alexa Ultrasound Assisted Synthesis of Diethyl-2,2’-Thiodiacetate with 2-Bromoethylacetate Under a New Polymer-Supported Phase-Transfer Catalyst in Solid-Liquid Condition | Open Access Journals
ISSN: 2150-3494
Chemical Sciences Journal
Make the best use of Scientific Research and information from our 700+ peer reviewed, Open Access Journals that operates with the help of 50,000+ Editorial Board Members and esteemed reviewers and 1000+ Scientific associations in Medical, Clinical, Pharmaceutical, Engineering, Technology and Management Fields.
Meet Inspiring Speakers and Experts at our 3000+ Global Conferenceseries Events with over 600+ Conferences, 1200+ Symposiums and 1200+ Workshops on
Medical, Pharma, Engineering, Science, Technology and Business

Ultrasound Assisted Synthesis of Diethyl-2,2’-Thiodiacetate with 2-Bromoethylacetate Under a New Polymer-Supported Phase-Transfer Catalyst in Solid-Liquid Condition

Rajendran V1* and Harikumar K2

1Head of The Department of Chemistry, Pachaiyappa’s College for Men, Kanchipuram, TamilNadu, India-631 501

2Sri Chandrashekarendra Saraswathi Viswa Mahavidyalaya, Kanchipuram, Enathur, Tamil Nadu, India-631 561

*Corresponding Author:

Rajendran V
Head of The Department of Chemistry
Pachaiyappa’s College for Men
Kanchipuram, TamilNadu, India
Fax no: +91-44-27268824
Email: [email protected]

Received date March 19, 2015; Accepted date March 20, 2015; Published date March 26, 2015

Citation: Rajendran V, Harikumar K (2015) Ultrasound Assisted Synthesis of Diethyl-2,2’-Thiodiacetate with 2-Bromoethylacetate Under a New Polymer- Supported Phase-Transfer Catalyst in Solid-Liquid Condition. Chem Sci J 6:094. doi: 10.4172/2150-3494.100094

Copyright: ©2015 Rajendran V, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Visit for more related articles at Chemical Sciences Journal

Abstract

In this work, a new polystyrene-bound single-onium phase-transfer catalyst was synthesized, and their catalytic activities were investigated in the synthesis of diethyl-2,2’-thio diacetate derivatives of thioether. The new insoluble polymer anchored mono-site phase-transfer catalyst showed significant high catalytic activity as compared to soluble single site phase-transfer catalysts. The comparative kinetic investigation reveals that the diethyl-2,2’-thio diacetate of ethyl-2-bromoacetate is faster than sodium sulfide in the presence of the new catalyst. This catalyst can be used several times with consistent catalytic activity. Experimental observations support an interfacial-type mechanism.

Keywords

Phase-transfer catalysis; Polymer-supported phasetransfer catalyst; Ultrasound irradiation; Ethyl-2-bromoacetate; Solidliquid reaction; Kinetics.

Introduction

Currently, phase-transfer catalysis (PTC) is a key green approach [1] for driving reactions involving immiscible reactants [2-6]. However, application of this methodology to organic reactions has encountered two major problems viz.: (i) the recovery and reuse of the catalyst and (ii) the separation and purification of the final product [7,8]. To circumvent the predicament of separation of catalyst from the reaction mixture, for the first time Regen [9] reported anchoring the phase-transfer catalysts to a polymer backbone and suggested the name “Triphase Catalysis”. Organic reactions catalyzed by triphase catalysts [10-13] have created remarkable attention for its application in synthesis of organic compounds. The vital considerations in the selection of the catalyst are economy of scale and efficiency of the PTC, particularly on the explicit industrial scale preparation of organic compounds. To cater these requirements, multi-site phase-transfer catalysts (MPTC) have been developed and revealed more efficient in activity than conventional single-site phase-transfer catalysts [14-18], and these catalysts are successfully employed for the generation of dihalocyclopropanes [19], which are generally prepared traditionally with lot of predicaments [20]. Synthesis of polymer-supported ‘multisite’ phase-transfer catalysts (PS-MPTC) of the ammonium type have not been explored much in contrast to the soluble ‘multi-site’ PTCs. However, the demand and requirement of these triphase catalysts are on the rise due to their popularity.

