Micellar and Phase Transfer Catalyses

Transcription

1 Micellar and Phase Transfer Catalyses Dr. Pallab Ghosh Associate Professor Department of Chemical Engineering IIT Guwahati, Guwahati India Joint Initiative of IITs and IISc Funded by MHRD 1/17

2 Table of Contents Section/Subsection Page No Micellar catalysis Kinetics of micellar catalysis Phase transfer catalysis 12 Exercise 16 Suggested reading 17 Joint Initiative of IITs and IISc Funded by MHRD 2/17

3 7.4.1 Micellar catalysis In early 1970s, it was observed that the reactions of organic compounds were significantly enhanced in the aqueous micellar solutions of ionic surfactants. The reaction rate was found to increase in the vicinity of the critical micelle concentration. The maximum rate in micellar solution was found to be several hundred times that of the rate in water. For some reactions, however, the presence of micelles was found to decrease the rate significantly. The former case was called micellar catalysis and the latter was called micellar inhibition. When micelles are present in solution, bimolecular reactions can occur at two places, viz. at the micellar region and in the bulk solution. The overall reaction rate is determined by the partitioning factor by which each of the reactants is assimilated into the micelles from the bulk solution. A promotion in the rate would occur when both reactants are preferentially located in the micelles. A decrease in rate would occur if one of the reactants is incorporated into the micelles such that it is inaccessible to the other reactant for reaction. Interests in micellar catalysis arose mainly due to two factors: (i) huge time, material and energy may be saved, and (ii) the process has important biochemical implications. Many processes in the living cell occur at interfaces, such as the active site of an enzyme on a membrane. As a result, chemical reactivity in biochemical reactions is critically dependent on the local microenvironment, local concentrations and relative orientations of the bound reactants at the cell surfaces. Thus, the realization that micelles could be realistic cell mimicks has become a major thrust behind the investigations on micellar catalysis. Micellar catalysis has been widely used for the hydrolysis of esters, aliphatic and aromatic nucleophilic substitution reactions and free radical reactions. The spectacular effect of micelles on organic reactions is due to electrostatic and hydrophobic effects. Electrostatic interactions may affect the rate by influencing the concentration of the reactant(s) near the site of reaction. To illustrate, a micelle made of a cationic surfactant in an aqueous solution has positively charged head-groups on its surface. It can attract negatively charged ions and bring it closer to the micellewater interface where it can react with the Joint Initiative of IITs and IISc Funded by MHRD 3/17

4 substrate. In addition, it can reduce the energy of activation of the reaction by delocalizing the charge developed in the transition state. Two important criteria must be satisfied for micellar catalysis to occur, viz. (i) the substrate must be solubilized in the micelle, and (ii) the locus of solubilization must be such that the reactive site of the substrate is accessible to the attacking reagent. The hydrophobic interactions govern the extent and locus of solubilization in the micelle. The orientation of the substrate in the micelle can affect the selectivity of a chemical reaction. To illustrate, Samant et al. (2006) have reported selective chlorination of phenol in which the major product was o-chloro phenol. The phenol molecule is solubilized in the micelle in such a manner that the polar OH group projects out of the micelle towards the bulk aqueous phase and a relatively non-polar aromatic ring remains penetrated inside the hydrophobic core as shown in Fig Fig Selective orientation of solubilized substrate in the micelle. Therefore, the electrophilic attack at the para position is hindered whereas the ortho position becomes favorable for attack by the electrophile Kinetics of micellar catalysis A simple kinetic analysis of micellar catalysis is presented here. Let us assume that the substrate S is solubilized in the micelle M only, and there is no interaction with the monomer surfactant molecules. The solubilization between the substrate and the micelle is in 1:1 stoichiometric ratio. Joint Initiative of IITs and IISc Funded by MHRD 4/17

