Ground-State versus Transition-State Effects on the α-Effect as Expressed by Solvent Effects

Ground-State versus Transition-State Effects on the α-Effect as Expressed by Solvent Effect

Alkaline Degradation of the Organophosphorus Pesticide Fenitrothion as Mediated by Cationic C₁₂, C₁₄, C₁₆, and C₁₈ Surfactants

The effect of varying surfactant chain length (C12, C14, C16, C18) on the alkaline hydrolysis of the organophosphorus pesticide fenitrothion was determined for the following series of inert counterion cationic surfactants:  dodecyltrimethylammonium bromide (DTABr), tetradecyltrimethylammonium bromide (TTABr), hexadecyltrimethylammonium bromide (CTABr), and octadecyltrimethylammonium bromide (OTABr). Plots of kobs versus [surfactant] at constant [KOH] showed saturation behavior at low total [Br-], and (constrained) S-shaped curvature was observed at high total [Br-]. kobs values increased with increasing surfactant chain length but decreased with added KBr. For systems exhibiting saturation behavior, further analysis of the results using the PPIE treatment as modified to account for HO-/Br- exchange allowed the evaluation of substrate binding constants, KS, and micellar rate constants, k2m. The binding constants increased with chain length (hydrophobicity), but ionic strength had no effect on KS. Meanwhile, because of the increased KS values as the surfactant chain length increased, the rate enhancements observed for fenitrothion degradation correspondingly increased. However, rate enhancements decreased with ionic strength because reactive counterions could not compete against the bromide anion for micellar binding sites. Low k2m/k2w ratios revealed that the observed rate enhancements were due to the so-called concentration effect rather than true catalysis. Finally, where the PPIE model failed (displaying S-shaped curvature), our results support the intervention of sphere-to-rod transitions that are favored at high ionic strength (>0.01 M Br-) and lower temperatures as the cause of the S-shaped curvature

Alkaline Degradation of the Organophosphorus Pesticide Fenitrothion as Mediated by Cationic C₁₂, C₁₄, C₁₆, and C₁₈ Surfactants

The effect of varying surfactant chain length (C12, C14, C16, C18) on the alkaline hydrolysis of the organophosphorus pesticide fenitrothion was determined for the following series of inert counterion cationic surfactants:  dodecyltrimethylammonium bromide (DTABr), tetradecyltrimethylammonium bromide (TTABr), hexadecyltrimethylammonium bromide (CTABr), and octadecyltrimethylammonium bromide (OTABr). Plots of kobs versus [surfactant] at constant [KOH] showed saturation behavior at low total [Br-], and (constrained) S-shaped curvature was observed at high total [Br-]. kobs values increased with increasing surfactant chain length but decreased with added KBr. For systems exhibiting saturation behavior, further analysis of the results using the PPIE treatment as modified to account for HO-/Br- exchange allowed the evaluation of substrate binding constants, KS, and micellar rate constants, k2m. The binding constants increased with chain length (hydrophobicity), but ionic strength had no effect on KS. Meanwhile, because of the increased KS values as the surfactant chain length increased, the rate enhancements observed for fenitrothion degradation correspondingly increased. However, rate enhancements decreased with ionic strength because reactive counterions could not compete against the bromide anion for micellar binding sites. Low k2m/k2w ratios revealed that the observed rate enhancements were due to the so-called concentration effect rather than true catalysis. Finally, where the PPIE model failed (displaying S-shaped curvature), our results support the intervention of sphere-to-rod transitions that are favored at high ionic strength (>0.01 M Br-) and lower temperatures as the cause of the S-shaped curvature

Micellar Catalyzed Degradation of Fenitrothion, an Organophosphorus Pesticide, in Solution and Soils^†

We report on a study of the decomposition of fenitrothion (an organophosphorus pesticide that is a persistent contaminant in soils and groundwater) as catalyzed by cetyltrimethylammonium (CTA+) micelles. The CTA micelles were associated with two types of counterions:  (1) inert counterions (e.g. CTABr) and (2) reactive counterions (e.g. CTAOH). The reactive counterion surfactants used were hydroxide anion (HO-) as a normal nucleophile and hydroperoxide anion (HOO-) and the anion of pyruvaldehyde oxime (MINA-) as two α-nucleophiles. The reactivity order followed:  CTABr < CTAOH < CTAMINA ≪ CTAOOH. Treatment of the rate data using the Pseudo-Phase Ion Exchange (PPIE) model of micellar catalysis showed the ratio k2M/k2W to be less than unity for all the surfactants employed. Rather than arising from a “true catalysis”, we attributed the observed rate enhancements to a “concentration effect”, where both pesticide and nucleophile were incorporated into the small micellar phase volume. Furthermore, the CTAOOH/CTAOH pair gave an α-effect of 57, showing that the α-effect can play an important role in micellar systems. We further investigated the effectiveness of reactive counterion surfactants in decontaminating selected environmental solids that were spiked with 27 ppb fenitrothion. The solids were as follows:  the clay mineral montmorillonite and SO-1 and SO-2 soils (obtained from the Canadian Certified Reference Materials Project). The reactive counterion surfactant solutions significantly enhanced the rate of fenitrothion degradation in the spiked solids over that obtained when the spiked solid was placed in contact with either 0.02 M KOH or water. The rate enhancements followed the order CTAOOH ≫ CTAMINA ∼ CTAOH > KOH ≫ water. We conclude that reactive counterion surfactants, especially with α-nucleophiles, hold great potential in terms of remediating soils contaminated by toxic organophosphorus esters