Thioethers, which have been widely employed as perfume additives [21] and inverse phase-transfer catalysts [22,23] are generally synthesized in a homogeneous reaction [24]. Phase-transfer catalysis (PTC) is well-recognized as a invaluable methodology in organic synthesis in recent years [25-27]. The advantages of the PTC method for synthesizing thioethers are increased reaction rate and selectivity, hydrophilic conditions and low energy requirement. Searching for a more effective condition to enhance the reaction or to elevate the conversion is the primary purpose in phase-transfer catalysis in combined with ultrasonic waves [28-33]. Inventing selective, efficient and eco-friendly methods for applications in complex organic synthetic manipulations constitutes a major chemical research effort. In this regard, several non-conventional methods are emerging that involve reactions in aqueous media [34,35] or those that are accelerated by exposure to microwave [36-38] or ultrasound [39-44] irradiation. These methods are now recognized as viable environmentally benign alternatives. Although, sonication methods have been initially applied to homogeneous reactions in a variety of solvents, this approach has now evolved into a useful technique in heterogeneous reactions. A vast majority of sonochemical applications in the synthesis deal with reactions involving metals [43-52] organic phase insoluble reagents, or their aqueous solutions [53-76]. Our interest was centered on first time evaluating the influence of ultrasound in association with phasetransfer catalyst on the rate of alkyl thioether formation. This work investigates the substitution of alkyl bromides (RBr) to sodium sulfide (Na2S), including linear and branched alkyl groups and the application of phase-transfer catalysts in combination with ultrasound obviously provides a more efficient synthetic approach for the preparation of thioethers. These PSPTC reactions were carried out in a soiled-liquid two-phase medium. In the absence of a phase-transfer catalyst and ultrasound, less than 5% conversion was detected even after 4 h of reaction. In contrast high yields of products were obtained in shorter reaction time using 4 mol% (based on the amount of alkyl bromide) of the phase-transfer catalyst, polymer-supported benzyl N-ethyl-N-di-isopropyl ammonium chloride (PSPTC) and ultrasound 40 kHz (300 W) conditions. Kinetics of the substitution of ethyl-2-bromoacetate to sodium sulfide, including the effect of amount of catalyst, agitation speed, quaternary ammonium salts, amount of sodium hydroxide, organic solvents, temperature and ultrasound frequency on the conversion were investigated in detail.

With all these antecedents, we decided to synthesize a new monoactive site onium salt with two active sites viz., polymersupported benzyl-N-ethyl-N-di-isopropyl ammonium chloride (PSPTC) by a facile method (Scheme 1).We investigated the catalytic efficiency of the new catalyst by following the comparative kinetics of thioether addition to sodium sulfide and ethyl-2-bromoacetate (EBA) under pseudo-first-order conditions (Scheme 2). Also, to the best of our knowledge, there is no systematic kinetic study of thioetherfication addition to these sodium sulfide involving polymer-supported monosite phase transfer catalysts. Kinetic investigation has been carried out by varying experimental parameters and from the obtained kinetic results, a plausible reaction mechanism has been proposed. Comparative catalytic efficiency of new onium salt and other onium salts (single-active site) were evaluated. (Schemes 3 and 4) shows the images of insoluble polymer-supported benzyl chloride and polymeric insoluble benzylethyldiisopropyl ammonium chloride (PSPTC).

chemical-sciences-journal-Synthesis-polymeric

Scheme 1: Synthesis of polymeric benzylethyldiisopropylammonium chloride.

chemical-sciences-journal-Preparation-diethyl

Scheme 2: Preparation of diethyl-2,2’-thidiacetate.

chemical-sciences-journal-polymer-supported

Scheme 3: SEM image of polymer-supported benzyl chloride.

chemical-sciences-journal-phase-transfer

Scheme 4: SEM image of polymer supported phase-transfer catalyst (PSPTC).

Experimental

Chemicals

All the reagents, including, sodium sulfide (Merck), ethyl-2- bromoacetate (Merck), polymer-supported benzyl chloride (Aldrich), biphenyl (Aldrich), tetrabutylammonium chloride (TBAC) (CDH), tetrabutylammonium bromide (TBAB) (CDH), benzyltriethylammonium chloride (BTEAC) (CDH), tetraethylammonium chloride (TEAC) (CDH), tetraethylammonium bromide (TEAB) (CDH), potassium hydroxide (CDH), potassium carbonate (CDH) , n-hexane (CDH), toluene (CDH), chlorobenzene (CDH), anisole (CDH), diethyl ether (CDH) and other reagents for synthesis were guaranteed grade (GR) chemicals and were used without further purification.

Instrumentation

1H NMR and 13C spectra were recorded on a Bruker 300 MHz and 75 MHz respective using TMS as an internal standard. Gas chromatography was carried out using a GC-Varian 3700 model. Ultrasonic water bath, Equitron, Media Instrument Manufacturing Company, Chennai, India-600004. The ultrasonic generator was a thermostatic bath equipped with dual frequencies (28/40 kHz) and electric power 300 W with 0.0126 W/mL of power density.

