Polyethyleneoxide as a phase transfer catalyst in the reaction between benzylchloride and potassium acetate

448

O. YE. FILIPI~DVAet aL

10. M. R. KRF~LAVSKII and I. V. SMUSHKOV, (book): Apparatura i metody rentgenovskogo analiza (Apparatu~ and Methods of X-ray Diffraction Analysis), issue 20, p. 45, Mashinostroyeniye, Leningrad, 1978 11. R. V. HEMMING, Chislennyye metody, p. 313, Fizmatgiz, Moscow, 1972 12. W. A, RACHINGER, J. Sci. Instr. 25: 254, 1948 13. D. K. KHAKIMOVA, V. G. NAGORNYI and A. A. SMOL'YANINOV, Kristaliografiya 18: 480, 1973 14. G. FORSYTH, M. MALCOLM and K. MULLER, Mashinnyye metody matematicheskikh, vychislenii, p. 86, Mir, Moscow, 1980 15. Yu. A. ZUBOV, V. L SELIKHOVA, V. S. SH1RETS and A. N. OZERIN, Vysokomol. soyed. A16: 1681, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 7, 1950, 1974)

Polymer Science U.S.S.R. Vol. 26,/,,To. 2, !0p. 448-455, 1984 Printed in Poland

0032-3950/84 $10.0+ .00 © 1985 Pergamon Press L t d .

POLYETHYLENEOXIDE AS A PHASE TRANSFER CATALYST IN THE REACTION BETWEEN BENZYLCHLORIDE AND POTASSIUM ACETATE* O. YE. FILn~POVA, I. N. TOPCHIEVA, V. V. LUTSENKO and V. P. Z t m o v M. V, Lomonosov State University, Moscow (Received 6 August 1982)

IR spectroscopy was used for the detailed kinetic study of the reaction between benzylchlo ride and potassium acetate in the presence of polyethyleneoxide of different M in two solvents- toluene and butanol. It was shown that solubilization of the insoluble reagent is the stage limiting reaction rate in a low-polarity solvent (toluene). The "toxic" effect was shown of polyethyleneoxide in toluene and views were expressed regarding the specific effect of this proeess when using phase transition for polymer catalysts. The actions of polyethyleneoxide and crown ethers as catalysts of phase transition were compared and it was shown that oligoethyleneoxides are more effective catalysts in low-polarity solvents, while in a polar solvent (butanol) the effects of polyethyleneoxide and d o w n ethers as catalysts of phase transition are the same. IT IS k n o w n t h a t synthetic linear p o l y m e r s - p o l y e t h y l e n e oxides (PEO), in additiorz to macrocyclic c r o w n ethers, are able to f o r m complexes with alkali a n d alkali-earth metal cations [1]. As a result oligo- a n d polyethyleneoxides are widely used in o r g a n i c heterogeneous reactions taking place with metal salts, where they function as catalysts o f phase transition (CPT) [2, 3]. * Vysokomol. soyed. A26: No. 2, 402-408, 1984.

Polyethyleneoxide

449

The action o f P E O and c r o w n ethers as c o m p l e x - f o r m i n g agents in C P T is often c o m p a r e d in the literature. This is, first o f all, because o f the similarity o f their chemical structure a n d c o n f o r m a t i o n properties o f PEO. The c o n f o r m a t i o n o f P E O [ 4 ] - a spiral w h i c h is the c o m b i n a t i o n o f two trans- and one g a u c h e - f o r m , ensures the simultaneous participation o f two and m o r e adjacent electron-donor groups in interaction with a metal cation [5]. It was shown that similar to macrocyclic polyesters, P E O f o r m c o m plexes o f given stoichiometry with alkali metal cations in solution [1, 6, 7]. Therefore, complexes o f c r o w n ethers with cations m a y be regarded as m o n o m e r analogues o f linear P E O complexes. It should be b o r n e in m i n d that in contrast with c r o w n ethers, each molecule o f which m a y effectively c o m b i n e alkali metal ions, in the case o f P E O only small sections o f macromolecules interact with cations, which are linked with each other by free polyether chain segments. C o m p a r i s o n o f the catalytic effect o f m o n o m e r (crown ether) and p o l y m e r (PEO) catalysts shows [3] that in m a n y reactions the catalytic efficiency o f P E O is c o m p a r a b l e with the efficiency o f dibenzo-18-crown-6 (DBC). At the same time, the availability, non-toxic nature a n d the possibility o f recovery by reprecipitation into diethyl ether [6] a r e u n d o u b t e d advantages o f using P E O as CPT. Analysis o f results in the literature only enables the most general idea about the b e h a v i o u r o f P E O as C P T to be formulated. Therefore, this study is aimed at carrying o u t a detailed study o f the m e c h a n i s m o f action o f P E O and c o m p a r i n g it with the action o f cyclic a n a l o g u e s - c r o w n ethers in a typical reaction o f nucleophilic substitut i o n - i n t e r a c t i o n o f potassium acetate with benzylchloride C6HsCH2CI + CH3COOK~ C6H5 CH200CCH3 + KCi

