Hydrolysis of 2,4-Dinitrochlorobenzene Catalyzed by Cationic Micelles

Micellar catalysis by nine cationic surfactants of the basic hydrolysis of 2,4-dinitrochlorobenzene(DNCB) was studied. The results obtained are as follows: (I) The second-order constants k₂ for the hydrolysis reaction of DNCB catalized by the cationic micelles increase by a factor of 11–100 than that in water. Plots of k₂ against the surfactant concentration show an S-type curve, and the catalytic effect is observed below the critical micelle concentration(CMC) of the surfactants. (2) For a series of surfactants, there is an optimal chain length for the alkyl of the surfactants to show the greatest catalytic effect. (3) The hydrolysis rate of DNCB decreases as the base concentration increases. (4) For the surfactants with the same hydrophilic and hydrophobic groups, chlorides have advantage over bromides in enhancing the reaction rate. These results can be interpreted in term of the changes in CMC, micelle size, solubilization capacity of the micelles, binding degree of counterion et al.     

Influence of Amine‐Based Cationic Gemini Surfactants on Catalytic Activity of α‐Chymotrypsin

The α‐chymotrypsin activity was tested in aqueous media with the presence of novel cationic amine–based gemini surfactant, with different spacer chain lengths and head group size, and also compared with the cationic cetyltrimethylammonium bromide (CTAB) and cetyltriphenylphosphonium bromide (CTPB) surfactants and aqueous buffer only. The p‐nitrophenyl acetate (PNPA) hydrolysis rate was monitored in the presence of the surfactant concentration at 30°C. Most of these gemini surfactants gave higher catalytic activity as compared to cationic CTAB and CTPB. The highest superactivity was measured in the presence of gemini 16‐12‐16, [dodecanediyl‐1,12‐bis(cetyldimethylammonium bromide)] surfactant at pH 7.5. The catalytic reaction follows the Michaelis–Menten mechanism. The catalytic rate constants, k_cat, show the same profile that the catalytic affinity; K_M being enhanced with increasing space chain length. The results are favorable for considering that the amine‐based gemini surfactant influences more than both the aqueous and cationic micellar media.     

Investigation on the basic hydrolysis of crystal violet in mixed reverse micelles formed with AOT and nonionic surfactants

 The kinetics and thermodynamics of the basic hydrolysis of crystal violet (CV) in mixed reverse micelles formed with anionic surfactant AOT and nonionic surfactants have been investigated. It was found that the mixed reverse micelles had inhibitory effects on CV hydrolysis compared with the normal aqueous solution, and the equilibrium constant K of the reaction in mixed reverse micellar systems is smaller than that in pure water. The influence of water content and surfactant composition in reverse micelles on the second-order rate constant k ₁ of the positive reaction, on the first-order rate constant k _-1 of the reverse reaction, as well as on the equilibrium constant K of the reaction has been studied, and the results obtained were interpreted in terms of the nature of surfactants and the properties of microenvironment where the reaction took place. Received: 24 October 1997 Accepted: 18 March 1998     

Effect of reactive and non-reactive counterion micelles upon the alkaline degradation of indomethacin

In the present paper, kinetics of alkaline degradation of well known drug, indomethacin (2-[1-(4-chlorobenzoyl)-5-methoxy-2-methylindol-3-yl]acetic acid), was studied in presence of excess [NaOH]. The rate of hydrolysis of substrate was independent of the [indomethacin] though it increased linearly with increasing the hydroxide ion concentration with a positive slope, suggesting the following rate law: k_obs = k₁[OH⁻]. Cationic surfactants having non-reactive ions (cetyltrimethylammonium bromide, CTAB and cetyltrimethylammonium sulfate (CTA)₂SO₄) first increased the rate constants at lower concentrations and then decreased it at higher concentrations while in case of the surfactant with reactive counterions (cetyltrimethylammonium hydroxide, CTAOH) the rate increases sharply at lower concentrations of surfactant until it reaches to a plateau in contrast to the appearance of maxima in case of CTAB and (CTA)₂SO₄. Anionic surfactant, sodium dodecyl sulfate (SDS), inhibited the reaction rate at all concentrations used in the study. Pseudophase ion-exchange model was used for analyzing the effect of cationic micelles while the inhibition by SDS micelles was fitted using the Menger–Portnoy model. The effect of salts (NaCl, NaBr and (CH₃)₄NBr) was also seen on the hydrolysis of indomethacin and it was found that all salts inhibited the rate of reaction. The inhibition followed the trend NaCl < NaBr < (CH₃)₄NBr.     