5 The solubilization process can be represented by the reaction depicted in Fig Fig Reaction scheme for micellar catalysis. The rate constant for the reaction of the substrate in the bulk phase is represented by b k mc and the rate constant for the reaction in the micelle is represented by m k mc. The overall rate of reaction can be expressed as the sum of the rates of reaction for the unsolubilized substrate and the solubilized substrate as, t o u b c m cskmc cskmc cskmc (7.4.1) where the overall rate constant for the reaction is k mc o, t c S is the total concentration of the substrate, c S u is the concentration of the unsolubilized substrate and c c S is the concentration of the solubilized substrate. The equilibrium constant for the interaction between the micelle and the substrate K mc is known as the binding constant. It is given by, From Eqs. (7.4.1) and (7.4.2) we get, c c K S mc u (7.4.2) cm cs cs t kmc o cs u kmc b kmc m KmccM cs u kmc b kmc m KmccM cs u (7.4.3) A total mole balance of the substrate, S, gives, t u c cs cs cs (7.4.4) From Eqs. (7.4.2) and (7.4.4) we get, t u cs c K S mc (7.4.5) u cm cs Joint Initiative of IITs and IISc Funded by MHRD 5/17

6 Rearranging Eq. (7.4.5) and substituting u cs t c S in equation (7.4.3) we get, b m o kmc kmckmcc k M mc (7.4.6) 1 KmccM The concentration of micelles can be expressed as, c c c CMC M N (7.4.7) where N is the aggregation number, c is the total concentration of surfactant and c CMC is the critical micelle concentration. Substituting c M from Eq. (7.4.7) into Eq. (7.4.6) and rearranging we get, N o b m b m b k K mc kmc kmc k mc kmc k mc mc c ccmc (7.4.8) o b The rate constants, k mc and k mc, can be determined experimentally. Therefore, the left side of Eq. (7.4.8) is known. o b If we plot 1 kmc kmc versus 1 c c CMC m b whose slope is N Kmc kmc k m b mc and the intercept is 1 kmc kmc, a straight line should be obtained. Since b m k mc is known from experiment, k mc and K mc can be determined. o The variation of the overall rate constant k mc with the concentration of cetyltrimethylammonium bromide for the hydrolysis of the dianion of 2,6- dinitrophenyl phosphate is shown in Fig Joint Initiative of IITs and IISc Funded by MHRD 6/17

7 Fig Variation of overall rate constant with concentration of CTAB for the hydrolysis of the dianions of 2,6-dinitrophenyl phosphate [Bunton (1973); adapted by permission from Elsevier Ltd., 1973)]. Example 7.4.1: The following data are available on the kinetics of reduction of toluidine blue by ascorbic acid in a micellar solution of cetyltrimethylammonium bromide in presence of 0.1 mol/dm 3 HCl (Arikan and Tunçay, 2005). 1, c ccmc dm 3 /mol 1 1,, s c c b o CMC kmc k mc dm 3 /mol 1, s b o kmc k mc The rate constant for reaction in absence of the micelles is s 1. The aggregation number is 50. Calculate the rate constant for reaction in the micelle and the binding constant. b 3 Solution: Given: k mc 5 10 s 1. From Eq. (7.4.8) we get, Joint Initiative of IITs and IISc Funded by MHRD 7/17

8 1 1 1 N b o b m k b m mc k mc kmc k mc kmc kmc cccmc Kmc The plot of 1 b o kmc k mc 1 against c ccmc is shown in Fig Fig Determination of binding constant and the rate constant for the reaction in the micelle from Eq. (7.4.8). Slope N Intercept Kmc Kmc dm 3 /mol 1 Intercept b m kmc kmc b m kmc kmc m kmc s 1 The surfactant concentration is usually below 10 mol/m 3. There will generally be little enhancement of the rate of reaction in the presence of micelles unless the m quantity, kmck mc, is large enough (e.g., 100 or more). Joint Initiative of IITs and IISc Funded by MHRD 8/17

