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Reaction of 5-ethylidene-2-norbomene with formic acid: cation exchange resins as catalysts
REACTIVE & FUNCTIONAL POLYMERS
ELSEVIER
Reactive
& Functional
Polymers
36 (1998) 41-49
Reaction of Sethylidene-2-norbomene with formic acid: cation exchange resins as catalysts Basudeb Saha * University Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India Received
28 April 1997: revised version received
15 July 1997; accepted
16 July 1997
Abstract
The esterification of formic acid with ethylidene norbomene (ENB), i.e. 5ethylidene bicyclo[2.2.l]hept-2-ene was carried out with and without cation exchange resins as catalysts. The effect of various parameters e.g. speed of agitation, catalyst loading, mole ratio of the reactants, temperature, concentration of formic acid in aqueous solutions were studied to optimise the reaction conditions; the reusability of catalysts was also studied. A comparison of the rate of esteri$cation between ENB and dicyclopentadiene (DCPD), with formic acid was made. 0 1998 Elsevier Science B.V. All righ& reserved. Keywords:
Esterification; Cation exchange resin; Formic acid; 5-Ethylidene bicyclo[2.2.l]hept-2-ene; 6-Ethyhdene-exo2-norbomyl formate; S-Ethylidene-exo-2-norbomyl formate; 6-Ethyltricyclo[2.2.1.02~6]heptan-3-yl formate
1. Introduction Esterification of ethylidene norbornene (ENB), i.e., 5-ethylidene bicyclo[2.2.l]hept-2-ene, with different aliphatic carboxylic acids is of academic interest and industrial relevance. ENB is a commercially available compound having an olefinic side chain and its esters are useful as plasticizers. Very little work has been done on the chemical properties of ENB and, especially, detailed investigations concerning the reactivity difference of the two double bonds in ENB have not yet been made using cationic exchange resins as catalysts. Some of the major advantages of using ion exchange resins as catalysts in this reaction are ease of separation of the catalyst from the reaction mixture, milder reaction conditions, higher yield * Corresponding ical Engineering, icestershire. LEl
author. Present Loughborough 1 3TU. UK.
address: Department of ChemUniversity. Loughborough, Le-
1381-5148/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PIZS1381-5148(97)00.112-0
and selectivity towards the ester, higher catalyst activity, catalyst reusability etc. The purpose of the present work is to obtain a better understanding of the type of stereoelectronic influence exerted on the reactivity of the norbomenyl ring double bond and to determine which of the two double bonds in ENB undergoes certain addition reaction preferentially. This work was undertaken to examine the performance of macroporous and microreticular ion exchange resins and acid-treated clay in the esterification of ENB with formic acid and to delineate the best operating conditions for predominantly producing the corresponding mono ester’of ENB. 2. Previous studies A careful survey of literature shows that no detailed studies have been made on the reaction of ENB with formic acid using cation exchange
42
B. Saha/Reactive & Functional Polymers 36 (1998) 41-49
resins as catalysts, However, studies of comparing the reactivity of the two double bonds in ENB have clarified that the Diels-Alder reaction takes place more readily on the norbomenyl ring double bond [l-3]. It is also reported that ENB undergoes addition of sulphur on the ring double bond [4], whereas singlet oxygen [5], peroxy acids [6] and chlorosulphonyl isocyanate [7] add to ENB on the ethylidene group. Moreover, the Prins reaction takes place on both double bonds in ENB [8] and in addition, ENB undergoes hydroformylation on its double bonds [9]. Bobyleva et al. [lo] have reported the addition of organic acids to ENB in the presence of homogeneous catalysts. The rates of addition of acetic acid and trifluoroacetic acid to unsaturated bridging bicyclic hydrocarbons was studied by Bobyleva et al. [ 111. Reactions of 5-vinyl-2norbomene and 5-ethylidene-2-norbomene with formic acid were reported by Inoue et al. [ 121. Saha and Sharma [13] have reported the esterification of formic acid with another cyclic olefinic compound, viz. cyclohexene in batch and distillation column reactors. Very recently, the reaction of dicyclopentadiene and formic acid with and without cation exchange resins as catalysts was studied by Saha and Sharma [ 141. In any event, earlier studies as stated above have not clearly revealed the chemical behaviour of ENB towards the esterification reaction using heterogeneous catalysts from the standpoint of regio- and stereochemistry. This has stimulated to study this reaction in detail with cation exchange resins as catalysts. 3. Experimental 3.1. Materials The raw materials used in the present work were of very high purity (assay >99%). ENB and tert-butyl catechol were obtained from Herdillia Unimers (India). Formic acid and toluene were supplied by s.d. Fine Chem. (India). Physical properties of ENB are given in Table 1. Indion 130 was obtained from Ion Exchange (India) and
Table 1 Physical properties of ENB Melting/freezing range Boiling point Decomposition temperature Flash point Auto-ignition temperature Odour Physical form (at 293 K) Density (at 293 K) viscosity (at 293 K)
193 K 421 K 2573 K 302 K 545 K pungent odour colourless liquid 896 kg/m3 1.1 m Pa s-’
Amberlyst 15 and Amberlite IR 120 were obtained from Fluka (Switzerland). Clay catalyst, Engelhard F 24 was obtained from Engelhard (USA). Cation exchange resin and acid-treated clay catalysts were washed with acetone and dried at 373 K under vacuum (l-2 mmHg) for 6 h before use. Physical properties of these catalysts are listed in Table 2. 3.2. Apparatus and procedure Reactions were carried out in a 1 x 10h4 m3 autoclave (Parr Instruments, USA) with an internal diameter of 0.055 m equipped with a temperature controller. The temperature was maintained at f 1 K of the desired value. A four-bladed pitched turbine impeller was employed for agitation. In a typical experiment, 50 g of ENB, formic acid (in the required molar ratio), toluene (50% w/w) and 2.5 g of catalyst were charged in the reactor. The temperature was raised to the desired value, taking care to agitate only the liquid and not suspending the catalyst. The agitation was increased sufficiently to suspend the catalyst particles completely when the desired temperature was reached. This was noted as the starting time of the reaction. Samples were withdrawn at a regular intervals and analysed. 3.3. Analysis Analysis of the samples was done in a gas chromatograph with a flame ionisation detector. A stainless-steel column 4 m long having an internal diameter of 2 mm and packed with 10%
B. Saha/Reactive
Table 2 Physical properties Physical
of different
43
Polymers 36 (1998) 41-49
catalysts
property
Shape Size (mm; min. 90%) Internal surface area (m’/g) Weight capacity (meq/g) Crosslinking density (% DVB) Porosity (~01%) Temperature stability (K) ND = data not available.