Activity of α‐Chymotrypsin in Cationic and Nonionic Micellar Media: Ultraviolet and Fluorescence Spectroscopic Approach

α‐Chymotrypsin (α‐CT) activity was measured in aqueous buffer with the following alkyltriphenylphosphonium bromide surfactants in the series cetyl, tetradecyl, and dodecyl as a tail length. For the sake of comparison with mixed micellar investigation on activity of α‐CT, cationic cetyltriphenylphosphonium bromide (CTPB) and nonionic surfactant Triton X‐100, Brij‐56, Brij‐35, Tween 20, and Igepal Co‐210 have been used. The p‐nitrophenyl acetate (PNPA) hydrolysis rate was determined at the surfactant concentration of both cationic and mixed micellar systems by a UV–vis spectrophotometer. The catalytic reaction follows the Michaelis–Menten mechanism, and the catalytic efficiency (k_cat/K_M) was evaluated for both homogeneous and mixed‐micellar media. The maximum catalytic efficiency was observed at 5 mM concentration of CTPB, but the highest catalytic efficiency, 572 M^?1 s^?1, was measured in the presence of mixed micellar (7.5 mM CTPB + 2.5 mM Tween‐20). The fluorescence (FL) spectra showed the differences of α‐CT conformations in the presence of cationic surfactants. The FL results suggest that the influence of cationic surfactant on proteolysis arises from the interaction with the α‐CT. The binding constant, k_sv, of α‐CT with cationic aggregates was determined in the buffer using the Stern–Volmer equation by the fluorescence spectroscopic approach.     

A study on the hydrolysis of bis(nitrophenyl)phosphate catalyzed by N‐methyldiethanolamine‐Ce(III) complex

The hydrolysis of bis(p‐nitrophenyl)phosphate (BNPP) catalyzed by N‐methyldiethanolamine‐Ce(III) complex in the presence and absence of cetyltrimethylammonium bromide (CTAB) and Brij35 surfactants at pH 7.20 and 303 K has been studied. The experimental results indicate that N‐methyldiethanolamine‐Ce(III) complex remarkably accelerates the hydrolysis of BNPP. The observed first‐order rate constant of the hydrolysis of BNPP catalyzed by N‐methyldiethanolamine‐Ce(III) complex at pH 7.20 and 303 K is 1.22 × 10^?2 s^?1, which is 1.09 × 10⁹ times of that of spontaneous hydrolysis of BNPP at pH 7. It is close to the activity of natural enzyme. A general quantitative treatment of the catalytic reaction involved a ternary complex as M_mL_lS has also been proposed in this paper. Applying this method to the catalytic hydrolysis of BNPP, we have obtained its thermodynamic and kinetic parameters. CTAB and Brij35 surfactant micelles obviously influence the rate constants of the catalytic hydrolysis of BNPP. Brij35 micelles promote the catalytic hydrolysis of BNPP, while CTAB micelles inhibit it. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 687–692, 2004     

Effects of head group of cationic surfactants on the hydrolysis of p‐nitrophenyl acetate catalyzed by α‐chymotrypsin