9 Since the binding constant, K mc, depends on the extent of hydrophobic bonding between surfactant and substrate, it can be expected that K mc will increase with increase in the chain length of both the surfactant and the substrate. However, if the hydrophobic group of the substrate is too long, it may be solubilized so deeply in the micelle that access to its reactive site by a reagent in an aqueous solution phase is hindered. In that case, solubilization will inhibit the reaction. In accordance with these principles, the alkaline hydrolysis of p-nitrophenyl esters in aqueous medium is catalyzed by the cationic surfactant micelles of n- alkyltrimethylammonium bromide and retarded by anionic sodium laurate micelles. Nonionic surfactants either decrease the rate or have no significant effect on the rates of hydrolysis of carboxylic acid esters. The ester is probabily solubiized at the micelle water interface. The transition state for alkaline hydrolysis of the ester linkage carries a negative charge due to the oncoming OH, and this charge can be stabilized by the adjacent positive charges of the hydrophilic heads of cationic micelles and destabilized by the adjacent negative charges in anionic micelles. The concentration of OH at the micelle water interface is increased by the multiple positive charges on the cationic micelles and decreased by the multiple negative charges on the anionic micelles. Both of these effects may account for the enhancement and diminution of reaction rates in the respective cases. These effects also explain the observation that the rate of reaction with neutral nucleophiles, such as morpholine, is not accelerated by cationic micelles. They also explain the inhibiting action of small concentrations of inorganic anions (viz., F, Cl, Br, NO 3, SO 4 2 ) on micellar catalysis by the cationic surfactants, because these anions compress the electrostatic double layer surrounding the positively charged hydrophilic head-groups, and thereby weaken their interaction with negative charges. Joint Initiative of IITs and IISc Funded by MHRD 9/17

10 The extent of rate enhancement by cationic surfactants as well as reduction by anionic surfactants increase as the chain length of the acyl group of the ester is increased following the order: p-nitrophenyl dodecanote > p- nitrophenylhexanoate >> p-nitrophenylacetate. In the case of certain other esters (e.g., ethyl benzoate and acetyl salicylate), anionic and cationic surfactant micelles both retard the rate of hydrolysis. These effects are attributed either to small binding constants between substrate and micelle, or to solubilization into micelles in such fashion as to remove the reaction site from the attacking reagent. The increase in the rate of acid catalyzed hydrolysis of esters in aqueous media by anionic micelles can be explained in similar fashion as being due to the stabilization of the positively charged transition state or to concentration of H + at the micelle water interface by the negatively charged adjacent hydrophilic headgroups. The plots of rate constant versus surfactant concentration often show a maximum at some surfactant concentration above the CMC. There are a number of explanations for this. First, the number of micelles increases with increase in the surfactant concentration. When the number of micelles exceeds that required to solubilize all of the substrate, there is a dilution of the concentration of substrate per micelle as the surfactant concentration is increased further. This causes a reduction in the rate constant. Second, the charged surface of an ionic micelle in aqueous medium may cause the adsorption of an oppositely-charged reactant on it, or even the solubilization of the reactant into the micelle. Such adsorption or solubilization of the reactant will result in a decrease in its activity in the solution phase. An increase in the concentration of surfactant over that required to effect substantially complete solubilization of the substrate may therefore result in a decrease in the rate constant, even in those cases where rate enhancement by micelles occurs. Aliphatic and aromatic nucleophilic substitution reactions are also subject to micellar effects, with results consistent with those in other reactions. In the reaction of alkyl halides with CN and S 2 O 2 3 in aqueous media, it has been found Joint Initiative of IITs and IISc Funded by MHRD 10/17

