Kinetics and mechanism of the aminolysis of phenyl and methyl 4-nitrophenyl thionocarbonates

The reactions of secondary alicyclic amines with the title substrates are s ubjected to a kinetic study in aqueous solution, 25.0 degrees C, ionic stre ngth 0.2 (KCl), by following spectrophotometrically the release of 4-nitrop henoxide ion. Under amine excess, pseudo-first-order rate coefficients (k(o bsd)) are found. For the reactions of phenyl C-nitrophenyl thionocarbonate (1), linear plots of k(obsd) VS [NH] (NH is the free amine) are obtained, e xcept 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 l inear. The Bronsted-type plot for 1 is linear with slope beta = 0.25, indic ating that the formation (k(1) step) of a tetrahedral addition intermediate (T+/-) is rate determining. For the aminolysis of 2 (except piperidine), k (-1) approximate to k(3)[NH] > k(2), where k(-1), k(3), and k(2) are the ra te coefficients for amine expulsion, amine deprotonation, and leaving group expulsion from T+/-, respectively. For the reaction of 2 with piperidine, k(-1) < k(3)[NH]; therefore, the k(1) 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 k(1), k(-1), or k(2) values. (ii) Substitution of MeO by PhO in 2 results in lower k(1) values due to steric hindrance. (iii) The change of 4-nitrophenoxy (NPO) by PhO in 2 lowers the k(1) values and enla rges those of k(-1). (iv) Secondary alicyclic amines are less reactive towa rd 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 k(1) values signif icantly. (vii) The change of NPO by Cl in 1 increases the k(1) values. (vii i) Substitution of C=S by C=O in 1 shifts the rate-limiting step from k(1) to k(2) due to a larger k(-1)/k(2) ratio by this change.