Ultrasonic process equipment

Ultrasonic energy is transmitted to the process vessel through the liquid medium, usually water in the tank. For safety purpose, the sonochemical reactor consisted of two layers stainless steel body. The sonochemical reactor configuration used in the present work is basically an ultrasonic bath. The internal dimension of the ultrasonic cleaner tank is 48 cm × 28 cm × 20 cm with liquid holding capacity of 5 liters. Two types of frequencies of ultrasound were used in these experiments, which are 28 kHz and 40 kHz with each output as 300 W. Both ultrasounds separately produces through a flat transducer mounted at the bottom of the sonicator. The reactor was a 250 mL three-necked Pyrex round-bottom flask. This reaction vessel was supported at the centre of the ultrasonic cleaning bath 2 cm above from the position of the transducer to get the maximum ultrasound energy. All the experimental parameters were done at 40 kHz with output power of 300 W.

Synthesis of a New PSPTC

Preparation of a new polymer-supported benzyl N-ethyl-Ndi- isopropyl ammonium chloride (PSPTC)

In the polymer-supported benzyl chloride (2 g) was swelled in acetonitrile (150 mL) for about 24 h and transferred into a 500 mL three- necked round bottomed flask with excess diisopropylethylamine (20 mL). The reaction mixture was stirred continuously using a mechanical stirrer equipped with a poly (tetrafluoroethylene) (PTFE) half-moon blade agitating at 600 rpm under a nitrogen atmosphere. The reaction was carried out at reflux condition for 50 h. The solvent was then completely removed under vacuo and the onium salt viz., polymer-supported benzyl N-ethyl- N-di-isopropyl ammonium chloride (PSPTC) was washed with diethyl ether, methanol and acetone (Scheme 2) was stored in a CaCl2 desiccator. The extent of quaternization, the amount of chloride ion present in the PSPTC was quantitatively estimated by Volhard’s method and found to be 1.1 mequiv-1.

FT-IR (KBr), cm-1: 1152(C–N+), 1443(C–C), 1605(aromatic C=C), 2931(alkyl stretching).

Synthesis of Diethyl-2,2’-Thiodiacetate under Mechanical Stirring

Synthesis of diethyl-2,2’-thiodiacetate under mechanical stirring. To the mixture of Na2S (6 g, 77 mmol) in water (10 mL) and the newly synthesized PSPTC (1.0 g), was added under overhead stirring to generate the sulphur anion. Then ethyl-2-bromo acetate (1.0 g, 6 mmol) in chlorobenzene (40 mL) was added slowly. The reaction mixture was heated at 60°C for 4 h with vigorous stirring. The crude product was isolated by simple extraction with diethyl ether (3 × 25 mL). The organic layer was collected and the solvent was evaporated under reduced pressure. The crude product was chromatographic (SiO2) employing hexane:ethyl acetate (9:1) as an eluent to obtain a pure monoderivative. The identity of the product was confirmed by 1H NMR and 13C NMR spectra of the product. m.pt. 306–308 -C; Yield: 88%; 1H NMR(400 MHz, CDCl3): d . 0.96–0.98 (t, 3H, CH3), 5.34–1.33–1.45 (m, 2H, CH2), 1.71–1.86 (m, 2H, CH2), 3.90–4.01 (t, 3H, CH3), 7.13, 8.18 (m, 4H, Ar–CH) 13C NMR (100 MHz, CDCl3): d. 14.12 (CH3),19.16 (CH3), 32.01 (CH2), 68.67 (CH2), 115.31, 121.72, 140.05,163.64 (Ar–CH).

Reaction Mechanism and Kinetic Model

The reaction was conducted in a 250 mL three-necked Pyrex round-bottom flask which permits agitating the solution, inserting the water condenser to recover organic reactant and taking samples and feeding the reactants. This reaction vessel was supported at the centre of the sonicator. Known quantities of chlorobenzene (30 mL, solvent), sodium sulfide powder (6 g in 10 mL water) and 0.2 g biphenyl (IS-internal standard) were introduced into the reactor. Then, 1.0 g of ethyl-2-bromo acetate (6 mmol) and 1.0 g of the newly synthesized PSPTC were introduced to the reactor to start the reaction. The reaction mixture was stirred at 300 rpm. The phase separation was almost immediate on arresting the stirring process. Samples were collected from the organic layer of the mixture (by stopping the stirring for 20-30 seconds each time) at regular time intervals. A pinch of anhydrous CaCl2 was placed in the sample vials to absorb any moisture present in the organic layer. Each run consisted of six samples taken over the period ranging from 5 to 30 minutes. The kinetics was followed by estimating the amount of ethyl-2-bromoacetate that disappeared and measured by a gas Chromatography (GC-Varian 3700 model). The analyzing conditions were as follows; Column, 30 m × 0.525 mm i.d. capillary column containing 100% poly(dimethyl siloxanen); injection temperature, 250°C; FID detector (300°C). Yields were determined from standard curve using biphenyl as an internal standard.