& Functional
Amberlyst- 15 (macroporous)
Indion 130 (macroporous)
Engelhard F-24 (acid-treated clay)
Amberlite (gelular)
beads 0.5 45.0 4.15 20-25 36.0 393
beads 0.55 ND 4.8 ND ND 393
granular 0.66 350.0 0.3 NA 32.0 ND
beads 0.5 ND 4.4 8 no permanent 393
-
IR- 120
porosity
NA = Not applicable.
OV- 17 supported on chromosorb WHP was used for the analysis. The injector and the detector temperature were kept at 523 K and the oven temperature was kept isothermal at 498 K. Nitrogen was passed as the carrier gas and its flow rate was maintained at 0.5 ml/s. Reaction samples were washed with excess water to remove unreacted formic acid and the organic layer was dried over anhydrous sodium sulphate before injecting into the gas chromatograph. 4. Results and discussion
4.1. Elimination of mass transfer limitations 4.1. I. Eflect of speed of agitation Some of the preliminary experiments showed that there was no effect of speed of agitation in the range of 12 to 20 rps on the overall rate of reaction. Hence, all the reactions were carried out at 18.33 rps, ensuring that there was no external mass transfer resistances. It was also confirmed that no attrition of the resin particle took place under the experimental conditions employed for this work. 4.1.2. Eflect of particle size The catalyst particles obtained from the manufacturer were screened and two different fractions were employed to comment on the macropore diffusional limitation. It is to be noted that the effect of particle size was studied using macroporous resins, Indion 130 and Amberlyst 15 as catalysts and no change in the rate of reaction
was observed within the range of particle size from 0.3 to 0.6 mm. Gelular resin (e.g. Amberlite IR 120) was not employed for this purpose. In the presence of polar reactant, swelling occurs leading to the easy accessibility of the reactants to the active sites and free mobility of all the components. It has been shown ~by many investigators that for reactions such as hydration of isobutylene and etherification of isobutylene with methanol, the microgel effectiveness factor for macroporous resin is unity [ 15-171. Sundmacher and Hoffmann [ 181 and Berg and Harris [ 191 have showed for MTBE synthesis lcatalysed by a macroporous resin that it is possible to find for larger particles effectiveness factors greater than unity caused by internal mass transfer limitations. A plateau of particle size withithe same effectiveness factor, perhaps in the higher range of catalyst particle (above 0.6 mm), is ~possible. Hence, it is necessary to conduct experibents for smaller particles (0.3 mm and less) for which it is sure that there is no internal mass transfer resistance [20]. Since the rate of reactkon is independent on the range of particle size Iexplored, it is confirmed that internal mass transport by diffusion in macropore does not occur tiithin this range. 4.2. Mechanism of reaction ENB reacts with formic acid to ive three predominant products, viz. 6-ethylide,gne-exo-2norbomyl formate (A), 5-ethylidene-+o-2-norbomyl formate (B) and 6-ethyltricyclo[2.2.1 .02,6]
B. Saha/Reactive & Functional Polymers 36 (1998) 41-49
44
+
Sethylidene-2-norbornene
HCOOH -
formic
IER
?&
‘7
acid 6-ethylidene-exe-2-norbornyl formate (A)
dOCH0 Sethylidene-exo-2-norbomyl formate (B)
+ OCHO 6-ethyltricyclo[2.2.1.02~6]heptan (C) -3-yl-formate Fig. 1. Network of the reaction between ENB and formic acid.
heptan-3-yl for-mate (C). In uncatalysed esterification, the formic acid molecule itself acts as a proton donor, whereas in the presence of cation exchange resin, which acts as a proton donor, it is assumed that this esterification reaction involves addition of a proton to the ENB molecule to form a carbonium-ion-like intermediate. The latter is simultaneously attacked by the lone pair of electrons of the oxygen atom of the hydroxyl group of the formic acid molecule to form the corresponding ester. The reaction network is shown in Fig. 1. Three structural isomers, A, B and C, can be distinguished readily by gas chromatography (GC). The product ratio of A + B to C was found to be -70 : 30. The GC reports also show that the ratio of A to B is 85 : 15. The mechanism of formation of A and B from ENB and formic acid is shown in Fig. 2. This particular positional orientation can be explained by preferential attack of H+ upon C-2 of ENB molecule, since C-2 (and C-8) has the richest electron density in the highest occupied frontier orbital of the homoconjugated diene system consisted of C-2, C-3, C-5 and C-8. Thus, C-2 would be the site of attack by electrophile with its LUMO [21]. The preference of exe-addition can be best explained by both the perturbation effect on the planarity of the rc system and the smaller steric hindrance [22].
In the formation of molecule C by the addition of formic acid to ENB, the following mechanistic explanation can be proposed. It is conceivable that protonation of ENB also occur on C-8 having the richest electron density as mentioned above would give the carbonium ion like intermediate or transition state (D) [23-251 as shown in Fig. 3. The molecular model of D suggests that the bond formation between C-2 and C-6 in D evidently renders the distance between C-2 and C-6 in D shorter than that between C-3 and C-5 in ENB molecule [26]. The torsional strain in D thus generated puts C-3 and C-5 in D far away from each other, resulting in diminishing the steric interference induced by en&-hydrogen attached to C-5 towards a nucleophilic attack by formate anion at C-3 from the en&-direction. Alternatively, it can be rationalised that once the carbonium ion like intermediate D is formed, there is no need for considerations of electronic distribution governed by the molecular orbital theory, thereby giving no special preference for the direction of the addition. 4.3. Uncatalysed reaction The reaction of ENB and 98% formic acid was conducted with toluene as solvent (50% w/w of the reaction mixture), and trace amount of tert-
B. Saha/Reactive
& Functional Polymers 36 (1998) 41-49
45
ENB HCOOH w
or
@I
(4
A = 6-ethylidene-exo-2-norbomyl
formate
B = Sethylidene-exo-2-norbomyl
formate
Fig. 2. Mechanism
+
A and B from ENB and formic acid.
of formation
H+
endo (D)
HCOOH G====== OCHO
((2 C = 6-ethyltricyclo[2.2.1.0*96~ Fig. 3. Mechanism
heptan-3-yl-formate
of formation
butyl catechol as polymerisation inhibitor. The polymerisation of ENB takes place at moderate temperature and at higher concentration, To get rid of this problem, the concentration of ENB (non-polar) was kept low by adding a suitable
of C from ENB and formic acid,
non-polar solvent, viz. toluene, during the course of reaction. Moreover, due to the diff rence in boiling point of toluene with those of o $ er components present in the reaction mixture, iit can be easily separated by fractional vacuum distillation.