(l)

lnitial substances. Benzylchloride previously distilled in vacuum, b.p. 63 ° (8 mmHg) was used in the study. Chemically pure potassium acetate, polyethyleneoxide with M=300, 400, 3000, 6000 and 40,000 of the firm "Merck" and PE0 with M = 100,000 by "Union Carbide" were used without subsequent purification. 18-Crown-6 and DBC were synthesized in the Physico-Chemical Institute of the UkrSSR Academy of Sciences. Benzylacetate was obtained by the interaction of benzyl alcohol and acetyl chloride. A product of b.p. 212-213 ° was used. The product of condensation of DBC with chloral (poly-DBC-chloral) was obtained by methods previously described [8]. Its intrinsic viscosity in chloroform was 0.17 dl/g. Solvents (toluene and butanol) were previously dried. Reaction methods and kinetic measurements. The reaction was carried out with a reagent ratio of benzylchloride : potassium acetate of 1 : 1"5. The initial concentration of benzylchloride was 0.7 mole/l, when carrying out the reaction in toluene and 1-4 mole/l, in butanol. Previously pulverized potassium acetate was added to a catalyst solution in 30 ml dry solvent placed in a two-necked flask provided with a stirrer and reflux condenser. The reaction was carried out at 110°. Benzylchloride was introduced into the flask after 30 minutes' heating and agitation of the reaction mixture. Reaction time was counted from this moment. Reaction kinetics were studied by IR spectroscopy according to the concentration loss of benzylchloride (absorption band corresponding to bond-stretching vibrations of the C - C I group at 815 era-~) and the concentration increase of benzylacetate (absorption band corresponding to bondstretching vibrations of the - C = O group at 1750 cm-~). Spectra were recorded using an IKS-22 speetrophotometer at room temperature and samples were therefore taken from the reaction mixture at certain time intervals. The solution filtered was poured into cells (NaCI) 40 gm thick,

450

O. YE. I:7ILIPPOVA e t

aL

According to modern ideas, CPT has a dual role [9]: it takes salt insoluble in organic medium into solution (solubilizing function) and increases the nucleophilic properties o f the anion (catalytic function). The interphase catalytic scheme may be presented as follows: C H 3 C O O - K + ( s o l i d ) + C P T (soluble) ~ I C P T ' K ] + C H 3 C O O - (soluble) I-CPT • K ] '+ C H a C O O - + C6 H S C H 2 C I ~ [CPT" K ] + CI- + C 6 H s C H z O O C C H 3 I-CPT • K ] + CI- ~ CPT + KCI

J0

Yield,% b

•

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Off

to

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0 II!

I

I

I

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b

~

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I

I ~

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7ime, hr

FIG. 1. Kinetic curves of the formation of benzylaeetate in reaction (I) in the presence of PEO of different M in toluene (a) and in butanol (b); the overall volume of the reaction mixture 30 ml. a: concentration of benzylehloride 0.7 mole/L; 0.415 base-mole/l. PEO, 3-15 × 10-2 mole potassium acetate. MPEo: 1--100,000, 2--40,000, 3--6000, 4--3000, 5--600, 6--400. b: concentration of benzylchloride 1-4 mole/L; 8-3 × 10-2 base mole PBO, 6.3 × 10-z mole potassium acetate. MpEo: 1 -100,000, 2--40,000, 3--400, 4--300, 5-without PEO.

