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The Dichlorocyclopropanation of Styrene and P-Chloromethyl Styrene Using Water Soluble Multi-Site Phase Transfer Catalyst – a Kinetic Study

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The Dichlorocyclopropanation of Styrene and P-Chloromethyl Styrene Using Water Soluble Multi-Site Phase Transfer Catalyst – a Kinetic Study Journal of Chemistry and Chemical Sciences, Vol. 5(8), 448-457, August 2015 ISSN 2229-760X (Print) (An International Research Journal), www.chemistry-journal.org ISSN 2319-7625 (Online) The Dichlorocyclopropanation of Styrene and P-Chloromethyl Styrene Using Water Soluble Multi-Site Phase Transfer Catalyst – A Kinetic Study Kannan Shanmugan and Kamalakannan Sasikala Department of Chemistry, Government Arts College, Chidambaram, Tamil Nadu, INDIA. (Received on: August 6, 2015) ABSTRACT The present study focuses the attention towards the utility of multi-site phase transfer catalyst (MPTC), is demonstrated by studying hydroxide-ion initiated reaction like dichlorocarbene addition to olefins. The formation of the product was monitored by GLC. Dichlorocyclopropanation of styrene and p-chloro methyl styrene catalyzed by multi-site phase transfer catalyst carried out in biphase medium under pseudo-first-order conditions by keeping aqueous sodium hydroxide and chloroform in excess. The effect of various experimental parameters on the rate of the reaction has been studied. Also thermodynamic parameters such as ∆S#, ∆G# and ∆H# were evaluated; based on the experimental results, a suitable mechanism is proposed. It also deals in greater detail on the kinetic aspects of chosen reactions. An attempt has been made to compare the ability of MPTC-I with MPTC-II and single-site PTC for dichlorocarbene addition to olefins like styrene and p-chloro methyl styrene. Keywords: Multi-site phase transfer catalysts, Dichlorocyclopropanation, Styrene, p-chloro methyl styrene, kinetics. INTRODUCTION The addition of dichlorocarbene to olefins1,2 has provided an exceptionally useful synthesis of gem -dihalocyclo alkanes. When acyclic olefins are employed, the dihalides are relatively stable but certain cyclic olefins have been found to undergo rearrangement to ring expanded products3. Literature reports of the generation and reaction of dichlorocarbene stress the necessity of operating under strictly anhydrous conditions because of the ready and rapid hydrolysis of dichlorocarbene. Many of these difficulties are eliminated when the reactions are August, 2015 | Journal of Chemistry and Chemical Sciences | www.chemistry-journal.org 449 Kannan Shanmugan, et al., J. Chem. & Cheml. Sci. Vol.5 (8), 448-457 (2015) carried out in bi - phase systems of concentrated sodium hydroxide in the presence of PTCs4. Since the addition of a dichlorocarbene to an olefin assuredly must be a highly exothermic process5,7, the demonstration of the electrophilic nature of the carbenes8,9 has provided amble evidence for the polar contribution to the transition state of the carbene - olefin reaction10. Similar studies employing PTCs for the generation of dichlorocarbene were reported by several authors11,12 . The kinetic experiments (followed by GC) of the dichlorocarbene addition to styrene and p-chloro methyl styrene were carried out under pseudo - first order conditions, taking chloroform and 15% aqueous sodium hydroxide in excess at 450C (Scheme 1,2. dichlorocarbene addition to styrene and p-chloro methyl styrene under PTC conditions). Dichlorocarbene addition to styrene and p-chloromethyl styrene under PTC conditions Effect of stirring speed The effect of varying the stirring speed on the rate of Dichlorocarbene addition to reaction using MPTC - I was studied in the range of 100 - 700 rpm. The rate of the reaction increases sharply as the stirring speed is increased up to 500 rpm in the case of styrene and p-chloro methyl styrene (Table.1, Fig.1). The effect of varying stirring speed is well documented for interfacial mechanisms, which are transfer rate - limited9,13,14 (the rate constant increases with stirring ) below a given stirring speed (600 – 700 rpm) and intrinsic reaction rate limited (the kobs is nearly a constant) above this stirring speed. Similar behavior is displayed by reactions with a real “ phase transfer’’(Stark’s Extraction mechanism) but with a much smaller limit of stirring speed between physical and chemical control (100 – 300 rpm).In the present study, the rate constants of the reaction increases as stirring speed increases and levels off to a constant value above the optimum stirring speed (500 rpm). Halpern et al.15 studied the kinetic details for the isomerization of allyl benzene under PTC conditions. The reaction rate increases with the increase in the stirring speed up to 300 rpm and then the rate becomes independent of stirring speed. The experimental observations are consistent with the hydroxide ion extraction mechanism. In a generally accepted phase transfer process, the reaction