46
B. Saha/Reactive
& Functional Polymers 36 (1998) 41-49
At 1: 1 mole ratio of ENB to 98% formic acid at 373 K, -60% conversion of ENB was realised after 5 h of reaction. This shows that formic acid itself in the reaction medium can catalyse the reaction. It is a well known concept that the mechanism of catalysis would have to be such that the free energy of activation is lowered by the presence of a catalyst. A catalyst is effective in increasing the rate of a reaction because it makes possible an alternative mechanism, each step of which has a lower free energy of activation than that for the uncatalysed process. It is worth noting that the position of an equilibrium in a reversible reaction is not changed by the presence of a catalyst. When macroporous cation exchange resin (e.g. Indion 130) was employed for this reaction, -77% conversion of ENB was observed at 373 K for 1 : 1 mole ratio of ENB to formic acid after 5 h of reaction. Hence, it is evident that the effect of catalyst is significant for this reaction.
+****
ENB:FA 1:4 1:l
?? ummmENB:FA
2
4
Time
6
(h)
Fig. 4. Effect of mole ratio on the reaction of ENB with formic acid. Catalyst, Indion 130; catalyst loading, 10% (w/w); temperature, 373 K.
60
4.4. Reaction catalysed by ion exchange resins and clays 4.4.1. Effect of mole ratio The mole ratio of ENB to formic acid was varied from 1: 1 to 1 : 4 using Indion 130 (10% w/w) catalyst at 373 K. For 1 : 1 mole ratio of the reactants, 76.7% conversion of ENB was observed after 5 h of reaction, whereas for 1 : 4 mole ratio of the reactants, 99.4% conversion of the ENB was realised under otherwise identical conditions. Therefore, an increase in conversion of ENB with an increase in the mole ratio of formic acid to ENB (limiting reactant) reveals that the reaction is limited by equilibrium. Hence, to increase the conversion of ENB, all further experiments were conducted at a mole ratio of 1 : 4 of ENB to formic acid. This is shown in Fig. 4. 4.4.2. Effect of catalyst The effect of catalyst was studied by carrying out the reaction at 1 : 4 mole ratio of ENB to 98% formic acid at 373 K with Indion 130, Amberlyst 15, Amberlite IR 120 and Engelhard F 24 as cat-
OY
Indion 130 Amberlyst 15 Amberlite IR 120 En&hard F 24
***++ 00000 aaaah 00000
20
0
I
1
I
2 Time
I
I
4
5
(2)
Fig. 5. Effect of catalyst on the reaction on ENB with formic acid. Catalyst loading, 10% (w/w); mole ratio (ENB : FA), 1: 4; temperature, 373 K.
alysts (10% w/w) using toluene as solvent (50% w/w) with trace amount of tert-butyl catechol as polymerisation inhibitor. As shown in Fig. 5, after 5 h of reaction, maximum conversion of ENB of -99% was obtained with Indion 130 and lowest conversion of -78% was observed with Engelhard F 24. Both Indion 130 and Amberlyst 15 showed similar level of conversion. Both In-
B. Saha/Reactive
& Functional
dion 130 and Amberlyst 15 (macroporous type of ion exchange resin) have higher capacity than Engelhard F 24 and hence the lower catalytic activity of the latter was observed. For gelular Amberlite IR 120, ‘swelling’ is necessary for the catalytic activity of the catalyst to be appreciable [27]. It is possible that the reactants do not ‘swell’ the catalyst substantially and thus exhibit lower catalytic activity. Furthermore, the lesser exchange capacity (meq/g) of Amberlite IR 120 compared to macroporous resins, Indion 130 and Amberlyst 15, may also suppress the conversion of ENB in the esterification reaction. 4.4.3. Effect of catalyst loading The effect of catalyst loading was studied with 1 : 4 mole ratio of ENB to formic acid at 373 K using Indion 130 catalyst with toluene as solvent (50% w/w) and tert-butyl catechol as polymerisation inhibitor. It is clear from Fig. 6 that the effect of catalyst loading is not much significant on the conversion of ENB. With an increase in catalyst loading from 5% to lo%, the conversion of ENB increased marginally from 98.3% to 99.4% with slight reduction on the selectivity towards esters (A and B).This can be attributed to the fact that beyond a certain catalyst loading,
17
Polymers 36 (1998) 41-49
there exists an excess of catalyst sites than actually required by the reactant molecules and hence there is levelling off of the reaction rate [28]. 4.4.4. E#ect of temperature The effect of temperature was studied at 1 : 4 mole ratio of ENB to 98% formic acid using 10% Indion 130 catalyst with toluene as solvent using trace amount of tert-butyl catechol as polymerisation inhibitor. Fig. 7 shows that with an increase in temperature from 353 to 373 K, the conversion of ENB increased from 85.5% to 99.4%, after 5 h of reaction. A typicall concentration profile of various components at 363 K for 1 : 4 mole ratio of ENB to formic acid using Indion 130 as catalyst is shown in Fig. 8,. 4.4.5. Effect of concentration oj’formic acid The effect of concentration of formic acid in the reaction with ENB was studied at I : 4 mole ratio of ENB to formic acid (concentration 90% and 99%) at 373 K with Indion 130 cataIyst (10% w/w) using trace amount of tert-butyl catechol as polymerisation inhibitor. It was observed that with an increase in the concentration of formic acid from 90% to 99%, the conversion of ENB was increased from -86% to -99% and the se100
****i373
04 0
1
4
2
Time
(h”,
Fig. 6. Effect of catalyst loading on the reaction of ENB with formic acid. Catalyst. Indion 130; mole ratio (ENB :FA), 1 :4, temperature, 373 K.
0
1
K
I
4
i
Time
5
(h”,
Fig. 7. Effect of temperature on the reaction of ENB #with formic acid. Catalyst, Indion 130; catalyst loading. 10% (W/W): mole ratio (ENB : FA). 1 : 4.