Definition

The conversion (X) of ethyl-2-bromo acetate (EBA) is defined as follows:

X = 1-{[EBA]0/[EBA]0,t}    (1)

Where [EBA]0 and [EBA]0,t EBA represent the concentration of ethyl-2- bromo acetate in the organic phase at a given time (t) t=0 and t>0, respectively.

Rate expression

The rate expression for this reaction may be expressed as;

Where [EBA]0,t is concentration of ethyl-2-bromo acetate in the organic phase and kapp is the apparent reaction rate constant. This reaction is carried out in a batch reactor, so the diminution rate of EBA with time (t) can be expressed as:

On integrating the Eq. (3) yields:

-ln(1-X) = kappxt

Using Eq. (4), we can get the kapp value experimentally by plotting –ln (1-X) against time, (t).

Results and Discussion

Effects of ultrasound and mechanical stirring on the reaction

To ascertain the influence of agitation speed on the rate of thioeitherfication of sodium sulfide, the speed of agitation was varied in the range of 0-500 rpm along with ultrasound irradiation (40 kHz, 300 W) using polymer-supported benzyl N-ethyl-N-di-isopropyl ammonium chloride (PSPTC) In principle, the homogeneous reaction is independent of the agitation speed. As shown in (Figure 1), that the rate of the reaction increases linearly as the agitation speeds increases from 0 to 300 rpm. Macroscopically, this result indicates that the reaction is carried out in a solid-liquid phase. In fact, the reaction proceeds by accompanying with the dissolving Na2S in chlorobenzene. The dissolving rate of Na2S in chlorobenzene is highly influenced by the agitation speed. In general, a high concentration of Na2S dissolving in organic solvent is obtained at a high agitation speed. Therefore, the conversion of ethyl-2-bromo acetate was increased with the increase in the agitation speed up to 300 rpm. For agitation speeds higher than 300 rpm, the conversion is influenced almost not at all by the agitation speed. The purpose of stirring is to provide a well mixing to dissolve Na2S in the chlorobenzene in the ethyl-2-bromo acetate presence of PSPTC to form Q2S. Then, the dissolved Q2S reacted with to produce ethyl-2-bromoacetate. Therefore, the agitation speed was set at 300 rpm for studying the reaction phenomena from which the resistance of mass transfer stays at a constant value [27-35]. The kapp values indicate that the mechanical effects brought up by the use of low frequency ultrasounds are responsible of the enhancement of the kinetics by harsh mixing, enhancement of mass transfer and in solid-liquid systems, high erosion of the solid particles occurs and consequently the surface area is increased. So on, further when the same reaction was carried out in conventional method, the observed kapp value (kapp=6.2 x 10-3, min-1) almost four fold lesser than in the presence of ultrasonication (40 kHz, 300 W, 300 rpm: kapp=24.63 x 10-3, min-1).

chemical-sciences-journal-Effect-ultrasound

Figure 1: Effect of ultrasound and mechanical stirring speeds on the synthesis of diethyl-2,2’-thiodiacetate: Na2S (9.25 mmol), ethylbromoacetate (6.35 mmol), 1.0 g PSPTC, 0.2 g biphenyl (internal standard), chlorobenzene 30 mL, 60°C, 40 kHz, 300 W.

Effect of the amount of newly prepared PSPTC

The experiments were conducted by varying the amount of the newly synthesized polymer-supported benzyl-N-ethyl- N-diisopropylammonium chloride (PSPTC) by keeping other experimental parameters are kept constant. The influence of the amount of PSPTC has been studied by varying the amount of PSPTC from 0.5 g to 2.5 g with respect to ethyl-2-bromo acetate (EBA) under ultrasound irradiation (40 kHz, 300 W). As shown in (Table 1) and (Figure 2), the rate of conversion is increased with increasing in the amount of PSPTC along with ultrasound irradiation (40 kHz, 300 W). The conversion about 75% in the higher concentration of polymer-supported monosite phase-transfer catalyst in 30 minutes of the reaction. The increase in the kapp value is attributed to the synergic effect of ultrasound might be enlarged [36]. The small amount of water can efficiently promote the solubilization of the solid reactant and thus enhance the formation of catalytic intermediate (Q2S). All the further experiments were done at 1.0 g of catalyst amount.

chemical-sciences-journal-amount-PSPTC

Figure 2: Effect of the amount of PSPTC on the synthesis of diethyl-2,2’- thiodiacetate: Na2S (9.25 mmol), ethylbromoacetate (6.35 mmol), 0.2 g biphenyl (internal standard), chlorobenzene 30 mL, 60°C, 300 rpm, 40 kHz, 300 W.