Main kinetic regularitiesof esterificationcatalysedby PEO. Kinetic curves of reaction (1) catalysed by PEO o f M ranging from 300 to 100,000 in two solvents o f different p o l a r i t i e s - t o l u e n e and b e n z e n e - a r e shown in Fig. 1. It can be seen that product yield in toluene for oligomers increases with an increase in Mr, while for high polymers, starting from M = 3 0 0 0 limiting values of yield cease to depend on M, being much lower than the quantitative value. These regularities are also retained in butanol, however, limiting product yield values increase considerably, approximating 100 ~o for fractions o f high molar mass. Therefore, two special features may be noted in the behaviour of PEO as CPT: differences in the kinetic behaviour o f oligomer and high molar mass PEO and differantes in the behaviour of PEO in s61vents o f different polarity.

Polyethyleneoxid~

451

To compare the effect of PEO of different M values, reaction orders were determined from reagent concentrations. In butanol, where reaction (1) takes place under homogeneous conditions first order rate is observed both for oligomer and for lfigh molar mass PEO according to concentration, which is typical of catalytic processes. In toluene first order in CPT concentration is only maintained for oligomer PEO. For PEO with M = 100,000 it decreases to 0.33. It may be assumed that with heterogeneous reaction the rate of solubilizatJon of - C H a C O O K , the insoluble reagent, has a marked effect on kinetic characteristics of the process. When using high molar mass PEO the rate is probably lower than for oligomer PEO. To verify this assumption, kinetics were studied of reaction (1) in the presence of high molar mass PEO under homogeneous conditions. A P E O - CH3COOK (1 molecule CHaCOOK for 7 units of PEO) complex was therefore previously formed by combining components in a general solvent-methanol and then removing the solvent. The complex formed is readily dissolved in toluene. Results of studying reaction kinetics in the presence of a soluble complex are shown in Fig. 2. The reaction for the catalyst was of first order. Therefore, the apparent anomalies in the behaviour of high-molar mass PEP, compared with oligomer PEO in non-polar solvents, are due to the superposition of the slow rate of solubilization on the measured rate of the reaction. It is interesting to compare rates of the reaction catalysed by high molar mass PEO in polar and non-polar solvents. To make such comparison accurate, let us examine two homogeneous reactions catalysed by a PEO complex with M = 100,000 with potassium acetate. Values of reduced reaction rates measured with the same concentrations of the soluble complex and related to concentrations of benzylchloride, are 1.35 x l0 -2 mole/1..min for the reaction in toluene and 1.0× l 0 - 2 molefl..min for the reaction taking place in butanol. To compare thc catalytic efficiency of PEO in various solvents, the concentration of reacting anions has to be evaluated. Comparison of limiting values of reaction product yield in toluene and in butanol indicates that the concentration of acetate-ions in toluene is ,--half that in butanol. Bearing in mind this fact we find that PEO in toluene is N3 times more effective catalyst than in butanol. A very different conclusion about the effect of the type of solvent on the rate of reaction (1) in the presence of PEO with M=33,000 was drawn previously [3]; this is, apparently, due to the fact that reaction rates described by various orders as regards polymer concentration, were compared with each other. The result obtained indicates that CHaCOOhas higher nucleophilic properties in non-polar medium, which is in agreement with general views about the reactivity of anions. We note that the difference in rates of similar reactions catalysed by crown ethers in solvents of varying polarity, reaches several orders of magnitude [10]. This is, possibly due to the varying state of CH3COO- functioning as counter-ion of the charged complex of crown-ether and PEO. In fact, as a result of combining potassium ions PEO acquires properties of a polyelectrolyte, which is seen from the effect of polyelectrolyte swelling and the ability of counter-ions to undergo specific combination (Fig. 3). The assumption concerning specific combination of the polymer chain of acetate ions inL

452

O. YE. FILI,PPOVA et al.

toluene, in contrast with butanol, was made on the basis that in the first solvent in the concentration range of PEO used in catalytic systems, the viscosity of PEO solution is lower in the presence of potassium salt than in its absence. In butanol the addition o f potassium acetate to the PEO solution increases the viscosity of the system, although polyelectrolyte swelling is observed in b o t h cases.