B. SahdReactive
& Functional Polymers 36 (1998) 41-49
?? =mmm
00000
56 g 3
**+r* 00000 WEster
ENB Formic
acic A
00000 Ester ?? ammmEster
6
DCPD ENB
i
B C
4
0 0
1
2
3
Time
4
5
6
(h)
Fig. 8. Concentration profile of various components. Catalyst, Indion 130; catalyst loading, 10% (w/w); mole ratio (ENB : FA), 1: 4; temperature, 373 K.
lectivity towards the ester A and B increased from -70% to -73%. At lower concentration of formic acid, for every single water molecule present in the resin phase, four sulphonic acid groups can be attached to it [29]. Thus, the availability of the active sites decreases which results in a decrease in the rate of reaction. Moreover, water molecule permeates easily inside the pore of ion exchangers resulting in adsorption of water in the resin matrix [30]. Hence, the presence of larger amount of water molecule in the reaction mixture decreased the catalytic activity of the resin particle, which in turn reduces the conversion of ENB . 4.4.6. Catalyst reusability Macroporous cation exchange resin, Indion 130, was reused up to five times for this reaction. However, no significant change in the conversion of ENB and the selectivity towards the ester was observed. Hence, we can infer that Indion 130 can be repeatedly used for this reaction without losing its catalytic activity. 4.4.7. Comparison of the rate of esteri$cation between DCPD and that of ENB with formic acid The comparison of the rate of esterification
between DCPD and ENB with 98% formic acid
0
1
2
Tim:
4
5
(h)
Fig. 9. Comparison of reaction rate between DCPD and ENB with formic acid. Catalyst, Indian 130; catalyst loading, 10% (w/w); mole ratio (DCPD/ENB : FA), 1: 4; temperature, 373 K.
is shown in Fig. 9. At 373 K with Indion 130 catalyst (10% w/w), at 1 : 1 mole ratio of DCPD to 98% formic acid using toluene (50% w/w) as solvent, 82.5% conversion of DCPD was realised, whereas -77% conversion of ENB was observed under otherwise identical conditions. Moreover, in the reaction with DCPD, the selectivity towards dicyclopentadienyl formate was lOO%, whereas for ENB -72% selectivity towards esters A and B was observed. Hence, in the reaction with ENB, essentially three products were formed. This proves that the mechanism of addition of carboxylic acid to the DCPD molecule is entirely different from that of ENB. In DCPD molecule, no addition of formic acid took place in the second double bond when excess of formic acid was added to the reaction medium. It was mentioned by Bruson and Riener [31] that when hydrogen chloride was bubbled through DCPD molecule in the presence of a small amount of water at 333-343 K, the addition of the hydrogen chloride took place readily to form mono hydrochloride. They also noted that this reaction product still contained one double bond, even though an excess of hydrogen chloride be used.
B. Saha/Reactive
& Functional
5. Conclusions The esterification of formic acid with ENB can be conveniently conducted in the presence of macroporous cation exchange resins, Indian 130 and Amberlyst 15 as catalysts. The cation exchange resins, Indion 130 and Amberlyst 15, were found to be better catalysts as compared to the acid-treated clay catalyst. The optimum conditions for selectively obtaining these esters have been delineated. The presence of larger amount of water molecule in the catalyst particle is detrimental to the rate of reaction. The macroporous resin, Indion 130, can be repeatedly used (five times) without losing its catalytic activity towards the esterification reaction. The rate of esterification between formic acid and DCPD was higher than that with ENB.
Acknowledgements The author is indebted to Professor M.M. Sharma for providing all sorts of facilities and giving valuable suggestions for this work. The author wishes to thank the University Grants Commission, New Delhi for awarding a Senior Research Fellowship during this work.
References [II S. Tsuchida, M. Ogawa, Sekiyu Gakkaishi 15 (1972) 193. PI N. Nakai, S. Iwasa, S. Ishii, M. Ogawa, Shikizai Kyokaishi 51 (1978) 132. [31 H. Kawachi, T. Akima, Jpn. Kokai 72 (1972) 31970. [41 T.C. Shields. A.N. Kurtz, J. Am. Chem. Sot. 91 (1969) 5415. [51 W.R. Adams, D.J. Trecker, Tetrahedron 28 (1972) 2361. [61 T.C. Shields, Can. J. Chem. 49 (1971) 1142.
Polymers 36 (1998) 41-49
49
[71 E.J. Morikoni, C.C. Jalandoni, J. Org. Chem. 35 (1970) 2073. VI VI. Isagulyants, V.R. Melikyan, V.V. Pokrovskaya. Doki Akad. Nauk Arm. SSR 55 (1972) 179. I91 H. Fischer, F. Roehrscheid, L. Brinkman, E. Reske, Ger. Offen. (1973) 2,163,753; cf., Chem. Abstr. 78 (1973) 160329. [lOI A.A. Bobyleva, N.F. Dibitskaya, N.A. Belikova, T. Pehk, E. Lippmaa, A.F. Plate, Zh. Org. Khim. 13 (1977) 2085. N.A. Btlikova, A.F. [ill A.A. Bobyleva, A.V. Dzhigirkhanova, Plate, Khim. 18 (1977) 247. H21 Y. Inoue, F. Tanimoto, H. Kitano, Bull. Chem. Sot. Jpn. 57 (1984) 2468. [I31 B. Saha, M.M. Sharma, React. Funct. Polym. 28 (1996) 263. 1141 B. Saha, M.M. Sharma, React. Funct. Polym. 34 (1997) 161. 1151 P. Leung, C. Zorilla, F. Recasens, J.M. Smith, AIChE J. 32 (1986) 1839. U61 S.K. Ihm, M.J. Chung, K.Y. Park, Ind. Eng. Chem. Res. 27 (1988) 41. [I71 A. Rehfinger, U. Hoffmann, Chem. Eng. Technol. 13 (1990) 150. [181 K. Sundmacher. U. Hoffmann, Chem. Eng. Sci. 49 (1994) 3077. [I91 D.A. Berg, T.J. Harris, Ind. Eng. Chem. Res. 32 11993) 2147. WI A. Rehfinger, Il. Hoffmann, Chem. Eng. Sci. 45 (1990) 1605. WI I. Fleming (Ed.), in: Frontier Orbitals and Organic Chemical Reactions, Wiley, New York, 1976, p. 27. WI H.C. Brown, W.J. Hammer, J.H. Kawakami, I. Rothberg, D.L. Vander Jagt, J. Am. Chem. Sot. 89 (1967) 6381. 1231 J.D. Roberts, CC. Lee, J. Am. Chem. Sot. 73 (1951) 5009. t241 J.D. Roberts, W. Bonnett, J. Am. Chem. Sot. 76 (1954) 4623. P51 S. Winstein, M. Shatavsky, Chem. Ind. 56 (1956) 7. Ml G. Wipff, K. Morokuma, Tetrahedron Lett. 21 (1980) 4445. u71 A. Chakrabarti, M.M. Sharma, React. Polym. 20 (1993) 1. WI J.M. Smith, Chemical Engineering Kinetics, 3rd ed., McGraw-Hill. Singapore, 198 1. Interaction: In~291 G. Zundel, Hydration and Intramolecular frared Investigations with Polyelectrolytic membranes, Academic Press, New York, 1969. [301 M. Ibora, C. Fite, J. Tejero, E Cunill, J.F. Izquierdo. React. Polym. 21 (1993) 65. [311 H.A. Bruson. T.W. Riener, J. Am. Chem. Sot. 67 (1945) 1178.