PSPTC (gm) kapp× 103 min-1    
(40 kHz, 300W)
0.5 12.24
1.0 24.63
1.5 29.02
2.0 41.68
2.5 47.82

Table 1: Effect of the amount of PSPTC on the synthesis of diethyl-2,2’- thiodiacetate: Na2S (9.25 mmol), ethylbromoacetate (6.35 mmol), 0.2 g biphenyl (internal Standard), chlorobenzene 30 mL, 60°C, 300 rpm, 40 kHz, 300 W.

Effect of the amount of ethyl-2-bromoacetate

To investigate the influence of ethyl-2-bromoacetate (EBA) on the kinetics of synthesis of ethyl-2-bromoacetate under ultrasonic irradiation condition (40 kHz, 300 W), the amount of EBA was varied from 0.25 g to 2.0 g. The results are shown in (Table 2). The data clearly indicates that the kapp value increases with increasing the amount of EBA. When the ethyl-2-bromoacetateconcentrations increased, the probability of finding the substrate with active-site of the catalyst and ultrasound enhanced the rate of the reaction [36,37]. It may be due to reduces the surface area between the solid and organic phases, and hence more reactants collide to each other simultaneously we get higher kapp value.

Ethyl-2-bromoacetate (EBA), g kapp×103 min-1    
  (40 kHz, 300W)
0.25 13.23
0.5 19.52
1.0 24.63
1.5 28.30
2.0 31.78

Table 2: Effect of the amount of ethyl-2-bromoacetate (EBA) on the rate of synthesis of diethyl-2,2’-thiodiacetate : Na2S (9.25 mmol), ethylbromoacetate (6.35 mmol), 0.2 g biphenyl (internal Standard), chlorobenzene 30 mL, 60°C, PSPTC 1 g, 300 rpm, 40 kHz, 300 W.

Effect of temperature

The effect of temperature on the reaction between sodium sulfide and ethyl-2-bromoacetate was studied under otherwise similar conditions. The temperature was varied from 40 to 80°C (Figure 3). The kinetic profile of the reaction is obtained by plotting -ln(1-X) versus time. It is obvious that the reactivity is increased with an increase in the temperature along with ultrasonic effect [38]. The reason is that the number of reactant molecules which possess higher activation energy at a higher temperature and thus the ultrasonic wave easily passes through the reactor [39,40]. The other point is that the collision of the reactants at higher temperature is also increased. Hence, the apparent rate constant is increased at higher temperature. Arrhenius plots were made in (Figure 4) of –lnkapp against 1/T to get activation energy of 53.32 kJ mol-1. From the literature survey, the dehydrobromination of (2-bromoethyl) benzene catalyzed by tetraoctylammonium bromide (TOAB), an extraction mechanism was proposed [41] due to lower Ea value (<43 kJ mol-1). In general, higher activation energy (more than 43 kJ mol-1) suggests an interfacial mechanism [37,42]. The activation energy for the heterogeneous ethylation of phenylactonitrile was reported to be 63.64 kJ mol-1 and for this an interfacial mechanism was proposed [43,44]. Further they concluded that the deprotonation of the substrate takes place at the inter-phase and consequently the organic anion is extracted and reacts in the bulk organic phase. The rate-determining step in the process is the anion exchange at the interphase. In our study, the observed Ea value is 53.32 kJ mol-1 (64.66 kJ mol-1 under silent condition). Hence, we proposed an interfacial mechanism for our present study [37,46].

chemical-sciences-journal-synthesis-diethyl

Figure 3: Effect of temperature on the synthesis of diethyl-2,2’-thiodiacetate: Na2S (9.25 mmol), ethylbromoacetate (6.35 mmol), 0.2 g biphenyl (internal standard), chlorobenzene 30 mL, 60°C, PSPTC 1 g, 300 rpm, 40 kHz, 300 W.

chemical-sciences-journal-Arrhenius-plot-synthesis

Figure 4: Arrhenius plot on the synthesis of diethyl-2,2’-thiodiacetate: Na2S (9.25 mmol), ethylbromoacetate (6.35 mmol), 0.2 g biphenyl (internal standard), chlorobenzene 30 mL, 60°C, PSPTC 1 g, 300 rpm, 40 kHz, 300 W.