Yield,% 1 00

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Ct

2.5 2

20

f.5

i

0

2 Time, hr FIG. 2

m

I

q

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0

8

FIG. 3

FIG. 2. Kinetic curves of the formation of benzylacetate in the reaction of benzylchloride with a :soluble complex of PEO (M= 100,000) with potassium acetate. Solvent-toluene. Concentration of benzylchloride 0"2 mole/L, overall volume of the reaction mixture 10 ml. Amount of complex: •-0.825, 2-0.618, 3-0-413 g. FIG. 3. Dependence of reduced viscosity on the concentration of PEO in toluene (a) and butanol (b) in the presence of potassium acetate (1) and without it (2). Therefore, it may be assumed that part of ions in toluene is combined with the p o l y m e r chain and is in the f o r m of ionic pairs because of the low dielectric constant of the solvent. As the reaction proceeds, combined acetate ions gradually change into the solution volume, where interaction with the substrate takes place. This effect is absent when using low-molar mass crown-ethers, where all acetate ions are highly reactive nucleophils of the same type. Comparison of catalytic properties of oligomer and high m.olar mass PEO on the initial sections of kinetic curves indicates that oligomer PEO are more effective CPT than high m o l a r mass PEO (Fig. 4). It may be assumed that the effectiveness of CPT is determined by complex-forming properties of PEO in relation to the potassium ion. In order to verify this assumption, results in the literature concerning the complex-forming properties of PEO of different molar masses and of crown-ethers were analysed. It was shown [11] that firstly, on transition from oligomer to polymer PEO the constant of complex-formation increases, approximating to a limiting value when M = 6 0 0 0 20,000 and secondly, constants of complex-formation of high molar mass PEO and 18-crown-6 are comparable. Comparison of these data with kinetic results shows that the effectiveness of CPT in this system is not determined by complex-forming properties

Polyethyleneoxide

453

,of PEO, but by other factors. It is possible that this factor in this system is the moderating effect of specific combination of the acetate-ion in the case of high-molar mass PEO.

.lo~c

-0"8

-o.q

I

Yield, % -2"G

60

/

1

O0 -3"0

20 I

lured FIG. 4

0

i

2

I

I

q

Time, hp

I

I

G

FIG. 5

Fro. 4. Dependence of reaction rate reduced to benzylchloride concentration on the concentration ,of CPT in toluene in logarithmic coordinates: 1 - D B C , 2 - P E O with M = 6 0 0 , 3 - P E O with M = 100,000. Concentration of benzylchloride 0.7 mole/l. FIG. 5. Kinetic curves of the formation of benzylacetate in reaction (l) in the presence of 18-crown-6 {1), D B C (2) and poly-DBC-ch/oral (3). Concentration of benzylchloride 0.7 m o l e / l , concentration of crown-ether 0.166 mole/l.; 3.15 x 10 -2 mole potassium acetate; overall volume of the reaction mixture 30 ml.

The second special kinetic feature of the reaction is the incomplete process in a non-polar solvent and the dependence of the limiting value of yield on the molar mass o f the polymer. One of the possible causes explaining this effect is the contamination o f the catalyst by reaction products [9]. This assumption is confirmed by the following experiment. After the reaction a new batch of PEO was introduced into the reaction mixture containing polyeth~leneoxide with M=100,000 as CPT and yield increased. To explain the problem which of the reaction products causes poisoning of the catalyst, a study was made o f the effect of preliminary addition of reaction products on reaction kinetics. Previous addition of benzylacetate has no effect on reaction kinetics. The solubility of KCI in toluene is so low that it is difficult to count on a considerable solubilization when introducing it into the reaction mixture. It may be assumed a t the same time that KCI formed during the reaction is combined with PEO and precipitates as a complex from the solution, which results in the removal of the catalyst from the reaction zone. The ease of poisoning PEO with inorganic salts has also been

454

O. YE. FILleeOVAet al,

noted in oxidation of stilbene by potassium permanganate in benzene in the presence of PEO [3]. Therefore, the assumption that KCI causes contamination of the catalyst, is very likely. Lower values of limiting yields when using oligomer PEO, compared with high-molar mass PEO are, apparently, due to lower lypophilic properties of PEO oligomers [1]. Comparison of the action of PEO and crown-ethers as CPT. It was interesting to compare kinetic features of the reaction catalysed by PEO and crown-ethers. Figure 5, shows typical kinetic curves of the reaction catalysed by DBC and 18-crown-6 which are cyclic analogues of PEO. According to results in the literature [12] and results obtained in this study, the equation of reaction rate, catalysed by crown-ethers in toluene takes the form: -- d [C6HsCH2C1] = £'[C6HsCH2CI]I"°[CHaCOOK]° [crown.ether] 1"° dt Limiting values of reaction product yield are independent of catalyst concentration and approximate to 1013~o. Comparison of the effectiveness of DBC and PEO indicates that the effectiveness of crown-ethers in toluene is intermediate between the effectiveness of PEO with M =600 and 100,000 (Fig. 4), while in butanol the effectiveness of these CPT is practically identical (rate constants of reaction (1), catalysed by DBC and PEO with M = 100,000 being 4 × 10 -2 and 2.6 × 10 -2 1.2/molC .see, respectively). When studying kinetic craves of reaction (1) catalysed by crown-ethers (Fig. 5) importance is attached to the fact that curves consist of two sections: the initial section, which is characterized by a steep slope and a flatter section. The same as when describing kinetic curves of the reaction catalysed by PEO it may be assumed that the break on kinetic curves is due to the contamination of the crown-ether as a result of the low solubility of the complex of crown-ether with KC1 [9]. We note that in the case of crown-ethers contamination of the catalyst does not lower the yield of the reaction preduct, but kinetically only shows in a reduction of reaction rate. It is obvious that on transition from "monomer" CPT to their polymeric derivatives, the process of combination of C1- becomes more sharply cooperative. Another confirmation of the accuracy of the assumption made was obtained as a result of a comparative study of kinetics of reaction (l) catalysed by DBC and its polymeric analogue using poly-DBC-chloral

