measured spectrophotometrically for the reaction of p-nitrophenyl acetate (PNPA) with morpholine, pipe razine and piperidine in MeCN-H2O mixtures of varying compositions. Th e rate of the present aminolysis decreases upon additions of MeCN into H2O up to near 30-40 mole % MeCN and remains nearly constant upon fur ther additions of MeCN. The reaction of PNPA with anionic nucleophiles , such as HO-, p-chlorophenoxide and butane-2,3-dione monoximate, has also exhibited two distinguishable reactivity zones, However, the reac tivity trend for the anionic nucleophiles is quite different from the one obtained for the amine system, e.g. an initial rate decrease in th e H2O-rich region followed by an increasing rate trend upon further ad ditions of MeCN in the MeCN-rich region. The rate behaviors shown by t he amine system in the MeCN-rich and by the anionic system in the H2O- rich region are unexpected based on the Hughes-Ingold rules. The prese nt unusual rate trends have been attributed to changes in the solvent structure and pK(a) of the nucleophiles upon the addition of MeCN into H2O. The effect of solvent appears to be more significant for the TS than the GS, and the TS structure is considered to become tighter in t he higher MeCN concentration.