Effect of ultrasonic frequency

Ultrasonic irradiation is defines as acoustic waves with frequencies in the 20 kHz-100 MHz range [47]. They create cavities generating locally high temperature and pressures [48-51] or strong electric fields [49,51-53]. Ultrasound is known to accelerate diverse types of organic reactions and it is established generous reactions, which are otherwise slow due to poor mass transfer are accelerated by sonication due to cavitations [50]. It has been reported that a combination of PTC and ultrasound is often better than either of the two techniques alone [51]. In such transfer of species across the interface and ultrasound merely facilitates this transfer, possibly by increasing the interfacial area across which this transfer occurs.

To ascertain the influence of various ultrasonic frequencies on the rate of the reaction with same output power of 300 W, the ultrasonic frequency was varied between 28 and 40 kHz under otherwise similar conditions using PSPTC as the catalyst and also we followed the reaction under silent condition (0 kHz). The kinetic profile of the reaction is obtained by plotting -ln(1-X) against time. In our experimental condition at 30 minutes, without ultrasonic irradiation (silent condition) the kapp values is 6.20 x 10-3, min-1 but in the presence of ultrasonic condition the kapp values are 17.51 x 10-3 min-1 and 24.63 × 10-3 min-1 for 28 kHz (300 W) and 40 kHz (300 W), respectively (Table 3). Hence, the overall kapp was increased by increasing the ultrasonic frequency in the order of 0 kHz (silent condition) <28 kHz (300W) <40 kHz (300W) for our system (Table 2). Similar trend was observed by Wang et al. [54,55].

Ultrasonic frequency (kHz) kapp×103 min-1
0 (Silent) 6.20
28 17.51
40 24.63

Table 3: Influence of the ultrasonic frequencies on the rate of synthesis of diethyl- 2,2’-thiodiacetate: Na2S (9.25 mmol), ethylbromoacetate (6.35 mmol), 0.2 g biphenyl (internal Standard), chlorobenzene 30 mL, 60°C, PSPTC 1 g, 300 rpm.

Effect of organic solvents

The influence of various organic solvents on the rate of thioeitherfication of sodium sulfide was followed under otherwise standard reaction conditions. Five organic solvents employed in this study are toluene, anisole, cyclohexane, chlorobenzene, and n-hexane. From the plot of -ln(1-X) against time, the kapp values are obtained. From the (Table 4), chlorobenzene possesses a higher kapp value among the five organic solvents, due to its higher dielectric constant. In another view the ultrasonic irradiation can enhance the rate in the presence of more polar solvents due to passing higher ultrasonic waves to the reactor and makes fruitful collision between the reactants, and hence we get higher kapp value for chlorobenzene solvent of this system and also this statement is not always true [56-62].

Solvents Dielectric constant (ε) kapp×103 min-1
(40 kHz, 300W)
Chlorobenzene 5.6 24.63
Anisole 4.3 21.76
Toluene 2.4 15.24
Hexane 2.2 11.52
Cyclohexane 2.0   9.68

Table 4: Influence of organic solvents on the rate of the synthesis of diethyl-2,2’- thiodiacetate: Na2S (9.25 mmol), ethylbromoacetate (6.35 mmol), 0.2 g biphenyl (internal Standard), PSPTC 1g, 60°C, PSPTC 1 g, 300 rpm, 40 kHz, 300 W.

Effect of volume of chlorobenzene

In a homogeneous reaction, the reaction follows the intrinsic kinetic law. The conversion or the reaction rate is directly proportional to the concentration of the reactants in 30 min of reaction. A dilute concentration of the reactant is obtained using a large amount of organic solvent. The conversion of ethyl-2-bromoacetate is increased with the decrease in the volume of chlorobenzene (Table 5). (Figure 5) shows the dependence of the % conversion on the volume of chlorobenzene. The kapp value is inversely proportional to the volume of chlorobenzene, as expected.