The kinetics of hydrolysis of p‐nitrophenyl acetate catalyzed by α‐chymotrypsin (α‐CT) has been studied in the presence of several cationic surfactants having different head groups maintaining the dodecyl hydrophobic residue and bromide counterion. The enzyme activity was tested in the presence of dodecyl trimethylammonium bromide (DTAB), dodecylpyridinium bromide (DPB), dodecyldimethylethanolammonium bromide (DDMEAB), dodecyldiethylethanolammonium bromide (DDEEAB), benzyldimethyldodecylammonium bromide (BDDAB), and dodecyltriphenylphosphonium bromide (DTPB) surfactants. The extent of superactivity depends upon head groups of surfactants. The activity of α‐CT depends on the surfactant concentration and it varies with the surfactant head group dimensions (DTPB > DDEEAB > DTAB > BDDAB > DDMEAB > DPB). For all surfactants, DTPB exhibits highest superactivity. The effects of surfactants on the apparent kinetic parameters like Michaelis constant K_m and the catalytic constant k_cat have been determined. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 377–381, 2009     

α-chymotrypsin superactivity in cetyltrialkylammonium bromide-rich media

α-Chymotrypsin (α-CT) activity was tested with N-glutaryl-l-phenylalanine p-nitroanilide in buffered media with added cationic surfactants. The effect of the commercial cetyltrimethylammonium bromide (CTABr) was compared with that of three other surfactants with ethyl (CTEABr), propyl (CTPABr), and butyl (CTBABr) head groups. These were synthesized and purified in this laboratory. Surfactant head groups provided distinct environments that largely modulated the catalytic performance. Larger alkyl head group hydrophobicity led to a marked enhancement of α-CT activity. CTBABr-rich media induced the highest superactivity. Kinetic measurements were performed in Tris-HCl buffer at a surfactant concentration either below or above CMC, and α-CT superactivity occurred in both media. Positive interactions between the enzyme and surfactants happened independently of thesupramolecular organization of the medium. The reaction followed the Michaelis-Menten kinetics. The substrate to micelle aggregates binding constant was used to calculate the substrate concentration available for catalysis. The k _cat to k _m ratio was in CTBABr-rich media always higher than in pure buffer and depended on the surfactant concentration. α-CT superactivity depended on the pH value of buffer solution. Enzyme inactivation followed a single-step mechanism in pure buffer and a series mechanism in the presence of a surfactant. The rate of activity decay obeyed a first-order kinetics.     

Kinetic Screening for the Acid‐Catalyzed Hydrolysis of Some Hydrophobic Fe(II) Schiff Base Amino Acid Chelates and Reactivity Trends in the Presence of Alkali Halide and Surfactant

Kinetics of acid‐catalyzed hydrolysis of some high‐spin Fe(II) Schiff base amino acid complexes were followed spectrophotometrically at 298 K under pseudo–first‐order conditions. The studied ligands were derived from the condensation of 5‐bromosalicylaldehyde with different four amino acids (phenylalanine, aspartic acid, histidine, and arginine). The acid hydrolysis reaction was studied in aqueous media and in the presence of different concentrations of the alkali halide (KBr) and cationic surfactant (cetyl‐trimethyl ammonium bromide, CTAB). The general rate equation was suggested to be rate = k_obs[complex], where k_obs = k₂[H⁺]. The increase in [KBr] enhances the reactivity of the reaction, and the addition of CTAB to the reaction mixture accelerates the reaction reactivity. The obtained kinetic data were used to determine the values of δ_mΔG^# (the change in the activation barrier) for the studied complexes when transferred from “water to water containing different [KBr]” and from “water to water containing altered [CTAB].”     

Hydrolysis of carboxylate and phosphate esters using monopyridinium oximes in cationic micellar media

