Abstract

The reactions of secondary alicyclic amines with the title substrates are subjected to a kinetic study in aqueous solution, 25.0 °C, ionic strength 0.2 (KCl), by following spectrophotometrically the release of 4- nitrophenoxide ion. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. For the reactions of phenyl 4-nitrophenyl thionocarbonate (1), linear plots of k(obsd) vs [NH] (NH is the free amine) are obtained, except for the reaction with piperazinium ion, which shows nonlinear upward plots. The aminolysis of methyl 4-nitrophenyl thionocarbonate (2) exhibits nonlinear plots of k(obsd) vs [NH], except that with piperidine, which is linear. The Bronsted-type plot for 1 is linear with slope ? = 0.25, indicating that the formation (k1 step) of a tetrahedral addition intermediate (T(±)) is rate determining. For the aminolysis of 2 (except piperidine), k-1 ? k3[NH] > k2, where k-1, k3, and k2 are the rate coefficients for amine expulsion, amine deprotonation, and leaving group expulsion from T(±), respectively. For the reaction of 2 with piperidine, k-1 < k3[NH]; therefore, the k1 step is rate limiting. By comparison of the reactions under investigation among them and with similar aminolyses, the following conclusions can be drawn: (i) The change of MeO by EtO in 2 does not affect the k1, k-1, or k2 values. (ii) Substitution of MeO by PhO in 2 results in lower k1 values due to steric hindrance. (iii) The change of 4-nitrophenoxy (NPO) by PhO in 2 lowers the k1 values and enlarges those of k-1. (iv) Secondary alicyclic amines are less reactive toward 2 than isobasic pyridines when the breakdown of T(±) is rate determining; this is mainly due to larger k-1 values for the former amines. (v) The change of PhO by NPO in 1 changes the mechanism from stepwise to concerted. (vi) Substitution of NPO by PhO in 1 does not alter the k1 values significantly. (vii) The change of NPO by Cl in 1 increases the k1 values. (viii) Substitution of C=S by C=O in 1 shifts the rate-limiting step from k1 to k2 due to a larger k-1/k2 ratio by this change.