Kinetics and Mechanism of the Aminolysis of Phenyl and Methyl 4-Nitrophenyl Thionocarbonates.
The reactions of secondary alicyclic amines with the title substrates are subjected to a kinetic study in aqueous solution, 25.0 degrees 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 Brönsted-type plot for 1 is linear with slope beta = 0.25, indicating that the formation (k(1) step) of a tetrahedral addition intermediate (T(+/-)) is rate determining. For the aminolysis of 2 (except piperidine), k(-)(1) approximately k(3)[NH] > k(2), where k(-)(1), k(3), and k(2) 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) < 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 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 k(1) values significantly. (vii) The change of NPO by Cl in 1 increases the k(1) values. (viii) 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.
Castro
Enrique A.
EA
Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile.
Saavedra
Claudia
C
Santos
José G.
JG
Umaña
María I.
MI