ELSEVIER
Reactive
& Functional
Polymers
36 (1998) 41-49
Reaction of Sethylidene-2-norbomene with formic acid: cation exchange resins as catalysts Basudeb Saha * University Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India Received
28 April 1997: revised version received
15 July 1997; accepted
16 July 1997
Abstract
The esterification of formic acid with ethylidene norbomene (ENB), i.e. 5ethylidene bicyclo[2.2.l]hept-2-ene was carried out with and without cation exchange resins as catalysts. The effect of various parameters e.g. speed of agitation, catalyst loading, mole ratio of the reactants, temperature, concentration of formic acid in aqueous solutions were studied to optimise the reaction conditions; the reusability of catalysts was also studied. A comparison of the rate of esteri$cation between ENB and dicyclopentadiene (DCPD), with formic acid was made. 0 1998 Elsevier Science B.V. All righ& reserved. Keywords:
Esterification; Cation exchange resin; Formic acid; 5-Ethylidene bicyclo[2.2.l]hept-2-ene; 6-Ethyhdene-exo2-norbomyl formate; S-Ethylidene-exo-2-norbomyl formate; 6-Ethyltricyclo[2.2.1.02~6]heptan-3-yl formate
1. Introduction Esterification of ethylidene norbornene (ENB), i.e., 5-ethylidene bicyclo[2.2.l]hept-2-ene, with different aliphatic carboxylic acids is of academic interest and industrial relevance. ENB is a commercially available compound having an olefinic side chain and its esters are useful as plasticizers. Very little work has been done on the chemical properties of ENB and, especially, detailed investigations concerning the reactivity difference of the two double bonds in ENB have not yet been made using cationic exchange resins as catalysts. Some of the major advantages of using ion exchange resins as catalysts in this reaction are ease of separation of the catalyst from the reaction mixture, milder reaction conditions, higher yield * Corresponding ical Engineering, icestershire. LEl
author. Present Loughborough 1 3TU. UK.
address: Department of ChemUniversity. Loughborough, Le-
1381-5148/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PIZS1381-5148(97)00.112-0
and selectivity towards the ester, higher catalyst activity, catalyst reusability etc. The purpose of the present work is to obtain a better understanding of the type of stereoelectronic influence exerted on the reactivity of the norbomenyl ring double bond and to determine which of the two double bonds in ENB undergoes certain addition reaction preferentially. This work was undertaken to examine the performance of macroporous and microreticular ion exchange resins and acid-treated clay in the esterification of ENB with formic acid and to delineate the best operating conditions for predominantly producing the corresponding mono ester’of ENB. 2. Previous studies A careful survey of literature shows that no detailed studies have been made on the reaction of ENB with formic acid using cation exchange
42
B. Saha/Reactive & Functional Polymers 36 (1998) 41-49
resins as catalysts, However, studies of comparing the reactivity of the two double bonds in ENB have clarified that the Diels-Alder reaction takes place more readily on the norbomenyl ring double bond [l-3]. It is also reported that ENB undergoes addition of sulphur on the ring double bond [4], whereas singlet oxygen [5], peroxy acids [6] and chlorosulphonyl isocyanate [7] add to ENB on the ethylidene group. Moreover, the Prins reaction takes place on both double bonds in ENB [8] and in addition, ENB undergoes hydroformylation on its double bonds [9]. Bobyleva et al. [lo] have reported the addition of organic acids to ENB in the presence of homogeneous catalysts. The rates of addition of acetic acid and trifluoroacetic acid to unsaturated bridging bicyclic hydrocarbons was studied by Bobyleva et al. [ 111. Reactions of 5-vinyl-2norbomene and 5-ethylidene-2-norbomene with formic acid were reported by Inoue et al. [ 121. Saha and Sharma [13] have reported the esterification of formic acid with another cyclic olefinic compound, viz. cyclohexene in batch and distillation column reactors. Very recently, the reaction of dicyclopentadiene and formic acid with and without cation exchange resins as catalysts was studied by Saha and Sharma [ 141. In any event, earlier studies as stated above have not clearly revealed the chemical behaviour of ENB towards the esterification reaction using heterogeneous catalysts from the standpoint of regio- and stereochemistry. This has stimulated to study this reaction in detail with cation exchange resins as catalysts. 3. Experimental 3.1. Materials The raw materials used in the present work were of very high purity (assay >99%). ENB and tert-butyl catechol were obtained from Herdillia Unimers (India). Formic acid and toluene were supplied by s.d. Fine Chem. (India). Physical properties of ENB are given in Table 1. Indion 130 was obtained from Ion Exchange (India) and
Table 1 Physical properties of ENB Melting/freezing range Boiling point Decomposition temperature Flash point Auto-ignition temperature Odour Physical form (at 293 K) Density (at 293 K) viscosity (at 293 K)
193 K 421 K 2573 K 302 K 545 K pungent odour colourless liquid 896 kg/m3 1.1 m Pa s-’
Amberlyst 15 and Amberlite IR 120 were obtained from Fluka (Switzerland). Clay catalyst, Engelhard F 24 was obtained from Engelhard (USA). Cation exchange resin and acid-treated clay catalysts were washed with acetone and dried at 373 K under vacuum (l-2 mmHg) for 6 h before use. Physical properties of these catalysts are listed in Table 2. 3.2. Apparatus and procedure Reactions were carried out in a 1 x 10h4 m3 autoclave (Parr Instruments, USA) with an internal diameter of 0.055 m equipped with a temperature controller. The temperature was maintained at f 1 K of the desired value. A four-bladed pitched turbine impeller was employed for agitation. In a typical experiment, 50 g of ENB, formic acid (in the required molar ratio), toluene (50% w/w) and 2.5 g of catalyst were charged in the reactor. The temperature was raised to the desired value, taking care to agitate only the liquid and not suspending the catalyst. The agitation was increased sufficiently to suspend the catalyst particles completely when the desired temperature was reached. This was noted as the starting time of the reaction. Samples were withdrawn at a regular intervals and analysed. 3.3. Analysis Analysis of the samples was done in a gas chromatograph with a flame ionisation detector. A stainless-steel column 4 m long having an internal diameter of 2 mm and packed with 10%
B. Saha/Reactive
Table 2 Physical properties Physical
of different
43
Polymers 36 (1998) 41-49
catalysts
property
Shape Size (mm; min. 90%) Internal surface area (m’/g) Weight capacity (meq/g) Crosslinking density (% DVB) Porosity (~01%) Temperature stability (K) ND = data not available.
