Kinetics and mechanism of the reactions of O‐aryl S‐(4‐nitrophenyl) dithiocarbonates with anilines in aqueous ethanol

Abstract

The reactions of O-(4-methylphenyl) S-(4-nitrophenyl) dithiocarbonate and O-(4-chlorophenyl) S-(4-nitrophenyl) dithiocarbonate with a series of anilines are subjected to a kinetic investigation in 44 wt% ethanol-water, at 25.0 -C and an ionic strength of 0.2 M. The reactions are followed spectrophotometrically at 420 nm (appearance of 4-nitrobenzenethiolate anion). Under excess amine, pseudo-first-order rate coefficients (k obs ) are found. For the reactions of both substrates with anilines, plots of k obs versus free amine concentration at constant pH are nonlinear upwards, according to a second-order polynomial equation. This kinetic behavior is in agreement with a stepwise mechanism consisting of two tetrahedral intermediates, one zwitterionic (T W ) and the other anionic (T S ), with a kinetically significant proton transfer from T W to an aniline to yield T S . The rate equation was derived from the proposed mechanism. By nonlinear least-squares fitting of the rate equation to the experimental data, values of the rate micro-coefficients involved in both steps were determined.

Reactions of O‐aryl S‐aryl dithiocarbonates with secondary alicyclic amines in aqueous ethanol. Kinetics and mechanism

“…The different behavior exhibited by all these reactions can be explained by the mechanism shown in Scheme . A similar reaction mechanism has been obtained for the aminolysis (SA amines) of some O ‐alkyl S ‐aryl dithiocarbonates,1 O ‐phenyl S ‐4‐nitrophenyl dithiocarbonate,8 alkyl aryl1 and diaryl thionocarbonates,5 and for the reactions of 1 and 2 with anilines 7…”

“…A value of ca . 4 × 10 9 s −1 M −1 has been estimated for k 3 in 44 wt% ethanol–water 7, 8, 13. It is noteworthy that this value is independent of the amine basicity and nature because the same amine that acts as a base is in the corresponding intermediate 1…”

“…For the tetrahedral intermediates T ± derived from the reactions of 1 and 2 with anilines a p K a value 5.4 and 6.0 units, respectively, lower than that of the respective amine has been estimated 7. Since these values should be independent of the amine basicity and nature,1 it means that the p K a of the tetrahedral intermediates formed with SA amines (intermediates 5 and 6 ) should also be 5.4 and 6.0 units, respectively, lower than that of the respective amine.…”

“…We have recently described the pyridinolysis of O ‐(4‐methylphenyl) S ‐(4‐nitrophenyl) dithiocarbonate ( 1 ), O ‐(4‐chlorophenyl) S ‐(4‐nitrophenyl) dithiocarbonate ( 2 ), and O ‐(4‐chlorophenyl) S ‐phenyl dithiocarbonate ( 3 ) in aqueous ethanol, showing that these reactions are stepwise and occur through a T ± intermediate 6. Also, a kinetic and mechanistic study of the reactions of dithiocarbonates 1 and 2 with anilines has demonstrated the presence of the above two (T ± and T − ) intermediates 7…”

“…On the other hand, the aminolysis of diaryl dithiocarbonates has received less attention 5–8. The reactions of SA amines with O ‐phenyl S ‐(4‐nitrophenyl) dithiocarbonate in aqueous ethanol have been shown to proceed through T ± and T − intermediates 8.…”