results of which are shown in Fig. 5. It is clearly seen that with identical concentrations. of DBC and its polymeric derivative (in terms of one unit) initial sections of kinetic

Polyethyleneoxide

455

curves fully coincide and the b r e a k is much m o r e sudden on transition from m o n o m e r i c crown-ether to its polymeric derivative. General considerations m a y be expressed concerning the use of PEO a s C P T in exchange reactions of ester synthesis on the basis of experimental data and results in the literature. When carrying out the reaction in non-polar media oligomer PEO ensure higher (compared with high-molar mass types) initial rates o f the process, exceeding the action of DBC. The m a i n shortcoming of PEO as CPT, compared with crownethers, is the much more clearly expressed tendency to contaminating the catalyst by hydrogen halide salt of potassium. Therefore, high reaction yield can only be achieved when using PEO concentrations considerably exceeding catalytic values. This special feature o f the polymer catalyst is, apparently, observed in any exchange reaction accompanied by the replacement of a more lypophilic anion by a less lypophilic one. Therefore, it is more promising to use PEO in reactions of addition, anionic polymerization, etc. When carrying out esterification in polar solvents kinetic behaviour and catalytic effectiveness of polymeric catalysts and crown-ethers are comparable. The authors are grateful to K. S. Kazanskii for discussing results. Translated by E. SEMERE REFERENCES

I. S. YANAGIDA, K. TAKAHASHI and M. OKAHARA, Bull. Chem. Soc. Japan 50: 1386, 1977 2. D. BALASUBRAMANIAN, Tetrahedron Letters, 37, 3543, 1974 3. A. HIRAO, S. NAKAHAMA, M. TAKAHASHI and N. YAMAZAKI, Makromolek. Chem. 179: 915, 1978 4. V. G. DASHEVSKII, Konformatsiya organicheskikh molekul tConformation of Organic Molecules), p. 346, Khimiya, Moscow, 1974 5. Z. N. MEDVED', N. A. STARIKOVA, O. G. TARAKANOV and A. K. ZHITINK1NA, Vysokomol, soyed. A19: 76, 1977 (Translated in Polymer Sci. U.S.S.R. 19: 1, 88, 1977) 6. J. M. HARRIS, N. H. HUNDLY, T. G. SHANNON and E. C. STRUCK, Polymer preprints 23: 193, 1982 7. D. K. DIMOV, L M. PANAYOTOV, V. N. LAZAROV and C. B. TSVETANOV, J. Polymer Sci. 20: 1389, 1982 8. T. TAKEKOSHI and J. L. WEBB, USA Pat. 3824215, 1974 9. V. WEBER and G. GOKEL', Mezhfaznyi kataliz v organicheskom sinteze, pp. 24, 116, Mir, Moscow, 1980 10. C. L. LIOTTA, H. P. HARRIS, M. MCDERMOTT, T. GONZALEZ and K. SMITH, Tetrahedron Letters, 28, 2417, 1974 11. G. N. ARKHIPOVICH, S. A. DUBROVSKII, K. S. KAZANSKII and A. N. SHUPIK, Vysokomol, soyed. 32?.3: 1653, 1981 (Translated in Polymer Sci. U.S.S.R. 23: 7, 1827, 1981) 12. W. M. MACKENZIE and D. C. SHERRINGTON, Polymer 21: 791, 1980