J . Org. Chem., Vol. 44, No. 6, 1979 947
Iodine Oxidation of Diethyl Sulfide
Entropy-Related Rate Accelerations in the Micelle-Bound Carboxylate-Catalyzed Iodine Oxidation of Diethyl Sulfide Paul R. Young* and K. C. Hou Department of Chemistry, University of Illinois at Chicago Circle, Chicago, Illinois 60680 Receiued September 25, 1978 Rate constants for the carboxylate-catalyzed iodine oxidation of diethyl sulfide in the presence of SDS micelles exhibit two linear first-order regions with respect to carboxylate ion concentration. This is in marked contrast to the oxidation in the absence of SDS where the reaction exhibits first- and second-order terms with respect to carboxylate. Second-order catalytic constants for acetate, propionate, butanoate, pentanoate, and hexanoate, when plotted aginst the micelle-induced pK, shifts for these buffers, give a linear plot with p = 3.8 and 0.7 for data in the presence of SDS a t low buffer and in the absence of SDS, respectively. Extrapolation of the nonmicelle data to hexanoate gives a 100-fold rate advantage for the micelle-catalyzed reaction. Activation parameters for the butanoatecatalyzed reaction are AH* = 5.5 kcal/mol, AS* = 9.1 eu for the micelle reaction a t low buffer and AH* = 5.5 kcali mol, AS* = 13.9 eu for the first-order nonmicelle process. The rate advantage for the micelle reaction is quantitatively accounted for by the AAS* = 4.8 eu. The 40-fold increase in micellar rate constants on going from acetate to hexanoate gives an average free-energy gain of -545 cal/mol per -CH2-, representing utilization of over 80% of the available hydrophobic binding energy for rate acceleration.
I t is well established that micellar aggregates can, in some instances, provide substantial rate enhancements for certain reactions relative to the rates that are observed in free solution.l,* The driving force for this catalysis can often be attributed to electrostatic or solvent-induced ground state destabilization which is i~elievedin the transition state and to entropy effects. All of these mechanisms undoubtedly play important roles in enzyme catalysis; however, it has been suggested that entropy loss may be especially important and rate accelerations of up to los have been attributed to this me~hanism.3,~ Surfactant micelles offer a system in which nonpolar interactions can be directly utilized to bind reactants and thereby reduce translational entropy losses associated with bi- or trimolecular reactions. This ability of micelles to serve as entropy sinks makes the study of catalyzed reactions on micellar surfaces especially important to the understanding of the mechanisms of enzymic rate accelerations. The iodine oxidation of sulfides to produce sulfoxides has been observed to be strongly catalyzed by carboxylate buffers and to contain both first- and second-order buffer-dependent terms.5 In order to investigate the possibility of micelle-induced, entropy-related effects in this reaction, we have investigated the iodine oxidation of diethyl sulfide catalyzed by carboxylate anions of increasing chain length, both in the presence and absence of the surfactant sodium dodecyl sulfate
(SDS).
Experimental Section Materials. Reagent grade organic acids and inorganic salts were used without further purification. Sodium dodecyl sulfate (SDS)was purified by recrystallization using the procedure of Duynstee and Grunwald.6 Glass-distilled water was used throughout. Stock solutions of organic acids, 50% ionized, were prepared by neutralizing solutions of the carboxylic acid with standard sodium hydroxide solution. Stock solutions of pentanoic and hexanoic acids were prepared in 2.4 X M solutions of SDS to aid solubility. Iodine solutions were prepared from the solid to be 0.01 M in 1M NaI. Diethyl sulfide solutions were prepared in methanol at 0.10 M. Kinetic Measurements. The rates of disappearance of 13- were followed a t 353 nm using a Hitachi 100-60 spectrophotometer equipped with an automatic cell changer and a digital printout. Constant temperature was maintained a t 15,25, or 42 "C by the use of a thermostated cell compartment; ionic strength was maintained a t less than 0.05. First-order rate constants were determined from semilogarithmic plots of A m - At against time and were typically linear for over 3 4 half-times. The pH of each solution was determined a t the completion of the experiment using a Corning 130 p H meter equipped with a combined glass electrode. Reactions were initiated by adding 10 p L of diethyl iiulfide solution (0.10 M) to thermostated cuvettes containing 3.0 mI. of the desired buffer solution and 10 fiL
0022-3263/79/1944-0947$01.00/0
of the stock iodine solution. SDS concentrations were maintained at 1.6 X M, which is twice the literature value for the critical micelle concentration.' Apparent pK,'s for the carboxylic acids were determined by potentiometric titration of 0.20 M solutions using standard 0.20 M sodium hydroxide solution.
Results The rate constants for the iodine oxidation of diethyl sulfide catalyzed by acetic, propionic, and butyric acids, 50%ionized, in the absence of micellar aggregates, show a nonlinear dependence on the concentration of carboxylate anion (Figure 1).Rate constants for first- and second-order processes were obtained from these data from plots of h,b,d/[RCOO-] vs. [RCOO-] and are listed in Table I. Rate constants for the carboxylate-catalyzed oxidation of diethyl sulfide in the M SDS also exhibit a nonlinear depresence of 1.6 X pendence on carboxylate concentration (Figure 2). At very low concentrations of carboxylate there is a rapid increase in the observed rate constant which breaks to a less steep, but nevertheless linear, second phase. Logarithmic plots of observed rate constants at the extremes of high and low buffer concentrations against log [RCOO-] are linear with unit slope. Rate constants for each first-order process (Table TI) were determined from plots such as these or from the initial and final slopes of plots of observed rate constants vs. buffer concentration (Figure 2). Agreement between the methods was good, and the probable errors in derived rate and activation parameters are estimated to be
