Degradation in aqueous solution of O-aryl N-arylthioncarbamates to aryloxide ions and isothiocyanates. A study of Leffler's assumption

Abstract

Rate constants for the title reaction have been measured in both forward and reverse directions leading to rates and equilibrium constants for all the steps in equation (i). The effects of varying the structures of A r A r -0 -C S -N H A r ' e A r -0 -C C N -A r ' s Ar-0- ExperimentalMateriafs.-0-4-Methoxyphenyl chlorothioformate was prepared by adding a solution of 4-methoxyphenol (8.3 g, 67 mmol) in 5% NaOH (60 ml) dropwise to a well stirred, cooled solution of thiophosgene (7.7 g, 67 mmol) in chloroform (40 ml). The reaction was stirred for 1 h at 0-5" and the chloroform layer washed with dilute HCl and water. The chloroform solution was dried with sodium sulphate, evaporated, and the residue distilled to yield a liquid, b.p. 92" at 0.5 Torr (lit.,3h 141-143" at 13 Torr), which solidified on cooling.0-4-Met hoxyphenyl N-arylthioncarbamates were prepared by adding the aniline (32 mmol) dissolved in carbon tetrachloride (5 ml) to a stirred, cooled solution of O-(Cmethoxyphenyl) chlorothioformate (3.3 g, 10 mmol) in carbon tetrachloride (50 ml). The mixture was stirred at room temperature for 24 h, filtered, and the filtrate evaporated to dryness. The thioncarbamate was recrystallised from a suitable solvent (see Table 1).Aryl isothiocyanates were prepared by adding a slight excess of thiophosgene to a mechanically stirred mixture of the appropriate aniline and water. Reaction was almost instantaneous and the isothiocyanate separated as an oil and was extracted with ether. The ether was dried (Na,SO,), evaporated, and the residual oil distilled under high vacuum. The materials prepared above had satisfactory analyses (Table 1) and their structures were confirmed by i.r. (Perkin-Elmer 297 instrument) and 100 MHz 'H n.m.r. spectra. The latter spectra were recorded by Dr. D. 0. Smith using a JEOL instrument, Buffers and reagents used in the kinetics were of analytical quality or were recrystallised or redistilled from bench grade materials. Water used throughout the investigation was distilled twice from glass.Method;F.-Kinetics were followed spectrophotometrically at an. appropriate wavelength determined using a repetitive wavelength scanning spectrophotometer (Unicam SP 800). Constant wavelength studies were carried out using a Unicam SP 800 machine or a Beckman D.B. instrument fitted with Servoscribe potentiometric recorders. The stock solution of substrate (thioncarbamate or isothiocyanate) in ethanol or

Enhanced reactivity in the ammonolysis of phenyl thiolacetates in aqueous medium

“…This is due to the fact that the PhO − is more electron‐donating than PhS − with σ R values of –0.40 and –0.23 respectively 35, 64–66, placing the carbonyl carbon of phenyl acetates less susceptible to nucleophilic attack. This is also in agreement with the observation of increasing acidity of NH in the series RNHCO 2 Ar < RNHCOSAr < RNHCS 2 Ar 66, 68.…”

Structure–reactivity correlations in the aminolysis of aryl chloroformates

“…This has been attributed to the greater ability of O Ϫ in T Ϯ , relative to S Ϫ , in forming a double bond and expelling a given nucleofuge [28,29]. Therefore, the results above mean that the greater nucleofugality rate brought about by the change of S Ϫ to O Ϫ in T Ϯ is not enough to provoke a change in the rate-determining step or to bring such instability to T Ϯ as to shift the mechanism from stepwise to concerted.…”

Kinetics and mechanism of the aminolysis of 4‐nitrophenyl and 2,4‐dinitrophenyl 4‐methylphenyl carbonates in aqueous ethanol

“…The value of k 2 for the intermediate 1 has been estimated as (0.9–1.5) × 10 7 s −1 5. Substitution of S − by O − in 1 (to yield 2 ) should result in a larger k 2 value, in view of the stronger driving force of O − , compared to that of S − , in a tetrahedral intermediate to form a double bond and expel a nucleofuge 21. …”