The reactions of p‐nitrophenyl acetate (PNPA) with a series of monopyridinium oximes, viz. 2‐PAM (2‐hydroxyiminomethyl‐1‐methylpyridinium iodide), 3‐PAM (3‐hydroxyiminomethyl‐1‐methylpyridinium iodide), and 4‐PAM (4‐hydroxyiminomethyl‐1‐methylpyridinium iodide) have been studied in the presence of cationic surfactants of same hydrophobic chain length (C₁₆) within the concentration range of 0.5–6.0 mM at pH 8.0 under the pseudo‐first‐order condition. The observed rate constant (k_obs) increases with increasing surfactant concentration culminating into a maximum, and this has been analyzed in detail following the concepts of micellar catalysis. The structure–activity relationship of the investigated oximes has been discussed, and 2‐PAM was found to be the most reactive among all the three investigated oximes for the cleavage of PNPA. Esterolytic decomposition of p‐nitrophenyldiphenyl phosphate with oximate ions (? CH?NO^?) was followed in cetyltrimethylammonium bromide micelles at pH 9.0, and 4‐PAM was the most reactive oxime for the micellar hydrolysis of phosphate ester. The apparent acid dissociation constants (pK_a) of the investigated oximes have been determined spectrophotometrically. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 569–578, 2011     

Dicationic surfactant based catalytic systems for alkaline hydrolysis of phosphonic acid esters

Use of aqueous micellar solutions of dicationic surfactants with the general formula [R(CH₃)₂N(CH₂)₆N(CH₃)₂R]²⁺2Br⁻ (R = n-C₁₀H₂₁ to n-C₁₆H₃₃) as the reaction medium for the alkaline hydrolysis of phosphonic acid esters has revealed a strong catalytic effect of the surfactants, which can increase the reaction rate by two orders of magnitude. This effect depends on the surfactant structure, shows itself at low surfactant concentrations, and is substrate-specific. The effect of the micelles on the phosphonate hydrolysis rate is largely determined by the hydrophobicity factor.     

Kinetics of the alkaline hydrolysis of fenuron in aqueous and micellar media

The kinetics of the hydrolysis of fenuron in sodium hydroxide has been investigated spectrometrically in an aqueous medium and in cationic micelles of cetyltrimethylammonium bromide (CTAB) medium. The reaction follows first‐order kinetics with respect to [fenuron] in both the aqueous and micellar media. The rate of hydrolysis increases with the increase in [NaOH] in the lower concentration range but shows a leveling behavior at higher concentrations. The reaction followed the rate equation, 1/k_obs = 1/k + 1/(kK[OH^?]), where k_obs is the observed rate constant, k is rate constant in aqueous medium, and k is the equilibrium constant for the formation of hydroxide addition product. The cationic CTAB micelles enhanced the rate of hydrolytic reaction. In both aqueous and micellar pseudophases, the hydrolysis of fenuron presumably occurs via an addition–elimination mechanism in which an intermediate hydroxide addition complex is formed. The added salts decrease the rate of reaction. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 638–644, 2007     

THE EFFECT OF SURFACTANT MICELLES ON THE ALKALINE HYDROLYSIS OF DIMETHYLFORMAMIDE

The alkaline hydrolysis of dimethylformamide has been studied at 40'C in micellar solutions of single surfactant (CTAB. SDS. Brij 35) with the analog thermoanalytical curve method of thermokinetics. A kinetic equation of micellar catalysis under the condition of highter reactant concentration than micellar concentration ([S]>[M]) has been derived from the pseudophase model of micellar catalysis and some relative assumptions, The kinetic parameters. k_m, k_2mand the association constant of reactant with micelle K₁, have been calculated in this way. the results indicate that these surfactant micelles exhibit catalytic effect on the reaction. This is attributed to the micropolarity and local concentration effect of micelles.     

Effects of Micellar Aggregates on the Kinetics and Mechanism of the Reaction between 4‐Nitrobenzenediazonium Ions and Some Amino Acids