& Functional
Amberlyst- 15 (macroporous)
Indion 130 (macroporous)
Engelhard F-24 (acid-treated clay)
Amberlite (gelular)
beads 0.5 45.0 4.15 20-25 36.0 393
beads 0.55 ND 4.8 ND ND 393
granular 0.66 350.0 0.3 NA 32.0 ND
beads 0.5 ND 4.4 8 no permanent 393
-
IR- 120
porosity
NA = Not applicable.
OV- 17 supported on chromosorb WHP was used for the analysis. The injector and the detector temperature were kept at 523 K and the oven temperature was kept isothermal at 498 K. Nitrogen was passed as the carrier gas and its flow rate was maintained at 0.5 ml/s. Reaction samples were washed with excess water to remove unreacted formic acid and the organic layer was dried over anhydrous sodium sulphate before injecting into the gas chromatograph. 4. Results and discussion
4.1. Elimination of mass transfer limitations 4.1. I. Eflect of speed of agitation Some of the preliminary experiments showed that there was no effect of speed of agitation in the range of 12 to 20 rps on the overall rate of reaction. Hence, all the reactions were carried out at 18.33 rps, ensuring that there was no external mass transfer resistances. It was also confirmed that no attrition of the resin particle took place under the experimental conditions employed for this work. 4.1.2. Eflect of particle size The catalyst particles obtained from the manufacturer were screened and two different fractions were employed to comment on the macropore diffusional limitation. It is to be noted that the effect of particle size was studied using macroporous resins, Indion 130 and Amberlyst 15 as catalysts and no change in the rate of reaction
was observed within the range of particle size from 0.3 to 0.6 mm. Gelular resin (e.g. Amberlite IR 120) was not employed for this purpose. In the presence of polar reactant, swelling occurs leading to the easy accessibility of the reactants to the active sites and free mobility of all the components. It has been shown ~by many investigators that for reactions such as hydration of isobutylene and etherification of isobutylene with methanol, the microgel effectiveness factor for macroporous resin is unity [ 15-171. Sundmacher and Hoffmann [ 181 and Berg and Harris [ 191 have showed for MTBE synthesis lcatalysed by a macroporous resin that it is possible to find for larger particles effectiveness factors greater than unity caused by internal mass transfer limitations. A plateau of particle size withithe same effectiveness factor, perhaps in the higher range of catalyst particle (above 0.6 mm), is ~possible. Hence, it is necessary to conduct experibents for smaller particles (0.3 mm and less) for which it is sure that there is no internal mass transfer resistance [20]. Since the rate of reactkon is independent on the range of particle size Iexplored, it is confirmed that internal mass transport by diffusion in macropore does not occur tiithin this range. 4.2. Mechanism of reaction ENB reacts with formic acid to ive three predominant products, viz. 6-ethylide,gne-exo-2norbomyl formate (A), 5-ethylidene-+o-2-norbomyl formate (B) and 6-ethyltricyclo[2.2.1 .02,6]
B. Saha/Reactive & Functional Polymers 36 (1998) 41-49
44
+
Sethylidene-2-norbornene
HCOOH -
formic
IER
?&
‘7
acid 6-ethylidene-exe-2-norbornyl formate (A)
dOCH0 Sethylidene-exo-2-norbomyl formate (B)
+ OCHO 6-ethyltricyclo[2.2.1.02~6]heptan (C) -3-yl-formate Fig. 1. Network of the reaction between ENB and formic acid.
heptan-3-yl for-mate (C). In uncatalysed esterification, the formic acid molecule itself acts as a proton donor, whereas in the presence of cation exchange resin, which acts as a proton donor, it is assumed that this esterification reaction involves addition of a proton to the ENB molecule to form a carbonium-ion-like intermediate. The latter is simultaneously attacked by the lone pair of electrons of the oxygen atom of the hydroxyl group of the formic acid molecule to form the corresponding ester. The reaction network is shown in Fig. 1. Three structural isomers, A, B and C, can be distinguished readily by gas chromatography (GC). The product ratio of A + B to C was found to be -70 : 30. The GC reports also show that the ratio of A to B is 85 : 15. The mechanism of formation of A and B from ENB and formic acid is shown in Fig. 2. This particular positional orientation can be explained by preferential attack of H+ upon C-2 of ENB molecule, since C-2 (and C-8) has the richest electron density in the highest occupied frontier orbital of the homoconjugated diene system consisted of C-2, C-3, C-5 and C-8. Thus, C-2 would be the site of attack by electrophile with its LUMO [21]. The preference of exe-addition can be best explained by both the perturbation effect on the planarity of the rc system and the smaller steric hindrance [22].
In the formation of molecule C by the addition of formic acid to ENB, the following mechanistic explanation can be proposed. It is conceivable that protonation of ENB also occur on C-8 having the richest electron density as mentioned above would give the carbonium ion like intermediate or transition state (D) [23-251 as shown in Fig. 3. The molecular model of D suggests that the bond formation between C-2 and C-6 in D evidently renders the distance between C-2 and C-6 in D shorter than that between C-3 and C-5 in ENB molecule [26]. The torsional strain in D thus generated puts C-3 and C-5 in D far away from each other, resulting in diminishing the steric interference induced by en&-hydrogen attached to C-5 towards a nucleophilic attack by formate anion at C-3 from the en&-direction. Alternatively, it can be rationalised that once the carbonium ion like intermediate D is formed, there is no need for considerations of electronic distribution governed by the molecular orbital theory, thereby giving no special preference for the direction of the addition. 4.3. Uncatalysed reaction The reaction of ENB and 98% formic acid was conducted with toluene as solvent (50% w/w of the reaction mixture), and trace amount of tert-
B. Saha/Reactive
& Functional Polymers 36 (1998) 41-49
45
ENB HCOOH w
or
@I
(4
A = 6-ethylidene-exo-2-norbomyl
formate
B = Sethylidene-exo-2-norbomyl
formate
Fig. 2. Mechanism
+
A and B from ENB and formic acid.
of formation
H+
endo (D)
HCOOH G====== OCHO
((2 C = 6-ethyltricyclo[2.2.1.0*96~ Fig. 3. Mechanism
heptan-3-yl-formate
of formation
butyl catechol as polymerisation inhibitor. The polymerisation of ENB takes place at moderate temperature and at higher concentration, To get rid of this problem, the concentration of ENB (non-polar) was kept low by adding a suitable
of C from ENB and formic acid,
non-polar solvent, viz. toluene, during the course of reaction. Moreover, due to the diff rence in boiling point of toluene with those of o $ er components present in the reaction mixture, iit can be easily separated by fractional vacuum distillation.
