Reactive anion rate constants

The existence of ion pairs of hydroxy aromatic anions with polar groups of cationic micelles was proposed by Zaitsev et al. [66] to explain the effective charge of anions close to zero, observed in acid-base photoreactions of hydroxyaromatics in CTAB solutions. Such a value for the effective charge was found by simulation of the values of the diffusion rate constants of hydrogen ions to excited anions of hydroxyaromatics to make the calculated diffusion-controlled protonation reaction of the excited anions rate constants close to experimentally observed ones. In aqueous solution, the excited anions are protonated with diffusional values of the rate constant with some nonsignificant steric factor [67,68]. The three-phase model can help to interpret the reactivity of polar and charged substances in micellar solutions. 

The operation of the nitronium ion in these media was later proved conclusively. "- The rates of nitration of 2-phenylethanesulphonate anion ([Aromatic] < c. 0-5 mol l i), toluene-(U-sulphonate anion, p-nitrophenol, A(-methyl-2,4-dinitroaniline and A(-methyl-iV,2,4-trinitro-aniline in aqueous solutions of nitric acid depend on the first power of the concentration of the aromatic. The dependence on acidity of the rate of 0-exchange between nitric acid and water was measured, " and formal first-order rate constants for oxygen exchange were defined by dividing the rates of exchange by the concentration of water. Comparison of these constants with the corresponding results for the reactions of the aromatic compounds yielded the scale of relative reactivities sho-wn in table 2.1. 

For two-equivalent couplers where the conversion of the leuco dye to image dye is rapid, the experimentally observed second-order rate constant, k, can be equated with kj, the rate of nucleophilic attack of coupler anion on oxidized developer. Thus when the pH of the process is specified, two parameters, piC and k, can be convenientiy used to characterize the molecular reactivity of a large variety of photographically weU-behaved couplers (40,54). 

The distinction between the rates of homo- and copolymerization apparently is misapprehended by some workers. For example, a recent review 141) discusses the results of McGrath 142) who reported butadiene to be more reactive in polymerization in hexane than isoprene, whether with respect to lithium polybutadiene or polyisoprene, although the homopropagation of lithium polyisoprene in hexane was found to be faster than of polybutadiene. The miscomprehension led to the erroneous statement14l) McGrath 142) results regarding the rate constants for butadiene and isoprene place in clear perspective the bizarre assertion 140) that butadiene will be twice as reactive as isoprene (in anionic co-polymerization). 

A kinetic study for the polymerization of styrene, initiated with n BuLi, was designed to explore the Trommsdorff effect on rate constants of initiation and propagation and polystyryl anion association. Initiator association, initiation rate and propagation rates are essentially independent of solution viscosity, Polystyryl anion association is dependent on media viscosity. Temperature dependency correlates as an Arrhenius relationship. Observations were restricted to viscosities less than 200 centipoise. Population density distribution analysis indicates that rate constants are also independent of degree of polymerization, which is consistent with Flory s principle of equal reactivity. 

Figure 1. Hydrolysis pH-rate profiles of phenyl acetate (lower) and a substituted 2-phenyl-l,3-dioxane (HND). Phenyl acetate profile constructed from data of Mabey and Mill (32), HND profile from data of Bender and Silver (33). Phenyl acetate reacts via specific-acid catalyzed, neutral, and base-catalyzed transformation pathways. The pseudo-first-order rate constant is given by Kobs = K(h+) [H+] + Kn + K(qh-) [0H—]. HND hydrolyzes only via an acid-catalyzed pathway the phenolate anion is some 867 times more reactive than its conjugate acid.

The extent to which the ion-radical pair suffers a subsequent (irreversible) transformation (with rate constant k characteristic of highly reactive cation radicals and anion radicals) that is faster than the reverse or back electron transfer (/cBET) then represents the basis for the electron-transfer paradigm that drives the coupled EDA/CT equilibria forward onto products (P)20 (equation 8). 

With these assumptions, ion exchange between a reactive anion, Y , and an inert anion, X , for example, was written in terms of (7).2 It then was relatively straightforward to write the concentration of reactive ion in the micelle in terms of an assumed constancy of fractional micellar charge, a, and the ion exchange parameter, K, and to analyse rates in terms of these parameters, the binding constants of the substrate, Ks, and the second-order rate constants, kw and A M (Romsted, 1977, 1984 Quina and Chaimovich, 1979 Bunton and Romsted, 1979). 

This hypothesis is satisfactory for nucleophilic reactions of cyanide and bromide ion in cationic micelles (Bunton et al., 1980a Bunton and Romsted, 1982) and of the hydronium ion in anionic micelles (Bunton et al., 1979). As predicted, the overall rate constant follows the uptake of the organic substrate and becomes constant once all the substrate is fully bound. Addition of the ionic reagent also has little effect upon the overall reaction rate, again as predicted. Under these conditions of complete substrate binding the first-order rate constant is given by (8), and, where comparisons have been made for reaction in a reactive-ion micelle and in solutions... 