11 that the sodium dodecyl sulfate micelles decrease the second-order rate constants and dodecyltrimethylammonium bromide increase them. The reactivity of methyl bromide in the cationic micellar phase has been found to be 30 to 50 times that in the bulk phase, and is negligible in the anionic micellar phase. A nonionic surfactant does not significantly affect the rate constant for the reaction between n-pentyl bromide and S 2 O 2 3. Micellar effects on nucleophilic aromatic substitution reactions follow similar patterns. The reaction of 2, 4-dinitrochlorobenzene or 2, 4-dinitrofluorobenzene with hydroxide ion in aqueous media is catalyzed by cationic surfactants and retarded by sodium dodecyl sulfate. Cetyltrimethylammonium bromide micelles increase the reactivity of dinitrofluorobenzene ~60 times, whereas sodium dodecyl sulfate decreases it by a factor of 2.5. On the other hand, a POE nonionic surfactant have no effect. Diquatemary ammonium halides of the gemini type are particularly effective micellar catalysts for nucleophilic substitution and decarboxylation reactions. The hydrolysis of long chain alkyl sulfates in aqueous solution is an example of a reaction where micellar effects can be observed without the complicating presence of a solubilizate. The rate of acid catalyzed hydrolysis is increased about 50 times by micellization because of the high concentration of H + on the negatively charged micellar surface. As the chain length of the alkyl group is increased, the rate constant increases, reflecting the lower CMC of the surfactant. On the other hand, alkaline hydrolysis of these compounds is retarded considerably by micelle formation. Micelle formation has a negligible effect on the neutral hydrolysis of these materials. A study of double-tailed (sodium dialkylsulfosuccinate) and double-headed (disodium monoalkylsulfosuccinate) surfactants revealed that these surfactants have no advantage over similar single-headed, single-tailed (sodium alkylsulfoacetate) surfactants. The second tail does not substantially increase the binding of the substrate (i.e., pyridine-z-azo-p-dimethylaniline) to the micelles, and the second head decreases, rather than increases, the binding of the reagent Joint Initiative of IITs and IISc Funded by MHRD 11/17

12 (Ni +2 ) to the micelle. The latter effect may be due to the competition of the additional Na + present. The presence of micelles can also result in the formation of different reaction products. A diazonium salt, in an aqueous micellar solution of sodium dodecyl sulfate, yield the corresponding phenol from reaction with OH in the bulk phase, but the corresponding hydrocarbon from material solubilized in the micelles. Micellar effects are also apparent in reactions involving free radicals. Surfactants have been used extensively for the enhancement or inhibition of industrially and biologically important free radical processes, such as emulsion polymerization and the oxidation of hydrocarbons and unsaturated oils. An investigation of the free radical oxidation of benzaldehyde and p- methylbenzaldehyde by oxygen in aqueous nonionic surfactant solutions indicated that the rate of oxidation is increased when the aldehyde is solubilized in the interior region of the micelles. As the alkyl chain length of the surfactant is increased, the oxidation rate of p- methylbenzaldehyde increases because of the increased solubilization of the aldehyde in the interior region of the micelle. However, the oxidation rate for benzaldehyde is not increased by this change in the structure of the surfactant. Spectroscopic observations have indicated that p-methylbenzaldehyde is solubilized in both the outer and inner regions of the micelles. Increase in the length of the alkyl chain of the surfactant increases the proportion of aldehyde in the inner region, whereas benzaldehyde is solubilized only in the polyoxyethylene region of the micelle Phase transfer catalysis The molecules or ions participating in a chemical reaction must come into intimate contact for a reaction to occur. It is therefore a common practice to carry out reactions in a homogeneous medium. Joint Initiative of IITs and IISc Funded by MHRD 12/17

13 In many cases, however, it is impractical to form a homogeneous medium, particularly when ionic compounds react with nonpolar organic compounds. For such cases, phase transfer catalysts offer a simple and efficient solution. Phase transfer catalysis (PTC) was first introduced in 1960s. At present, hundreds of reactions are carried out by this method. It has found wide use in many pharmaceutical and agrochemical industries. Phase transfer catalysis is a general methodology which is applicable to reactions in which inorganic and organic anions and other reactive species react with organic compounds in heterogeneous two phase systems. One phase acts as a reservoir of the reacting anion. The other phase is the reservoir of the organic reactant and the catalyst. The reacting anions are continuously introduced into the organic phase in the form of lipophilic ion pairs in which the lipophilic cation is supplied by the catalyst. Most often tetra-alkylammonium cations are used as PTC. The mechanism proposed by Starks (1971) for liquidliquid phase transfer catalysis is illustrated in Fig for nucleophilic substitution of alkyl halide, R X, in an aqueousorganic two-phase system in the presence of catalytic amounts of a quaternary onium salt, Q X, and an excess of a metal salt. Fig Mechanism of Starks (1971). The catalyst transfers the reacting anion, Y, into the organic phase as lipophilic and unsolvated ion pair, Q Y, which is highly effective. The leaving group, X, is then returned to the aqueous phase. Joint Initiative of IITs and IISc Funded by MHRD 13/17

