Kinetic Study of the Hydrolysis of Carboxylate Ester with Acetohydroxamic Acid in Non-Ionic Microemu

IJIRST –International Journal for Innovative Research in Science & Technology| Volume 1 | Issue 6 | November 2014 ISSN (online): 2349-6010

Kinetic Study of The Hydrolysis of Carboxylate Ester with Acetohydroxamic Acid in Non-Ionic Microemulsion System Supriya Biswas Professor Department of Applied Chemistry SSCET, junwani, Bhilai, Chhattisgarh (INDIA)

Deepak Kumar Pandey Assistant Professor Department of Chemistry Pt. DPC Govt. College, Saja, Chhattisgarh (INDIA)

Abstract In recent year a kinetic study of nucleophilic substitution reaction in microemulsion has witnessed a significant growth of interest. Hydrolysis of P-nitrophenyl acetate with Acetohydroxamic acid has been studied in non-ionic microemulsion prepared from combination of weight percent of non-ionic surfactant (Triton x-100), co-surfactant n-butanol, oil-phase n-hexane and water. Conductivity behavior on the other hand depends mainly on the weight percent and composition of aqueous phase. The pseudo first-order rate constant increase with the increasing weight percentage of water (W0). Kinetic model of nucleophilic substitution reactions, have been discussed.

Keywords: Acetohydroxamic acid, Nucleophilic substitution, Microemulsion, PNPA. _______________________________________________________________________________________________________

I. INTRODUCTION Current interest in studying the reactions of α-nucleophiles has received major importance in many applications of this highly reactive species [1]. For example, in the development of protocols for environmental decomposition of sites polluted with organophosphorus insecticides, α-nucleophiles such as oximates [2,3], hydroxamates [4], hydroperoxide [5], iodosobenzoate [6] and hydroxybenzotriazoles [7] and tetrazole[8] etc., has been demonstrated to be highly effective. The need to develop efficient means to destroy stickpiles of organophosphorus nerve agents have led a number of groups to investigate different approaches toward enhancing decomposition of these agents [8]; α-nucleophiles can accelerate these decomposition. In this regard, we have been studying the esterolytic cleavage of carboxylate and phosphate esters using hydroxamate ion in cationic micellar media [9].

In resent year the microemulsion has witnessed a significant growth of interest due to its role as unique micro reactors.

Microemulsions are clear thermodynamically stable dispersion of two immiscible liquids containing appropriate amounts of surfactants or surfactant and co surfactants. This dispersion may be either oil-in-water (O/W) or water-in-oil (W/O). The oil in water type is a dispersion of a water immiscible liquid (oil) in an aqueous phase, and the water in oil type is dispersion of water or an aqueous solution in water immiscible liquid oil [10]. During the past few years the chemical and electrochemical preparation of the nano-material employing microemulsion is emerging as a potential area of interest. Extensive reviews on the physicochemical and electrochemical characterization of microemulsions are available [11]. Aqueous surfactant dispersions such as micelles, microemulsions, vesicles etc., in addition to their ability to support rate enhancements towards electrolytic reaction, provide means for solubilisation of hydrophobic compounds in water rich media. The rate enhancement exerted by micelle is referred to as micellar catalysis. The analogous effect occurring in microemulsion may be called ‘microemulsion catalysis’.

Large solubilisation capacities of microemulsion offer wider scope for their useful exploitation as reaction media. Oil in water (O/W) micro-emulsion is clear dispersion in water which generally contains an apolar solvent, a surfactant and co-surfactant.

The formation of microemulsion is generally spontaneous when requisite quantities of water, oil, surfactant and co-surfactant are mixed in specific proportions. In recent year microemulsion are novel reaction media for the hydrolytic and organic reaction [12]. Investigation in microemulsion structure and property by the conductivity measurement [13].

Hydroxamate ions (I) are α-effective nucleophiles whose reactivity are higher than that predicted by relationship between nucleophilicity and basicity [14]. These are effective deacylating and dephosphorylating agent [15]. Nucleophilic dephosphorylation of paraoxon and other carboxylate ester by the hydroxamate ions in cationic micellar media [16]. The nucliophilic substitution reaction of carboxylate and phosphate esters with hydroxamate ions in micro-emulsions all ready discussed on [17].

