We investigated the stoichiometry, kinetics, and mechanism of arsenite [As(III)] oxidation by ferrate [Fe(VI)] and performed arsenic removal tests using Fe(VI) as both an oxidant and a coagulant. As(III) was oxidized to As(V) (arsenate) by Fe(VI), with a stoichiometry of 3:2 [As(III):Fe(VI)]. Kinetic studies showed that the reaction of As(III) with Fe(VI) was first-order with respect to both reactants, and its observed second-order rate constant at 25 degrees C decreased nonlinearly from (3.54 +/- 0.24) x 10(5) to (1.23 +/- 0.01) x 10(3) M(-1) s(-1) with an increase of pH from 8.4 to 12.9. A reaction mechanism by oxygen transfer has been proposed for the oxidation of As(III) by Fe(VI). Arsenic removal tests with river water showed that, with minimum 2.0 mg L(-1) Fe(VI), the arsenic concentration can be lowered from an initial 517 to below 50 microg L(-1), which is the regulation level for As in Bangladesh. From this result, Fe(VI) was demonstrated to be very effective in the removal of arsenic species from water at a relatively low dose level (2.0 mg L(-1)). In addition, the combined use of a small amount of Fe(VI) (below 0.5 mg L(-1)) and Fe(III) as a major coagulant was found to be a practical and effective method for arsenic removal.

Second-order rate constants (k(N)) have been measured spectrophotometrically for the reaction of 4-nitrophenyl X-substituted benzoates with a series of alicyclic secondary amines in H(2)O containing 20 mol % dimethyl sulfoxide at 25.0 degrees C. The magnitude of the k(N) values increases with increasing the basicity of amines and with increasing the electron-withdrawing ability of the acyl substituent X. The Hammett plots obtained are not linear but show a break or curvature as the acyl substituent X becomes electron withdrawing for all the amines studied, while the Bronsted-type plots are linear with large beta(nuc) values for all the substrates investigated. The nonlinear Hammett plots suggest a change in the rate-determining step upon changing the acyl substituent X, whereas the linear Bronsted-type plots indicate that the rate-determining step does not change upon changing amine basicity. The Yukawa-Tsuno plots obtained are also linear with positive rho(X) and large r values, suggesting that the nonlinear Hammett plots are not due to a change in the rate-determining step upon changing the acyl substituent X, but due to resonance demand of the pi-electron donor substituent on the acyl moiety. The magnitude of the rho(X) and beta(nuc) values increases with increasing the basicity of amines and with increasing the electron-withdrawing ability of the acyl substituent X, respectively, while that of the r values decreases with increasing rho(X) values and amine basicity.

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A kinetic study is reported for nucleophilic substitution reactions of 2,4-dinitro-1-fluorobenzene (DNFB) with a series of secondary amines in MeCN and H2O at 25.0 degrees C. The reaction in MeCN results in an upward curvature in the plot of k(obsd) vs [amine], indicating that the reaction proceeds through a rate-limiting proton transfer (RLPT) mechanism. On the contrary, the corresponding plot for the reaction in H2O is linear, implying that general base catalysis is absent. The ratios of the microscopic rate constants for the reactions in MeCN are consistent with the proposed mechanism, e.g., the facts that k2/k(-1) < 1 and k3/k2 > 10(2) suggest that formation of a Meisenheimer complex occurs before the rate-limiting step and the deprotonation by a second amine molecule becomes dominant when [amine] > 0.01 M, respectively. The Brønsted-type plots for k1k2/k(-1) and k1k3/k(-1) are linear with betanuc values of 0.82 and 0.84, respectively, which supports the proposed mechanism. The Brønsted-type plot for the reactions in H2O is also linear with betanuc = 0.52 which has been interpreted to indicate that the reaction proceeds through rate-limiting formation of a Meisenheimer complex. DNFB is more reactive toward secondary amines in MeCN than in H2O. The enhanced basicity of amines as well as the increased stability of the intermediate whose charges are delocalized through resonance are responsible for the enhanced reactivity in the aprotic solvent.