rate becomes on the stirring speed. The interfacial area per unit volume of dispersion increased linearly with increasing speed till a stage is reached where there is no significant August, 2015 | Journal of Chemistry and Chemical Sciences | www.chemistry-journal.org Kannan Shanmugan, et al., J. Chem. & Cheml. Sci. Vol.5 (8), 448-457 (2015) 450 increase in the interfacial area per unit volume of dispersion with the corresponding increase in the speed. Thus increasing the stirring speed changes the particle size of the dispersed phase. Above certain stirring speed (500 rpm), the particle size does not change. The constancy of the rate constants is observed not because the process is necessarily reaction rate limited but because the mass transfer rate has reached constant value. Therefore Fig. 1 (Table.1) are indicative of an interfacial mechanism and not of a real “Phase Transfer’’. Effect of Substrate Amount Kinetic experiments were performed by varying the substrate amount of ranging from 5.31 - 31.86 mmol of styrene and 4.64 - 27.82 mmol of p-chloro methyl styrene maintaining other reactants such as chloroform and 15% NaOH in excess (Table. 1,Fig.2). Pseudo - first order rate constants are obtained from the linear plots of log (a-x) verses time. The observed rate constants decrease as the amount of substrate increases. It is clear from Table.1,the molar ratio of the substrate to catalyst increases considerably for small increments in substrate amount. The decrease in rate constants may be attributed to the decrease in the ratio of the number of active sites of the catalyst to the corresponding amount of substrate present. August, 2015 | Journal of Chemistry and Chemical Sciences | www.chemistry-journal.org 451 Kannan Shanmugan, et al., J. Chem. & Cheml. Sci. Vol.5 (8), 448-457 (2015) Table.1.Effect of MPTC-1 on the rates of dichlorocarbene addition to styrene and p-chloromethyl styrene reactions 4 -1 Type of variation variable parameters Rates of the reaction (kobs x 10 , S ) ----------------------------------------------- Styrene p-chloromethyl styrene Stirring speed (rpm) 100 0.43 2.22 200 0.54 3.17 300 0.73 3.74 400 1.68 4.80 500 1.84 4.81 600 1.85 4.81 700 1.85 4.83 800 1.87 4.85 Substrate 5.31 2.54 7.47 Amount 10.62 1.89 3.53 (mmols) 15.93 1.56 3.42 21.24 1.24 2.60 26.55 0.88 1.40 31.86 0.65 1.30 Catalyst 0.10 0.36 2.98 Amount 0.15 0.63 2.41 (mmols) 0.20 1.22 4.91 0.25 1.46 5.90 0.30 2.59 7.23 [NaOH] (M) 3.41 0.53 2.73 4.41 0.87 4.42 5.49 1.29 4.93 6.65 1.59 5.37 7.89 1.88 6.61 Temperature 313 0.44 3.57 (K) 318 0.72 4.31 323 0.89 4.88 228 1.35 6.56 333 1.67 7.24 Effect Catalyst Amount The amount of catalyst (MPTC - I) used to study the rate of dichlorocarbene addition was varied in the range of 0.10 - 0.30 mol% of the catalyst (based on the substrate amount). The rate constant of the reaction is proportional to the amount of catalyst added. Control August, 2015 | Journal of Chemistry and Chemical Sciences | www.chemistry-journal.org Kannan Shanmugan, et al., J. Chem. & Cheml. Sci. Vol.5 (8), 448-457 (2015) 452 experiments were carried out and these showed absolutely no conversion even after three hours of the reaction. The linear dependence of reaction shows that the reaction is believed to proceed through the extraction mechanism. A bilograthmic plot on the observed rate constant against the concentration of the catalyst gives a straight line over a wide range of catalyst concentration 0.10 - 0.30 mol% for styrene and p-chloro methyl styrene (Table.1, Fig. 3). The slope of 4.52 for dichlorocarbene addition to styrene was found to be identical with the slope of the same reaction carried out in the presence of benzyl triethylammonium chloride (BTEAC) by Balakrishnan et al.16. This result suggests that the chemical reaction is not the slope rate - determining step. A slope of 1.46 for p-chloro methyl styrene were obtained graphically (Fig. 3).The fore given observation enables one to predict that the carbanions formed cannot leave the phase boundary to go into the organic phase since their counter ions (Na+) are strongly solvated in the aqueous phase and poorly in the organic phase. In the “absorbed’’ state, the carbanions are very unreactive being able to react only with strong electrophiles. The “multi -site’’ quaternary ammonium cations were able to the organic phase soluble ion - pairs with carbanions thus enabling them to pass into the organic phase for further transformation. The remarkable increase in yield of the dichlorocarbene adduct reflects the ability of the quaternary salt to cause : CCl2 to be generated in or transferred to be the organic phase where its reaction rate with the substrate is much greater than with water as reported by Stark’s in the study of dichlorocarbene addition to cyclohexene using tridecyl methyl ammonium chloride17. Effect of Sodium hydroxide Concentration The rate of dichlorocarbene addition reaction, strongly depends18 on the concentration of NaOH.
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