In the present work we report the nucleophilic hydrolysis of p-nitrophenyl acetate (PNPA) by Acetohydroxamic acid in Triton x-100, n-butanol, n-hexane cationic microemulsion media (Fig 1).

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Kinetic Study of The Hydrolysis of Carboxylate Ester with Acetohydroxamic Acid in Non-Ionic Microemulsion System (IJIRST/ Volume 1 / Issue 6 / 001)

II. EXPERIMENT The non-ionic surfactant Triton x-100, substrate p-nitrophenyl acetate, Acetolhydroxamic acid was purchased from sigma Aldrichals. The medium chain alcohol n-butanol (merk), were used as a co-surfactant. Aliphatic oil such as n-hexane (merk) was employed. All the microemulsions were prepared by the titration of initial composition of n-hexane (oil), surfactant & cosurfactant with aqueous buffer, for all kinetic run concentration of Acetohydroxamic acid were mentioned 1.0×10 -4 and 1.0×10-3 M respectively. Buffer employed was phosphate (pH 7.7) the pKa of Acetohydroxamic acid were determined by pH metrically using systronic (Type-335) pH-meter.

A. Preparation of Microemulsions Microemulsions were prepared by mixing the appropriate composition and stirred vigorously until clear. The basic composition of microemulsion depends on the percentage composition of water, surfactant, co-surfactant and oil. All the microemulsion solution based on different W0 value (Table-1).

ME W0 Water(%) Surfactant(%) n-Butanol(%) n-Hexane(%) 1 5 74 20 4.5 1.5 2 6 79 15 4.5 1.5 3 7 84 10 4.5 1.5 4 8 90 5 4.5 1.5 5 9 91 4 4.5 1.5 6 10 92 3 4.5 1.5 7 11.6 93 2 4.5 1.5 8 12.6 94 1 4.5 1.5 Table – 1. Composition by weight % of non-ionic microemulsions. For all microemulsion pH maintained as per-reaction condition required using 0.066 phosphate buffers. ME 1-8 were prepared with Triton x-100 in same composition.

B. Conductivity measurement The electrical conductivity of microemulsions was measured using a dcm-200 conductivity meter equipped with CD-06 platinum conductance electrode coated with platinum black. The conductivity was measured to 27±0.1°c.

C. Kinetic measurement All the kinetic reactions were followed at 27±°C with UV-300 spectrophotometer. The rate of nucleophilic reaction with PNPA were determined by following the increasing in the absorption of p-nitrophenoxide anion (400nm) for all the kinetic run the absorption time result fit very well to the first order rate equation.

In (A∞ - A0) = In (A∞ - At) - kt III.

RESULT & DISCUSSION

A. Effect of Conductivity Conductivity is also another important indication of structural variation and property of microemulsions. We have also measured the electric conductivity of several micro-emulsions. The increase in microemulsion conductivity observed when the water content is raised above the percolation threshold may result from a progressive aqueous droplets interlinking and clustering process. The conductivity change Table-2 depends on water percentage and surfactant. The conductivity data indicate the increasing the graph in Fig.2 with increasing the water concentration, showing the higher conductivity with high concentration of water. The conductivity depends on the total volume fraction of aqueous phase and its composition .

ME W0 Triton x-100 1 5 4.4216 2 6 4.8291 3 7 5.4161 4 8 5.6121 5 9 5.9142 6 10 6.1211 7 11.6 6.5232 8 12.6 7.4128 Table - 2 Conductivity (mScm-1) measurements of microemulsions

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8.5 8 7.5 7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

triton x100

5 6 7 8 9 10 11.6 12.6

Conducitivity, mScm-1

Kinetic Study of The Hydrolysis of Carboxylate Ester with Acetohydroxamic Acid in Non-Ionic Microemulsion System (IJIRST/ Volume 1 / Issue 6 / 001)

Water Concentration (W0) Fig. 2. Conductivity variation with the increase in chain length of measured at 27± 0.1°C. the linear oil phase employed.