We have explored the kinetics and mechanism of the reaction between 4‐nitrobenzenediazonium ions (4NBD), and the hydrophilic amino acids (AA) glycine and serine in the presence and absence of sodium dodecyl sulfate (SDS) micellar aggregates by means of UV/VIS spectroscopy. The observed rate constants k_obs were obtained by monitoring the disappearance of 4NBD with time at a suitable wavelength under pseudo‐first‐order conditions. In aqueous acid (buffer‐controlled) solution, in the absence of SDS, the dependence of k_obs on [AA] was obtained from the linear relationship found between the experimental rate constant and [AA]. At a fixed amino acid concentration, k_obs values show an inverse dependence on acidity in the range of pH 5–6, suggesting that the reaction takes place through the nonprotonated amino group of the amino acid. All kinetic evidence is consistent with an irreversible bimolecular reaction with k=2390±16 and 376±7 M ^?1 s^?1 for glycine and serine, respectively. Addition of SDS inhibits the reaction because of the micellar‐induced separation of reactants originated by the electrical barrier imposed by the SDS micelles; k_obs values are depressed by factors of 10 (glycine) and 6 (serine) on going from [SDS]=0 up to [SDS]=0.05M . The hypothesis of a micellar‐induced separation of the reactants was confirmed by ¹H‐NMR spectroscopy, which was employed to investigate the location of 4NBD in the micellar aggregate: the results showed that the aromatic ring of the arenediazonium ion is predominantly located in the vicinity of the C(β) atom of the surfactant chain, and hence the reactive ? N group is located in the Stern layer of the micellar aggregate. The kinetic results can be quantitatively interpreted in terms of the pseudophase kinetic model, allowing estimations of the association constant of 4NBD to the SDS micelles.     

Reactivity Tracers of the Hydrolysis of Fe(II)‐Bis(salicylidene) Complexes: Initial–Transition States Analysis and Enhanced Impact of Tensides on the Kinetic Trends

The kinetics of the acid hydrolysis reaction of Fe(II)‐bis(salicylidene) complexes were followed under pseudo–first‐order conditions ([H⁺] >> [complex]) at 298 K. The ligands of the studied azomethine complexes were derived from the condensation of salicylaldehyde with different five α‐amino acids. The hydrolysis reactions were studied in acidic medium at different ratios (v/v) of aqua–organic mixtures. The decrease in the dielectric constant values of the reaction mixture enhances the reactivity of the reaction. The transfer chemical potentials of the initial and transition states (IS–TS) from water into mixed solvents were determined from the solubility measurements combined with the kinetic data. Nonlinear plots of logk_obs versus 1/D (the reciprocal of the dielectric constant) suggest the influence of the solvation of IS–TS on the reaction reactivity. Furthermore, the acid hydrolysis reactions were screened in the presence of different concentrations of cationic and anionic tensides. The addition of surfactants to the reaction mixture accelerates the reaction reactivity. The obtained kinetic data were used to determine the values of δ_mΔG^# (the change in the activation barrier) for the studied complexes when transferred from “water to various ratios (v/v) of water–co‐organic binary mixtures” and from “water to water containing different [surfactant].” It was found that the reactivity of the acid hydrolysis reaction was controlled by the hydrophobicity of the studied chelates.     

Study of the bromide oxidation by bromate in zwitterionic micellar solutions

The redox reaction Br⁻ + BrO₃⁻ has been studied in aqueous zwitterionic micellar solutions of N‐tetradecyl‐N, N‐dimethyl‐3‐ammonio‐1‐propanesulfonate, SB3‐14, and N‐hexadecyl‐N,N‐dimethyl‐3‐ammonio‐1‐propanesulfonate, SB3‐16. A simple expression for the observed rate constant, k_obs, based on the pseudophase model, could explain the influences of changes in the surfactant concentration on k_obs. The kinetic effect of added NaClO₄ on the reaction rate in SB3‐14 micellar solutions has also been studied. They were rationalized by considering the binding of the perchlorate anions to the sulfobetaine micelles and their competition with the reactive bromide ions for the micellar surface. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 388–394, 2000     

Use of solvent isotope effect to identify an intermediate carbanion in the β-elimination reactions from N-[2-(4-pyridyl)ethyl]quinuclidinium and N-[2-(2-pyridyl)ethyl]quinuclidinium induced by acetohydroxamate/acetohydroxamic acid buffers