Hydrogen−Deuterium Exchange Studies in Platinum(II) Complexes of 1-Methylimidazole¹

A kinetic study of hydrogen−deuterium (H/D) exchange in Pt(II)−1-methylimidazole complexes has been performed in D2O/NaOD solution, at 60 °C, by means of 1H NMR spectroscopy. Isotopic exchange has been observed at C(2)−H, C(4)−H, and C(5)−H of the imidazole moiety. Kinetic data analysis of H/D exchange in the complexes cis-[Pt(NH3)2(MeIm)2]Cl2 (3), trans-[Pt(NH3)2(MeIm)2]Cl2 (4), [Pt(en)(MeIm)2]Cl2 (5), and [Pt(MeIm)4](ClO4)2 (6) revealed that Pt(II) enhances C(2)−H exchange in the coordinated 1-methylimidazole (MeIm) by ca. 102 to 103, relative to the neutral substrate. However, it is ca. 104−105 times less effective compared to H+ and CH3+. In complex 5, C(5)−H exchange is ca. 6 times faster than C(4)−H exchange, in contrast with expectations based on an inductive/field effect by Pt(II). The observation of the relative reactivity order, C(5)−H exchange > C(4)−H exchange in 5, is discussed by consideration of contributing resonance structures of the intermediates formed upon abstraction of C(4)−H and C(5)−H, which would place the positive charge at the more favorable N(1)−CH3 position. An X-ray crystal structure determination of 6 was also performed. The complex crystallizes in the triclinic space group P1̄, with a = 8.219(1) Å, b = 9.424(1) Å, c = 9.139(3) Å; α = 107.68(2)°, β = 83.72(2)°, γ = 114.87(1)°; and Z = 1. The complex has a center of symmetry with the 1-methylimidazole ligands coordinated in a square-planar arrangement around the Pt(II) atom

Hydrogen−Deuterium Exchange Studies in Platinum(II) Complexes of 1-Methylimidazole¹

A kinetic study of hydrogen−deuterium (H/D) exchange in Pt(II)−1-methylimidazole complexes has been performed in D2O/NaOD solution, at 60 °C, by means of 1H NMR spectroscopy. Isotopic exchange has been observed at C(2)−H, C(4)−H, and C(5)−H of the imidazole moiety. Kinetic data analysis of H/D exchange in the complexes cis-[Pt(NH3)2(MeIm)2]Cl2 (3), trans-[Pt(NH3)2(MeIm)2]Cl2 (4), [Pt(en)(MeIm)2]Cl2 (5), and [Pt(MeIm)4](ClO4)2 (6) revealed that Pt(II) enhances C(2)−H exchange in the coordinated 1-methylimidazole (MeIm) by ca. 102 to 103, relative to the neutral substrate. However, it is ca. 104−105 times less effective compared to H+ and CH3+. In complex 5, C(5)−H exchange is ca. 6 times faster than C(4)−H exchange, in contrast with expectations based on an inductive/field effect by Pt(II). The observation of the relative reactivity order, C(5)−H exchange > C(4)−H exchange in 5, is discussed by consideration of contributing resonance structures of the intermediates formed upon abstraction of C(4)−H and C(5)−H, which would place the positive charge at the more favorable N(1)−CH3 position. An X-ray crystal structure determination of 6 was also performed. The complex crystallizes in the triclinic space group P1̄, with a = 8.219(1) Å, b = 9.424(1) Å, c = 9.139(3) Å; α = 107.68(2)°, β = 83.72(2)°, γ = 114.87(1)°; and Z = 1. The complex has a center of symmetry with the 1-methylimidazole ligands coordinated in a square-planar arrangement around the Pt(II) atom