B. Effect of Water Content The effect of water content (W0) of the microemulsion system and the nucleophilic substitution is one of the parameter that have and it is reported that altering the molar ratio of water to surfactant (W 0). The rate of nucleophilic reaction in oil-in-water microemulsion strongly depends on the water pool size, which is proportional to the ratio of the water and the surfactant concentration, W0 = H2O/ Triton x-100. The water content W0 are presented in Table-1 the hydrolysis of PNPA with AHA of surfactant/n-butanol/n-hexane was investigated. As present Table-3 the rate constant kobs increase with W0 from ME1 to ME8.

The observed super-activity becomes high, especially for high W 0 value. It is deduced form these W0 dependences that the reaction occurs in the water pool of the ME. It is suggested that rate of PNPA in ME depends on W 0, i.e. the size of the water pool. On the other hand the hydrophilic substrate dissolve in the water pool and reacts with nucleophile in the water pool.

C. Rate of Reaction An increased degree of hydrolysis was noted for the reactions of p-nitrophenyl acetate with the Acetohydroxamic acid in microemulsion are thermodynamically stable, optically transparent dispersion of oil and water, stabilized by an interfacial film of surfactant. Micro-emulsion has the ability to stabilize and disperse the maximum amount of substrate, enhancing the possibility of hydrolysis by the Acetohydroxamic acid. These features arise from the fact that each of the components of reaction (water, hydrocarbon, surfactant, co-surfactant, substrate and nucleophile) has its own residence site and its own role in the reaction. This kind of self-assembly inherent to microemulsions, result in the formation of an organized system that help in assisting hydrolysis.

Synergistic effects of the nucleophilic catalysis and microemulsions are important in bringing about effective hydrolysis of the carboxylate esters.

Microemulsions were prepared by mixing n-hexane, surfactant Triton x-100 and n-butanol (co-surfactant) and titrating the slurry with water, agitating mildly to give a clear solution (Table-1). The pH of microemulsion solution was determined by directly immersing a glass electrode in to microemulsion solutions. The pH of the microemulsion was the same as that of the aqueous solutions, which indicate that the pH of the water content is close to that of initial buffer system. Buffer used was 0.066M phosphate buffer. Table-1 records a few of the microemulsion formulations that we prepared. All the microemulsions contained large weight percentage of water and therefore, regarded as O/W microemulsions in which micro-droplets of n-hexane (oil) are dispersed in aqueous buffer. Presence of a non-ionic surfactant Triton x-100 and co-surfactant (n-butyl alcohol), helps to stabilized these microemulsions recipes. Table-3 shows the kinetic rate data for the nucleophilic reactivity of the PNPA with Acetohydroxamic acid. All reaction were carried out under the reaction conditions, [PNPA]=1.0×10 -4M, [AHA]= 1.0×10-3M, pH 7.7 at 27°C. One of our aims was to treat reaction of anionic nucleophiles in microemulsions, using kinetic models which have been applied to bimolecular ionic reaction in micelle. The variation of the rate constants for nucleophilic attack by the Acetohydroxamic acid in microemulsions, based on quaternary ammonium chloride surfactants, can be accounted for in terms of the concentration of Acetohydroxamic acid in the microemulsion droplets.

Rate Constant Triton x-100 103kobs(s-1) K2(M-1 S-1) 1 5 1.00 1.95 2 6 1.15 3.01 3 7 1.45 3.65 4 8 1.76 4.05 5 9 2.11 5.71 6 10 2.31 6.04 7 11.6 2.40 7.63 8 12.6 2.69 7.91 Table - 3 Summary of the kinetic rate data for the nucleophilic reactions of PNPA with Acitohydroxamic acid in O/W non-ionic microemulsion media at 27 °C. [PNPA] = 1.0× 10-4M, [AHA] = 1.0 × 10-3M, pH 7.7.

