The influence of the structure of tetracoordinate phosphorus acid esters on the catalytic effect of the sodium dodecyl sulfate-hexanol-water ternary reverse micellar system

Abstract

The catalytic effect of the sodium dodecyl sulfate--hexanol--water ternary, reverse micellar system in the alkaline hydrolysis of O-alkyl O'-aryl chloromethylphosphonates as a function of the substrate structure was studied. The micellar effect is mainly determined by a change in the electronic properties of the substituents, while the hydrophobicity plays a secondary role. The kinetic data were examined in the framework of the pseudo-phase model of micellar catalysis. The rate constants of the reaction in the surface layer and the partition constants of the reactants were calculated.Key words: reverse micelles, pseudo-phases, surface layer, kinetics, hydrolysis, phosphonates.Reverse micelles are widely known as biomimetic structures that work according to the "guest--host" principle. ! The main qualitative regularities of catalysis of chemical reactions in these systems are poorly studied and only a few quantitative studies were performed. We have previously z,3 shown that in the reverse micellar system sodium bis(2-ethylhexyl) sulfosuccinate (AOT)--decane--water the alkaline hydrolysis of phosphonates proceeds more slowly than in water, and in the system sodium dodecyl sulfate (SDS)--hexanol--water the reaction is accelerated. To reveal the main reasons for catalysis of reactions and to solve the problem of substrate specificity in these organized media, the range of substrates under study should be enlarged. With this purpose, we studied the kinetics of the hydrolysis of O-alkyl O'-aryl chloromethylphosphonates (!--10) (Scheme I) at different ratios Z = [hexanoll/[SDS] and W = [H20]/ISDS ] and examined the kinetic data in the framework of the pseudo-phase model of micellar catalysis. ExperimentalCompounds 1--10 were prepared by a known procedure. 4 The synthesized esters were triply distilled. The purity of compounds 5--10 was confirmed by elemental analysis (Table 1). Phosphonates 1--4 have been synthesized by us previously, s SDS (high-purity grade) was doubly recrystallized From ethanol. Reverse micellar systems were prepared by the mixing of three components according to the phase diagram 6,7 followed by shaking to the formation of a transparent solution. The molar ratios W= [H2OI/ISDS ] and Z= [hexanoll/ [SDS] were varied in the intervals of 9.8--37 and 5--49, respectively. The reaction kinetics was studied spectrophotometrically on a Specord M-400 XRO/P u + H20 X = NO 2, R = Et (1), Bu n (2), n-CsH13 (3), n-Cell17 (4); R = Et, X = Br (5), H (6), Et (7), Bu n (8), n-CsH~7 (9), iso-C12H25 (|0)instrument by the change in the optical density of the anion of the leaving group. The initial concentration of the substrate was 5-10 -5 mol L -l, and the concentration of the nucleophile much exceeded that of the substrate. The observed rate constants (kobs) were calculated using the weighted least-squares method from the dependence ln(D,~-D) = -kobs t + const, where D and D= were the optical densities of the solution at the moment t and at the end of the reaction. The kinetic data were examined according to the ps...