Solvent Effect on the α-Effect for the Reactions of Aryl Acetates with Butane-2,3-dione Monoximate and <i>p</i>-Chlorophenoxide in MeCN−H₂O Mixtures

Second-order rate constants have been measured spectrophotometrically for the nucleophilic reactions of three substituted phenyl acetates with butane-2,3-dione monoximate (Ox-) as an α-nucleophile and p-chlorophenoxide (ClPhO-) as corresponding normal nucleophile, in MeCN−H2O mixtures of varying compositions at 25.0 ± 0.1 °C. The reactivity of Ox- toward the aryl acetates decreases upon addition of MeCN to the reaction medium up to ca. 30 mol % MeCN, followed by a gradual increase in rate upon further addition of MeCN. A similar result has been obtained for the reaction of ClPhO- with the aryl acetates. However, the decrease in rate is more significant for the less reactive ClPhO- than for the more reactive Ox-. Thus, for all the aryl acetates studied, Ox- exhibits a sizable α-effect (kOx−/kClPhO−) whose magnitude increases as the mol % MeCN in the reaction medium increases. The relative basicities (ΔpKa) of Ox- and ClPhO- have been determined spectrophotometrically using piperazine as a reference base. The ΔpKa values increase on increasing the mol % MeCN in the medium for both Ox- and ClPhO-. The difference in the relative basicities of these nucleophiles (ΔΔpKa) becomes larger with increasing mol % MeCN. The plots of log kOx−/kClPhO− vs ΔΔpKa for the three substrates are linear with near-unit slope, indicating that the difference in the relative basicity of the nucleophiles is largely responsible for the increasing α-effect with medium composition in this system

Enhanced Bistability of a Photochromic Microparticle in Condensed Medium

Enhanced Bistability of a Photochromic Microparticle in Condensed Mediu

Solvent Effect on the α-Effect: Ground-State versus Transition-State Effects; a Combined Calorimetric and Kinetic Investigation

In a study of the solvent effect on the α-effect, second-order rate constants (kNu−) have been determined spectrophotometrically for reactions of a series of substituted phenyl acetates with butan-2,3-dione monoximate (Ox-, α-nucleophile) and p-chlorophenoxide (p-ClPhO-, reference nucleophile) in DMSO−H2O (DMSO = dimethyl sulfoxide) mixtures of varying compositions at 25.0 ± 0.1 °C. The magnitude of the α-effect, kOx−/kp-ClPhO−, increases as the DMSO content in the medium increases up to 40−50 mol %, reaching 500, one of the largest α-effect values, and then decreases on further addition of DMSO, resulting in a bell-shaped α-effect profile regardless of the nature of the substrates. The magnitude of the α-effect is found to be significantly dependent on the substrates (or, more quantitatively, on βnuc). Thus, βnuc is an important predictor of the magnitude of the α-effect. The bell-shaped α-effect profile found in the present system is attributed to the differential change in the sensitivity of the medium effect on the Ox- and p-ClPhO- systems but not due to a change in the reaction mechanism or to a drastic change in the basicity of the two nucleophiles on addition of DMSO to the medium. Through application of calorimetric measurements of ground-state solvation combined with the diagnostic βnuc values, it is shown that the transition-state effect is more dominant than the ground-state effect as the origin of the α-effect in the present system

Pitfalls in Assessing the α-Effect: Reactions of Substituted Phenyl Methanesulfonates with HOO⁻, OH⁻, and Substituted Phenoxides in H₂O

Toward resolving the current controversy regarding the validity of the α-effect, we have examined the reactions of Y-substituted phenyl methanesulfonates 1a−1l with HOO−, OH−, and Z-substituted phenoxides in the gas phase versus solution (H2O). Criteria examined in this work are the following: (1) Brønsted-type and Hammett plots for reactions with HOO−and OH−, (2) comparison of βlg values reported previously for the reactions of Y-substituted phenyl benzenesulfonates 2a−2k with HOO− (βlg = −0.73) and OH− (βlg = −0.55), and for those of 1a−1l with HOO− (βlg = −0.69) and OH− (βlg = −1.35), and (3) Brønsted-type plot showing extreme deviation of OH− for reactions of 2,4-dintrophenyl methanesulfonate 1a with aryloxides, HOO−, and OH−, signifying extreme solvation vs different mechanisms. The results reveal significant pitfalls in assessing the validity of current interpretations of the α-effect. The extreme negative deviation by OH− must be due, in part, to the difference in their reaction mechanisms. Thus, the apparent dependence of the α-effect on leaving-group basicity found in this study has no significant meaning due to the difference in operating mechanisms. The current results argue in favor of a further criterion, i.e., a consistency in mechanism for the α-nucleophiles and normal nucleophiles