46
B. Saha/Reactive
& Functional Polymers 36 (1998) 41-49
At 1: 1 mole ratio of ENB to 98% formic acid at 373 K, -60% conversion of ENB was realised after 5 h of reaction. This shows that formic acid itself in the reaction medium can catalyse the reaction. It is a well known concept that the mechanism of catalysis would have to be such that the free energy of activation is lowered by the presence of a catalyst. A catalyst is effective in increasing the rate of a reaction because it makes possible an alternative mechanism, each step of which has a lower free energy of activation than that for the uncatalysed process. It is worth noting that the position of an equilibrium in a reversible reaction is not changed by the presence of a catalyst. When macroporous cation exchange resin (e.g. Indion 130) was employed for this reaction, -77% conversion of ENB was observed at 373 K for 1 : 1 mole ratio of ENB to formic acid after 5 h of reaction. Hence, it is evident that the effect of catalyst is significant for this reaction.
+****
ENB:FA 1:4 1:l
?? ummmENB:FA
2
4
Time
6
(h)
Fig. 4. Effect of mole ratio on the reaction of ENB with formic acid. Catalyst, Indion 130; catalyst loading, 10% (w/w); temperature, 373 K.
60
4.4. Reaction catalysed by ion exchange resins and clays 4.4.1. Effect of mole ratio The mole ratio of ENB to formic acid was varied from 1: 1 to 1 : 4 using Indion 130 (10% w/w) catalyst at 373 K. For 1 : 1 mole ratio of the reactants, 76.7% conversion of ENB was observed after 5 h of reaction, whereas for 1 : 4 mole ratio of the reactants, 99.4% conversion of the ENB was realised under otherwise identical conditions. Therefore, an increase in conversion of ENB with an increase in the mole ratio of formic acid to ENB (limiting reactant) reveals that the reaction is limited by equilibrium. Hence, to increase the conversion of ENB, all further experiments were conducted at a mole ratio of 1 : 4 of ENB to formic acid. This is shown in Fig. 4. 4.4.2. Effect of catalyst The effect of catalyst was studied by carrying out the reaction at 1 : 4 mole ratio of ENB to 98% formic acid at 373 K with Indion 130, Amberlyst 15, Amberlite IR 120 and Engelhard F 24 as cat-
OY
Indion 130 Amberlyst 15 Amberlite IR 120 En&hard F 24
***++ 00000 aaaah 00000
20
0
I
1
I
2 Time
I
I
4
5
(2)
Fig. 5. Effect of catalyst on the reaction on ENB with formic acid. Catalyst loading, 10% (w/w); mole ratio (ENB : FA), 1: 4; temperature, 373 K.
alysts (10% w/w) using toluene as solvent (50% w/w) with trace amount of tert-butyl catechol as polymerisation inhibitor. As shown in Fig. 5, after 5 h of reaction, maximum conversion of ENB of -99% was obtained with Indion 130 and lowest conversion of -78% was observed with Engelhard F 24. Both Indion 130 and Amberlyst 15 showed similar level of conversion. Both In-
B. Saha/Reactive
& Functional
dion 130 and Amberlyst 15 (macroporous type of ion exchange resin) have higher capacity than Engelhard F 24 and hence the lower catalytic activity of the latter was observed. For gelular Amberlite IR 120, ‘swelling’ is necessary for the catalytic activity of the catalyst to be appreciable [27]. It is possible that the reactants do not ‘swell’ the catalyst substantially and thus exhibit lower catalytic activity. Furthermore, the lesser exchange capacity (meq/g) of Amberlite IR 120 compared to macroporous resins, Indion 130 and Amberlyst 15, may also suppress the conversion of ENB in the esterification reaction. 4.4.3. Effect of catalyst loading The effect of catalyst loading was studied with 1 : 4 mole ratio of ENB to formic acid at 373 K using Indion 130 catalyst with toluene as solvent (50% w/w) and tert-butyl catechol as polymerisation inhibitor. It is clear from Fig. 6 that the effect of catalyst loading is not much significant on the conversion of ENB. With an increase in catalyst loading from 5% to lo%, the conversion of ENB increased marginally from 98.3% to 99.4% with slight reduction on the selectivity towards esters (A and B).This can be attributed to the fact that beyond a certain catalyst loading,
17
Polymers 36 (1998) 41-49
there exists an excess of catalyst sites than actually required by the reactant molecules and hence there is levelling off of the reaction rate [28]. 4.4.4. E#ect of temperature The effect of temperature was studied at 1 : 4 mole ratio of ENB to 98% formic acid using 10% Indion 130 catalyst with toluene as solvent using trace amount of tert-butyl catechol as polymerisation inhibitor. Fig. 7 shows that with an increase in temperature from 353 to 373 K, the conversion of ENB increased from 85.5% to 99.4%, after 5 h of reaction. A typicall concentration profile of various components at 363 K for 1 : 4 mole ratio of ENB to formic acid using Indion 130 as catalyst is shown in Fig. 8,. 4.4.5. Effect of concentration oj’formic acid The effect of concentration of formic acid in the reaction with ENB was studied at I : 4 mole ratio of ENB to formic acid (concentration 90% and 99%) at 373 K with Indion 130 cataIyst (10% w/w) using trace amount of tert-butyl catechol as polymerisation inhibitor. It was observed that with an increase in the concentration of formic acid from 90% to 99%, the conversion of ENB was increased from -86% to -99% and the se100
****i373
04 0
1
4
2
Time
(h”,
Fig. 6. Effect of catalyst loading on the reaction of ENB with formic acid. Catalyst. Indion 130; mole ratio (ENB :FA), 1 :4, temperature, 373 K.
0
1
K
I
4
i
Time
5
(h”,
Fig. 7. Effect of temperature on the reaction of ENB #with formic acid. Catalyst, Indion 130; catalyst loading. 10% (W/W): mole ratio (ENB : FA). 1 : 4.
