Ea. Castro et al., STEPWISE MECHANISM OF THE AMINOLYSIS OF O-ETHYL S-(4-NITROPHENYL) THIOCARBONATE, Journal of organic chemistry, 59(13), 1994, pp. 3572-3574
The reactions of secondary alicyclic amines with the title substrate (
NPTC) are subjected to a kinetic study in water, 25.0 degrees C, ionic
strength 0.2 M (KCl). The reactions are first order in both the amine
and NPTC. The Bronsted-type plot for the second-order rate coefficien
t is biphasic, with slopes beta(1) = 0.2 and beta(2) = 0.8 at high and
low pK(a) values, respectively. The Bronsted plot is explained by the
presence of a zwitterionic tetrahedral intermediate and a change in t
he rate-determining step. The center of the Bronsted curvature is pK(a
)(o) = 10.7. A semiempirical equation based on the above hypothesis ac
counts for the experimental results. The value of pK(a)(o) = 10.7 is l
arger than those found in the same aminolyses of O-ethyl 4-nitrophenyl
dithiocarbonate and S-(4-nitrophenyl) thioacetate, favoring amine exp
ulsion from the tetrahedral intermediate (relative to 4-nitrobenzeneth
iolate ion expulsion) in the NPTC reaction. By comparison of the stepw
ise reaction under study with the concerted aminolysis of O-ethyl S-(2
,4-dinitrophenyl) thiocarbonate it is concluded that the introduction
of a second nitro group to the tetrahedral intermediate formed in the
present reactions destabilizes the intermediate to such an extent that
it becomes a transition state and a concerted mechanism is enforced.