The situation is different for reactions of very hydrophilic ions, e.g. hydroxide and fluoride, because here overall rate constants increase with increasing concentration of the reactive anion even though the substrate is fully micellar bound (Bunton et al., 1979, 1980b, 1981a). The behavior is similar for equilibria involving OH" (Cipiciani et al., 1983a, 1985 Gan, 1985). In these systems the micellar surface does not appear to be saturated with counterions. The kinetic data can be treated on the assumption that the distribution between water and micelles of reactive anion, e.g. Y, follows a mass-action equation (9) (Bunton et al., 1981a). 

In the discussions of micellar effects thus far there has been essentially no discussion of the possible effect of micellar charge upon reactivity in the micellar pseudophase. This is an interesting point because in most of the original discussions of micellar rate effects it was assumed that rate constants in micelles were affected by the presence of polar or ionic head groups. It is impracticable to seek an answer to this question for spontaneous reactions of anionic substrates because they bind weakly if at all to anionic micelles (p. 245). The problem can be examined for spontaneous unimolecular and water-catalysed reactions of non-ionic substrates in cationic and anionic micelles, and there appears to be a significant relation between reaction mechanism and the effect of micellar charge upon the rate of the spontaneous hydrolysis of micellar-bound substrates. 

The second-order rate constant for the methylation of sodium 9-fluorenone oximate in 33.5% acetonitrile/66.5% t-butyl alcohol solution was found to decrease with increasing concentration of the salt, suggesting an equilibrium (13) between the reactive free anion [109] and the less reactive ion pair [110]... 

The unusual rate enhancement of nucleophiles in micelles is a function of two interdependent effects, the enhanced nucleophilicity of the bound anion and the concentration of the reactants. In bimolecular reactions, it is not always easy to estimate the true reactivity of the bound anion separately. Unimolecular reactions would be better probes of the environmental effect on the anionic reactivity than bimolecular reactions, since one need not take the proximity term into account. The decarboxylation of carboxylic acids would meet this requirement, for it is unimolecular, almost free from acid and base catalysis, and the rate constants are extremely solvent dependent (Straub and Bender, 1972). 

PNPA in dry aprotic media and the reaction was efficiently suppressed by minute amounts of water (much less than 1 M). In the reaction of tetraethylammonium N-methylmyristohydroxamate [63 R = C13H27] in acetonitrile, for instance, an increase in water concentration from 3.3 mM to 960 mM caused the rate to decrease from 845 M I s-1 to 8.5 M-1 s-1. Thus, the value of the rate constant is almost meaningless without accurate determination of water concentration. This finding contrasts with the previously held view that the reactivity of anions in dipolar aprotic solvents is... 

We have examined the competing isomerization and solvolysis reactions of 1-4-(methylphenyl)ethyl pentafluorobenzoate with two goals in mind (1) We wanted to use the increased sensitivity of modern analytical methods to extend oxygen-18 scrambling studies to mostly aqueous solutions, where we have obtained extensive data for nucleophilic substitution reactions of 1-phenylethyl derivatives. (2) We were interested in comparing the first-order rate constant for internal return of a carbocation-carboxylate anion pair with the corresponding second-order rate constant for the bimolecular combination of the same carbocation with a carboxylate anion, in order to examine the effect of aqueous solvation of free carboxylate anions on their reactivity toward addition to carbocations. 

Molecular mechanics and ab initio calculations on the cyclopentadienyl cation have been carried out an allylic stmcture is favoured. Calculations referring to the initiation of polymerization of 1,1-disubstituted cyclopropanes by cations (also neutrals and anions) are reported. Rate constants for the solvolyses of (69) show reasonable Yukawa-Tsuno correlations, interpreted in terms of the less reactive substituents... 

The monoanionic species is most reactive, but its associated rate constant for intramolecular general acid catalysis is only 65 times greater than that for the unionized species. Most of the large rate enhancement in comparison with the dimethyl ester is due to participation by one carboxyl group, as is the case with the unionized acetal [77]. If the carboxylate anion of the monoanionic species is electrostatically stabilizing the incipient carbonium ion in the reaction [8], its effect on the rate must be small. 

Our hypothesis Is that PB solutions owe their great reactivities with OP chemicals to the highly nucleophilic perhydroxyl anion, H02, produced by the dissociation of PB to HO2 and boric acid. We have tested this hypothesis by calculating blmolecular substitution rate constants, kunni... 

Reactivities of pentacyanoferrates(II) in micelles and reversed micelles have been studied. The hexadecyltrimethylammonium cation causes a modest increase in rate constant for the anion-anion reaction [Fe(CN)5(4-CNpy)] + CN. This can equally well be interpreted according to the pseudophase model developed from the Olson-Simonson treatment of kinetics in micellar systems or by the classical Bronsted equation. 

---