B. SahdReactive
& Functional Polymers 36 (1998) 41-49
?? =mmm
00000
56 g 3
**+r* 00000 WEster
ENB Formic
acic A
00000 Ester ?? ammmEster
6
DCPD ENB
i
B C
4
0 0
1
2
3
Time
4
5
6
(h)
Fig. 8. Concentration profile of various components. Catalyst, Indion 130; catalyst loading, 10% (w/w); mole ratio (ENB : FA), 1: 4; temperature, 373 K.
lectivity towards the ester A and B increased from -70% to -73%. At lower concentration of formic acid, for every single water molecule present in the resin phase, four sulphonic acid groups can be attached to it [29]. Thus, the availability of the active sites decreases which results in a decrease in the rate of reaction. Moreover, water molecule permeates easily inside the pore of ion exchangers resulting in adsorption of water in the resin matrix [30]. Hence, the presence of larger amount of water molecule in the reaction mixture decreased the catalytic activity of the resin particle, which in turn reduces the conversion of ENB . 4.4.6. Catalyst reusability Macroporous cation exchange resin, Indion 130, was reused up to five times for this reaction. However, no significant change in the conversion of ENB and the selectivity towards the ester was observed. Hence, we can infer that Indion 130 can be repeatedly used for this reaction without losing its catalytic activity. 4.4.7. Comparison of the rate of esteri$cation between DCPD and that of ENB with formic acid The comparison of the rate of esterification
between DCPD and ENB with 98% formic acid
0
1
2
Tim:
4
5
(h)
Fig. 9. Comparison of reaction rate between DCPD and ENB with formic acid. Catalyst, Indian 130; catalyst loading, 10% (w/w); mole ratio (DCPD/ENB : FA), 1: 4; temperature, 373 K.
is shown in Fig. 9. At 373 K with Indion 130 catalyst (10% w/w), at 1 : 1 mole ratio of DCPD to 98% formic acid using toluene (50% w/w) as solvent, 82.5% conversion of DCPD was realised, whereas -77% conversion of ENB was observed under otherwise identical conditions. Moreover, in the reaction with DCPD, the selectivity towards dicyclopentadienyl formate was lOO%, whereas for ENB -72% selectivity towards esters A and B was observed. Hence, in the reaction with ENB, essentially three products were formed. This proves that the mechanism of addition of carboxylic acid to the DCPD molecule is entirely different from that of ENB. In DCPD molecule, no addition of formic acid took place in the second double bond when excess of formic acid was added to the reaction medium. It was mentioned by Bruson and Riener [31] that when hydrogen chloride was bubbled through DCPD molecule in the presence of a small amount of water at 333-343 K, the addition of the hydrogen chloride took place readily to form mono hydrochloride. They also noted that this reaction product still contained one double bond, even though an excess of hydrogen chloride be used.
B. Saha/Reactive
& Functional
5. Conclusions The esterification of formic acid with ENB can be conveniently conducted in the presence of macroporous cation exchange resins, Indian 130 and Amberlyst 15 as catalysts. The cation exchange resins, Indion 130 and Amberlyst 15, were found to be better catalysts as compared to the acid-treated clay catalyst. The optimum conditions for selectively obtaining these esters have been delineated. The presence of larger amount of water molecule in the catalyst particle is detrimental to the rate of reaction. The macroporous resin, Indion 130, can be repeatedly used (five times) without losing its catalytic activity towards the esterification reaction. The rate of esterification between formic acid and DCPD was higher than that with ENB.
Acknowledgements The author is indebted to Professor M.M. Sharma for providing all sorts of facilities and giving valuable suggestions for this work. The author wishes to thank the University Grants Commission, New Delhi for awarding a Senior Research Fellowship during this work.
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Polymers 36 (1998) 41-49
49
[71 E.J. Morikoni, C.C. Jalandoni, J. Org. Chem. 35 (1970) 2073. VI VI. Isagulyants, V.R. Melikyan, V.V. Pokrovskaya. Doki Akad. Nauk Arm. SSR 55 (1972) 179. I91 H. Fischer, F. Roehrscheid, L. Brinkman, E. Reske, Ger. Offen. (1973) 2,163,753; cf., Chem. Abstr. 78 (1973) 160329. [lOI A.A. Bobyleva, N.F. Dibitskaya, N.A. Belikova, T. Pehk, E. Lippmaa, A.F. Plate, Zh. Org. Khim. 13 (1977) 2085. N.A. Btlikova, A.F. [ill A.A. Bobyleva, A.V. Dzhigirkhanova, Plate, Khim. 18 (1977) 247. H21 Y. Inoue, F. Tanimoto, H. Kitano, Bull. Chem. Sot. Jpn. 57 (1984) 2468. [I31 B. Saha, M.M. Sharma, React. Funct. Polym. 28 (1996) 263. 1141 B. Saha, M.M. Sharma, React. Funct. Polym. 34 (1997) 161. 1151 P. Leung, C. Zorilla, F. Recasens, J.M. Smith, AIChE J. 32 (1986) 1839. U61 S.K. Ihm, M.J. Chung, K.Y. Park, Ind. Eng. Chem. Res. 27 (1988) 41. [I71 A. Rehfinger, U. Hoffmann, Chem. Eng. Technol. 13 (1990) 150. [181 K. Sundmacher. U. Hoffmann, Chem. Eng. Sci. 49 (1994) 3077. [I91 D.A. Berg, T.J. Harris, Ind. Eng. Chem. Res. 32 11993) 2147. WI A. Rehfinger, Il. Hoffmann, Chem. Eng. Sci. 45 (1990) 1605. WI I. Fleming (Ed.), in: Frontier Orbitals and Organic Chemical Reactions, Wiley, New York, 1976, p. 27. WI H.C. Brown, W.J. Hammer, J.H. Kawakami, I. Rothberg, D.L. Vander Jagt, J. Am. Chem. Sot. 89 (1967) 6381. 1231 J.D. Roberts, CC. Lee, J. Am. Chem. Sot. 73 (1951) 5009. t241 J.D. Roberts, W. Bonnett, J. Am. Chem. Sot. 76 (1954) 4623. P51 S. Winstein, M. Shatavsky, Chem. Ind. 56 (1956) 7. Ml G. Wipff, K. Morokuma, Tetrahedron Lett. 21 (1980) 4445. u71 A. Chakrabarti, M.M. Sharma, React. Polym. 20 (1993) 1. WI J.M. Smith, Chemical Engineering Kinetics, 3rd ed., McGraw-Hill. Singapore, 198 1. Interaction: In~291 G. Zundel, Hydration and Intramolecular frared Investigations with Polyelectrolytic membranes, Academic Press, New York, 1969. [301 M. Ibora, C. Fite, J. Tejero, E Cunill, J.F. Izquierdo. React. Polym. 21 (1993) 65. [311 H.A. Bruson. T.W. Riener, J. Am. Chem. Sot. 67 (1945) 1178.