Solvent isotope effect is a useful technique for identifing and characterizing an intermediate carbanion in the base-induced -elimination reaction from N-[2-(4-pyridyl)ethyl]quinuclidinium, 1, and N-[2-(2-pyridyl)ethyl]quinuclidinium, 2. While at high [buffer]k _obs(D₂O) > k _obs(H₂O) due to the presence of a primary kinetic solvent isotope effect on the reprotonation of the intermediate carbanion by BD, at low [buffer] no solvent isotope effect is observed, and k _obs(D₂O) k _obs(H₂O). The data are in agreement with a reversible E1cb mechanism in which carbon deprotonation occurs from NH⁺, the substrate protonated at the nitrogen atom of the pyridine ring. In the absence of solvent isotope effect at low [buffer], and with the similarity of the results obtained with the two isomers, 1 and 2, the significance of an intramolecular proton transfer in the intermediate carbanion can be excluded in these processes.     

Micellar catalyzed hydrolysis of mono-2,3-dichloroaniline phosphate

The kinetics of micellar catalyzed hydrolysis of mono-2,3-dichloroaniline phosphate in the presence of different surfactants has been studied at 303?K. The rate of reaction has been found to be first order with respect to both [substrate] and [HCl]. The cationic micelles of cetylpyridinium chloride (CPC), anionic micelles of di-octyl sodium sulphosuccinate (AOT), and non-ionic micelles of polyoxyethylene sorbitan monooleate (Tween 80) enhanced the rate of reaction to a maximum value and after that the increase in concentration of surfactant decreased the reaction rate. The applicability of different kinetic models has been tested to explain the observed micellar effects. The various thermodynamic activation parameters (E_a, ΔH^≠, ΔS^≠, ΔG^≠) have been evaluated. The added salts viz. KCl, KNO₃, K₂SO₄ enhanced the rate of reaction in the presence of CPC, AOT, and Tween 80 micelles. The kinetic parameters were determined from the rate (surfactant) profile and a suitable mechanism consistent with the experimental finding has been proposed.     

Micellar Effects on the Reaction between an Arenediazonium Ion and the Antioxidants Gallic Acid and Octyl Gallate

The effect of sodium dodecyl sulfate (SDS) micelles on the reaction between the 3‐methylbenzenediazonium (3MBD) ion and either the hydrophilic antioxidant gallic acid (GA) or the hydrophobic analogue octyl gallate (OG) have been investigated as a function of pH. Titration of GA in the absence and presence of SDS micelles showed that the micelles do not alter the first ionization equilibrium of GA. Analysis of the dependence of the observed rate constant (k_obs) with pH shows that the reactive species are GA^2? and OG^?. Kinetics results in the absence and presence of SDS micelles suggest that SDS aggregates do not alter the expected reaction pathway. SDS Micelles inhibit the spontaneous decomposition of 3MBD as well as the reaction between 3MBD and either GA or OG, and upon increasing the SDS concentration, with k_obs approaching the value for the thermal decomposition of 3MBD in the presence of SDS. Our results are consistent with the prediction of the pseudophase model and show that the origin of the inhibition for the reaction with GA is different to that for the reaction with OG; in the former case, the observed inhibition can be rationalized in terms of the micelle‐induced electrostatic separation of reactants in the micellar Stern layer, whereas the observed inhibition in the reaction with OG is a consequence of the dilution effect caused by increasing SDS concentration, decreasing the local OG^? concentration in the Stern layer.     

The α‐effect in micelles: Nucleophilic substitution reaction of p‐nitrophenyl acetate with N‐phenylbenzohydroxamate ion

Pseudo‐first‐order rate constants have been determined for the nucleophilic substitution reactions of p‐nitrophenyl acetate with p‐chlorophenoxide (4‐ClC₆H₄O^?) and N‐phenylbenzohydroxamate (C₆H₅CON(C₆H₅)O^?) ions in phosphate buffer (pH 7.7) at 27°C. The effect of cationic, (CTAB, TTAB, DTAB), anionic (SDS), and nonionic (Brij‐35) surfactants has been studied. The k_obs value increases upon addition of CTAB and TTAB. The effect of DTAB and other surfactants on the reaction is not very significant. The micellar catalysis and α‐effect shown by hydroxamate ion have been explained. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 26–31, 2006