14 Various factors such as partition and structure of the catalyst, reactivity of the ion pairs in the organic medium of low dielectric constant and hydration of anions are important in the phase transfer catalysis process. Mąkosza (1975) proposed an interfacial reaction mechanism for phase transfer catalysis. Three steps are involved in this mechanism as shown in Fig Fig Mechanism of Mąkosza (1975). The first step involves the transfer of ionic reactant, M Y, from the aqueous phase and the catalyst, Q X, from the organic phase into the interfacial region. The second step involves the reaction of the ionic reactant with the catalyst in the interfacial region to form the intermediate catalytic reactant, Q Y. In the third step, the intermediate catalytic reactant transfers into the organic phase to react with R X to produce the product, R Y, and the catalyst, Q X. In situations where the complex is Q OH, the reaction with R X occurs at or near the interface because it is highly hydrophilic and has extremely low solubility in the organic phase. The interfacial reaction mechanism has been applied to carbanion reactions, condensation polymerization and C-alkylation of active methylene compounds such as activated benzylic nitriles, activated hydrocarbons and activated ketones. The kinetic criteria to distinguish between the two mechanisms are as follows. The extraction mechanism of Starks is characterized by: (i) increasing rate with increasing liophilicity of catalyst, (ii) independence of reaction rates on stirring speed above a certain value, (iii) first-order or fractional-order dependence of reaction rate on the catalyst concentration, and (iv) pseudo-first or second-order Joint Initiative of IITs and IISc Funded by MHRD 14/17

15 kinetics if the reaction in the organic phase is rate controlling, or zero-order kinetics if diffusion across the interface is rate controlling. The interfacial mechanism is characterized by: (i) increasing rate with increasing electrostaticity of catalyst, (ii) dependence of reaction rate on agitation rate, (iii) fractional kinetic order with respect to the catalyst concentration, and (iv) substrate acidity (pk a ) in the range of 16 to 23. Joint Initiative of IITs and IISc Funded by MHRD 15/17

16 Exercise Exercise 7.4.1: Answer the following questions clearly. i. What is micellar catalysis? Explain how a micellar solution can expedite the rate of a chemical reaction. ii. Explain how micellar reaction can alter the selectivity. iii. Explain why the plots of rate constant versus surfactant concentration often show a maximum at some surfactant concentration above the CMC. iv. Explain how the electrical charge on the micellar surface influence reaction. v. What is micellar inhibition? Where can it be useful? vi. Explain phase transfer catalysis. Mention five applications. vii. Explain the mechanisms proposed by Starks and Mąkosza for phase transfer catalysis. Exercise 7.4.2: Show that for micellear inhibition, the rate of reaction does not decrease until the critical micelle concentration is reached, and the rate constant falls to a limiting value with increasing surfactant concentration. Joint Initiative of IITs and IISc Funded by MHRD 16/17

17 Suggested reading Textbooks M. J. Rosen, Surfactants and Interfacial Phenomena, John Wiley, New Jersey, 2004, Chapter 4. P. Ghosh, Colloid and Interface Science, PHI Learning, New Delhi, 2009, Chapter 12. Reference books J. Fendler and E. Fendler, Catalysis in Micellar and Macromolecular Systems, Academic Press, New York, 1975, Chapters 13. M. N. Khan, Micellar Catalysis (Surfactant Science Series, vol. 133), CRC Press, Boca Raton, 2006, Chapter 3. Journal articles B. Arikan and M. Tunçay, Dyes and Pigments, 64, 1 (2005). B. S. Samant, Y. P. Saraf, and S. S. Bhagwat, J. Colloid Interface Sci., 302, 207 (2006). C. A. Bunton, Progr. Solid State Chem., 8, 239 (1973). C. M. Starks, J. Am. Chem. Soc., 93, 195 (1971). M. Mąkosza, Pure Appl. Chem., 43, 439 (1975). Joint Initiative of IITs and IISc Funded by MHRD 17/17