ME

W0

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Kinetic Study of The Hydrolysis of Carboxylate Ester with Acetohydroxamic Acid in Non-Ionic Microemulsion System (IJIRST/ Volume 1 / Issue 6 / 001)

The first order rate constant for the reaction of PNPA have been measured spectrophotometrically under pseudo-first-order conditions at 27°C, with the nucleophile in large excess over the substrates in each cases. The overall first-order rate given by kobs = k0obs + kNu [Nu-] (1) k0obs = kH2O + kOH- [OH-] (2) Since the hydroxamate ion are α-effect nucleophiles [18], so that competition with other nucleophile i.e. OH- and H2O is not expected and kobs is simply given by kobs = kNu- [Nu-] (3) Catalysis by the hydroxamate ion is dependent upon the ionization state of Acetohydroxamic acid, Eq. (4) may be written to describe kobs kobs = k2 [AHA]TαAHA- (4) Where k2 is second order rate constant, [AHA]T the analytical concentration of Acetohydroxamic acid and α AHA- is the fraction of hydroxamate ion ionized (αAHA-= Ka/Ka + H+). The second order rate constants determined in the way are summarized in Table-1with pseudo first order rate constants.

Kinetic rate data for the reaction of PNPA with AHA shows that the pseudo-first-order rate constant increasing with increasing water contents. On going from ME 1 (1.00 × 10 3 s-1) to ME 8 (2.69 × 103 s-1), the rate of reaction shows 2.69 time rate enhancement for the Triton x-100-AHA microemulsion system. O/W microemulsions are regarded as swollen micelle and reactivity in micelle and microemulsions are quite similar. During the nucleophilic reactivity in the non-ionic microemulsion the substrate molecules is located into hydrophobic droplet and anionic nucleophiles reside in water content. This fact is support by the decrease in rate of reaction of PNPA with AHA with decreasing water content. Schematic representation of an idealized microemulsion can be given as; water and nucleophile defined to the aqueous pseudo-phase, n-hexane to the oil pseudo-phase and non-ionic surfactants (Triton x-100) to the inter-phase, n-butanol and substrate are distributed between the inter-phase and the oil pseudo-phase. Second-order rate constant, k2, were calculated for the nucleophilic reaction of PNPA with Acetohydroxamic acid, based on the assumption that reaction occurs only in inter-phase and thus involve Acetohydroxamic acid, bound to CTA+ electro statically. Thus k2 is defined by Eq. (4), where [AHA]TαAHA-is actual concentration of hydroxamate ion in the inter-phase, and can be written as K2 = kobs/[AHA-] (5) Nucleophilic reaction of anionic nucleophiles in the non-ionic micelle/microemulsion is governed by the basicity and strength of incorporation of nucleophiles in to the micelle/microemulsion.

IV. CONCLUSSION In the present study the effect of the oil phase, the surfactant and the co-surfactant on the nucleophile hydrolysis of PNPA by hydroxamate ion are investigated. The electric conductivity and ion-transport behaviour of O/W microemulsion are determined by the weight percentage and composition of aqueous phase. Stable microemulsions behave as a well defined electrolytic medium for redox electrochemistry. The electron transfer property as measured by the peak separation value are measured by the limiting peak current value are found to exhibit very little variations in all the microemulsions investigated except one system.

This is indeed true whether we employ a predominantly water soluble reactant or an oil soluble reactant. This observation is true only for simple redox reactions. The medium effects may be significant for coupled chemical processes involving electrogenerated cat-ion or anion radicals. Such questions definitely deserve further investigation. The nucleophilic hydrolysis of PNPA by hydroxamate ion was determined as a function of water contents (W 0) and various concentration of organic solvents in the surfactant (Triton x-100)/ n-butanol/n-hexane ME system. Among all the ME system the water content are most important factor that determine the reaction rate. During the nucleophillic reactivity in the non-ionic microemulsion the substrate molecule located in to hydrophobic droplet and anionic nucleophile reside in water content. This fact support to the actual result of kinetic study by the decrease in rate of reaction of PNPA with AHA with decreasing water content. In the current study on the base of result we concluded that the value of first and second order rate constant increase as the surfactant concentration decrease in all the ME system. This work is very important for elucidating the kinetics and mechanism of the PNPA with AHA in microemulsions.

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