PTC (5 mol% with respect ethyl-2-bromoacetate) kapp×103 min-1 (40 kHz, 300W)
TBAB 16.86
TBAC 16.31
BTEAC 15.58
TEAB 14.33
TEAC 13.56

Table 5: Effect of various PTC’s (5mol% based on ethylbromoacetate) on the rate of synthesis of diethyl-2,2’-thiodiacetate: Na2S (9.25 mmol), ethylbromoacetate (6.35 mmol), 0.2 g biphenyl (internal Standard), chlorobenzene 30 mL, 60°C, PSPTC 1 g, 300 rpm, 40 kHz, 300 W.

chemical-sciences-journal-solvent-chlorobenzene

Figure 5: Effect of the volume of solvent chlorobenzene on the synthesis of diethyl-2,2’-thiodiacetate: Na2S (9.25 mmol), ethylbromoacetate (6.35 mmol), 0.2 g biphenyl (internal standard), 60°C, PSPTC 1 g, 300 rpm, 40 kHz, 300 W.

Effect of different phase-transfer catalysts

Comprehensive comparative kinetic studies for the thioeitherfication of sodium sulfide by ethyl-2-bromoacetate (EBA) were carried out using 5 mol% of various onium salts viz., polymer-supported benzyl-N-ethyl-N-diisopropylammonium chloride (PSPTC), tetraethylammonium chloride (TEAC), tetraethylammonium bromide (TEAB), tetrabutylammoniun chloride (TBAC), tetrabutylammonium bromide (TBAB), benzyltriethylammonium chloride (BTEAC), and benzyltriethylammonium bromide (BTEAB). The pseudo-first order rate constant are evaluated for all the catalyst from the plot of -ln(1-X) versus time and are presented in (Table 6) with ultrasonic condition. The order of the relativities of these quaternary ammonium salts are in the order TBAB>TBAC>BTEAC>TEAB>TEAC. It is thus concluded that the order of the reactivity is TEA cation>BTEA cation>TBA cation, which is consistent with the result [42] obtained by Wang and Rajendran combined with ultrasonic dichlorocarbene addition to 1,7-octadiene under PTC condition. The result indicates that the kapp value depends on organophilisity of the onium salts along with ultrasonic irradiation condition (Table 5) [36,63-68].

Volume of chlorobenzene (mL) kapp×103 min-1   (40 kHz, 300W)
30 24.63
40 19.36
50 15.02
60 12.22
70 10.56
80 9.76

Table 6: Effect of the volume of chlorobenzene on the rate of synthesis of diethyl- 2,2’-thiodiacetate: Na2S (9.25 mmol), ethylbromoacetate (6.35 mmol), 0.2 g biphenyl (internal Standard), 60°C, PSPTC 1 g, 300 rpm, 40 kHz, 300 W.

Effect of water

The critical effect of traces of water on solid-liquid phase transfer reactions has been studied in detail by several groups [4,69-77]. In solid-liquid system, the addition of small amount of water can be useful in forming the catalytic intermediate for conducting intrinsic reactions. The effect of water on this SLPTC was explored using under 300 rpm and ultrasonic irradiation (40 kHz, 300 W). (Figure 6) is the plot of conversion versus volume of water (mL) for different amounts of water added, showing that pseudo-first-order kinetic equation can be successfully used to describe the ultrasound assisted-SLPTC system. Without adding water, only 47% of product yield was obtained in 60 minute of reaction; but merely with 1mL of water in the system, the product yield largely increased to 85.3%. Continuing to increase the quantity of water, the reaction rate was increased for further 1mL and then gradually reduced. When the amount of water was added to 30 mL, the solid-liquid system became a liquid–liquid type, and the product yield greatly decreased to 30%, much lower than that in SLPTC. The increase of reaction rate with small amount of water was mainly due to water solubilizing a small part of solid reactant to increase the production of (Q+)2S2- in the inter-phase, thus enhancing the intrinsic reaction rate. When more water was added, the effective concentration of (Q+)2S2- in the inter-phase would be decreased and the reaction rate was reduced. In the absence of water, the reaction rate of solid reactant and PSPTC in the organic phase was slow, resulting in small product yield 49% after 30 min of reaction. With only 1 mL of water, the formation of (Q+)2S2- greatly increased and concentrated in the inter-phase and leads to the bulk organic phase where the intrinsic reaction take place.

chemical-sciences-journal-amount-water

Figure 6: Effect of the amount of water on the synthesis of diethyl-2,2’- thiodiacetate: Na2S (9.25 mmol), ethylbromoacetate (6.35 mmol), 0.2 g biphenyl (internal standard), 60°C, PSPTC 1 g, chlorobenzene 30 mL, 300 rpm, 40 kHz, 300 W.