A kinetic study is reported for the reaction of the anionic nucleophiles OH-, CN-, and N 3 - with aryl benzoates containing substituents on the benzoyl as well as the aryloxy moiety, in 80 mol % H2O - 20 mol % dimethyl sulfoxide at 25.0 degrees C. Hammett log k vs sigma plots for these systems are consistently nonlinear. However, a possible traditional explanation in terms of a mechanism involving a tetrahedral intermediate with curvature resulting from a change in rate-determining step is considered but rejected. The proposed explanation involves ground-state stabilization through resonance interaction between the benzoyl substituent and the electrophilic carbonyl center in the two-stage mechanism. Accordingly, the data are nicely accommodated on the basis of the Yukawa-Tsuno equation, which gives linear plots for all three nuceophiles. Literature reports of the mechanism of acyl transfer processes are reconsidered in this light.

Pseudo-first-order rate constants (k(obs)) have been measured spectrophotometrically for reactions of O-4-nitrophenyl thionobenzoate (2) with a series of primary and acyclic secondary amines. The plots of k(obs) vs amine concentration are linear for the reaction of 2 with primary amines. The slope of the Brønsted-type plot for the reaction of 2 with primary amines decreases from 0.77 to 0.17 as the amine basicity increases, indicating that the reaction proceeds through a zwitterionic addition intermediate in which the rate-determining step changes from the breakdown of the intermediate to the reaction products to the formation of the intermediate as the amine basicity increases. On the other hand, for reactions with all the acyclic secondary amines studied, the plot of k(obs) vs amine concentration exhibits an upward curvature, suggesting that the reaction proceeds through two intermediates, e.g., a zwitterionic addition intermediate and an anionic intermediate. The microscopic rate constants (k(1), k(-)(1), k(2), and k(3) where available) have been determined for the reactions of 2 with all the primary and secondary amines studied. The k(1) value is larger for the reaction with the primary amine than for the reaction with the isobasic acyclic secondary amines, while the k(-)(1) value is much larger for the latter reaction than for the former reaction. The k(3) value for the reaction with secondary amine is independent of the amine basicity. The small k(2)/k(-)(1) ratio is proposed to be responsible for the deprotonation process observed in aminolyses of carbonyl or thiocarbonyl derivatives.

The origin of the alpha-effect has been probed through a combination of calorimetric and kinetic studies involving butane-2,3-dione monoximate as alpha-nucleophile and p-chlorophenoxide as normal nucleophile in the reaction with p-nitrophenyl acetate in DMSO-H(2)O mixtures, which has been shown to exhibit a bell-shaped profile in the alpha-effect with solvent composition. The study, involving determination of enthalpies of solution and activation parameters, has allowed a dissection of contributions to the alpha-effect of ground-state destabilization and transition-state stabilization in these DMSO-H(2)O solvent media. It has been found that over the solvent composition 0-50 mol % DMSO desolvation of the alpha-nucleophile is the main driving factor to the increasing alpha-effect. However, in solvent mixtures covering 50-90 mol % DMSO the thermodynamic activation parameters suggest an interplay of factors that result in the bell-shaped alpha-effect profile. Discussion is presented that includes possible medium-dependent nonsynchronicity of nucleophile desolvation and bond formation for the alpha-nucleophile.

Second-order rate constants have been measured for the reactions of 2,4-dinitrophenyl X-substituted benzoates (1a-f) with a series of primary amines in 80 mol % H(2)O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Brønsted-type plot for the reactions of 1d with primary amines is biphasic with slopes beta(1) = 0.36 at the high pK(a) region and beta(2) = 0.78 at the low pK(a) region and the curvature center at pK(a) degrees = 9.2, indicating that the reaction proceeds through an addition intermediate with a change in the rate-determining step as the basicity of amines increases. The corresponding Brønsted-type plot for the reactions with secondary amines is also biphasic with beta(1) = 0.34, beta(2) = 0.74, and pK(a) degrees = 9.1, indicating that the effect of amine nature on the reaction mechanism and pK(a) degrees is insignificant. However, primary amines have been found to be less reactive than isobasic secondary amines. The microscopic rate constants associated with the aminolysis have revealed that the smaller k(1) for the reactions with primary amines is fully responsible for their lower reactivity. The electron-donating substituent in the nonleaving group exhibits a negative deviation from the Hammett plots for the reactions of 1a-f with primary and secondary amines, while the corresponding Yukawa-Tsuno plots are linear. The negative deviation has been ascribed to stabilization of the ground state of the substrate through resonance interaction between the electron-donating substituent and the carbonyl functionality.