Discussion of the Reaction between Sodium Sulfide and Ethyl-2-Bromoacetate

Known quantities of polymer-supported mono-site phase transfer catalyst (PSPTC) viz., polymer-supported benzyl-N-ethyl-Ndiisopropylammonium chloride (PSPTC) (1.0 g), ethyl 2-bromoacetate (1.0 g, 6 mmol) and 0.2 g biphenyl (internal standard in the analysis) in chlorobenzene solvent (30 mL) were added slowly to the mixture of well powdered sodium sulfide (6 g, 77 mmol) in 10 mL water and agitated (300 rpm) in a 250 mL three-necked Pyrex round – bottom batch reactor, which was immersed in a constant temperature (60°C) ultrasonicator (40 kHz, 300 W). During the reaction, 0.2 mL of the organic sample was withdrawn at the chosen time and diluted into 3 mL of chlorobenzene. The samples analyzed by a gas chromatography (GC-Varian 3700 model). The product viz., (ethyl-2-bromoacetate) was determined from standard curve using biphenyl as an internal standard. In the present solid-liquid system, the mechanism (Scheme 5) is illustrated by the reaction of organic substrate (ethyl-2-bromoacetate, EBA) with the active intermediate Q2S. Without an extra addition of water, the polymer-supported mono-site phase transfer catalyst (QX) diffuses to the surface of the solid particles of Na2S and converts it into the form of Q2S, which then dissolves in the bulk organic phase.

chemical-sciences-journal-Mechanism-formation

Scheme 5: Mechanism of the formation of diethyl-2,2’-thiodiacetate.

For the reaction of solid reactant Q2S and organic substrate, ethyl- 2-bromoacetate (EBA) catalyzed by the polymer-supported mono-site phase transfer catalyst (QX), the overall reaction is:

QX

Na2S (S)+EBA (org) → (CH3-CH2-COOCH2)2S(org)+NaCl(S)     (1)

Where (S) indicates the solid phase.

The following steps are contained in the overall reaction:

Reaction of Na2 S with QX produce of Q2S at near the solid-liquid interface. This reaction is usually reversible and fast compared with the intrinsic organic reaction. This reaction is written as:

Na2S+QX ↔ Q2S (org)+NaX(org)    (2)

Equilibration of KX

The product KX (org) is also in equilibrium with its solid parts and is limited by the solubility in the organic phase:

NaX (org) ↔ NaX(S)    (3)

(i) Reaction of Q2S with benzyl chloride, EBA to form the desired product viz., ethyl-2-bromoacetate, (CH3-CH2-COOCH2)2S which is represented in the equation (4).

Q2S (org)+EBA(org) → (CH3-CH2-COOCH2)2S (org)+QCl(S)    (4)

Conclusion

In the present study, the rate of the reaction was controlled to study the kinetic aspects of the formation of the ethyl-2-bromoacetate from sodium sulfide and ethyl-2-bromoacetate (EBA) under ultrasonic-PSPTC condition. The apparent reaction rates were observed to obey the pseudofirst order kinetics, performing the reaction in ultrasonic condition resulted in shorter reaction time. The reaction mechanism and the apparent rate constants were obtained from the experimental results, the apparent rate constants are found to be directly dependent on each kinetic variables, viz., (PSPTC), ultrasonic frequency, stirring speed and temperature. However it decreases with increase in the volume of water and volume of chlorobenzene. Energy of activation was calculated from the Arrhenius plot. Based on the experimental evidence, an interfacial mechanism has been proposed. Combination of ultrasound and PSPTC resulted in better efficacy as compared to the individual operations.

Acknowledgements

The authors would like to thank, The Pachaiyappa’s Trust, Chennai, Tamil Nadu, India-600 030 and Sri Chandrashekarendra Saraswathi Viswa Mahavidyalaya, Deemed University, Enathur, Kanchipuram, Tamil Nadu, India - 631561, for their grant of permission to do this research work. The Author VR acknowledge also thanks The University Grants Commission for financial support for this research work.

References

Select your language of interest to view the total content in your interested language
Post your comment

Share This Article

Relevant Topics

Recommended Conferences

Article Usage

  • Total views: 12370
  • [From(publication date):
    June-2015 - Aug 22, 2017]
  • Breakdown by view type
  • HTML page views : 8522
  • PDF downloads :3848
 

Post your comment

captcha   Reload  Can't read the image? click here to refresh

Peer Reviewed Journals
 
Make the best use of Scientific Research and information from our 700 + peer reviewed, Open Access Journals
International Conferences 2017-18
 
Meet Inspiring Speakers and Experts at our 3000+ Global Annual Meetings

Contact Us

 
© 2008-2017 OMICS International - Open Access Publisher. Best viewed in Mozilla Firefox | Google Chrome | Above IE 7.0 version
adwords