Base-catalyzed Ring-opening Reactions of Epoxides*

Transcription

1 Base-catalyzed Ring-opening Reactions of Epoxides* Yoshio Ishii** Shizuyoshi Sakai** and Tatsuo Sugiyama** Summary: The effects of solvents, catalysts (C), and reaction temperature on the base-catalyzed ring-opening reactions of phenyl glycidyl ether (E) and substituted phenols or benzoic acids (P) have been examined to discuss the mechanism. "Termolecular complex mechanism" proposed previously by us were related to "simple ionic mechanism" as of old. are clearly seen. Such results are not explained by "simple ionic mechanism", and "termolecular complex mechanism" becomes apposite. Which mechanism is operative is determined mainly by solvent, active hydrogen compound, and catalyst used. Ring-opening reactions of epoxides are of importance in the petrochemistry such as preparations of polyethers, polyglycols and non-ionic surfactants, or hardening of epoxy resins. Many studies had been done regarding these reactions1), but the mechanism remained as obscure one. Early in 1914, D. R. Boyd and E. R. Marle2) studied the base-catalyzed reactions, in which they examined the reactions of ethylene oxide or propylene oxide with phenols in aqueous ethanol using NaOH as catalyst, and found that these reactions were of the first-order kinetics about epoxide, and these reaction constants are negative. They suggested "simple and complete" ionic mechanism, which was hitherto generally accepted. We have studied the reactions of ethylene oxide and propylene oxide with active hydrogen compounds, and found that complete ionic mechanism fell into confusions in many cases. For example, in the absence of solvent, the third-order kinetics -de/dt=kecep held apparently in the re - actions between higher alcohols (P) and ethylene oxide or propylene oxide (E) in the presence of alcoxide catalyst (C; ROM), and the rates were affected by the change * Received November 14, ** Faculty of Engineering, Nagoya University. Furocho, Chikusaku, Nagoya, Japan. of R and M of alcoxide)3,4). And the rate of polyadditions of ethylene oxide to substituted phenols was also affected by the kinds of substituents5). These results show that the ionization in transition state is not "simple and complete" but "complex and incomplete". Now we have studied further the reactions of phenyl glycidyl ether (PGE) and phenols or benzoic acids in various solvents as a model reaction of epoxides. The results obtained show clearly that "simple ionic mechanism" of Boyd and Marle holds when the ring-opening of epoxide is carried out in polar and ionizing solvents such as ethanol, but not so in other solvents. In some cases, our "termolecular complex mechanism" holds. Which mechanism is operative is determined by reaction conditions, i. e., by the natures of solvents, catalysts, or reactants used. Reaction Schema and Kinetics Experimental data usually relate to bimolecular or termolecular kinetics, and generalizations of reaction schema are required for complete analysis of base-catalyzed ringopening reactions of epoxides. Our "termolecular complex mechanism" is assumed as below: Complex salt, Be, formed from free phenol and free base, attacks epoxide, E, and be-

2 Ishii, Sakai and Sugiyama: Base-catalyzed Ring-opening Reactions of Epoxides comes a complex compound [PEB]. Structure of transition complex [PEB] is difficult to be assured, but formula [I] discussed From (1), (2) and (3), we get the rate equations (4), (4-1), and (4-2). early5) is most probable at present. This formula resembles that of F. Patat6). Complex [PEB] converts immediately into substituted phenoxyethanol and free base. abse does not dissociate into free ions throughout the reaction. In "simple ionic mechanism" suggested by Boyd and Marle2), complex salt, Bc, dissociates into free phenoxide ion Pi and its counter cation Bi, and the phenoxide ion reacts with epoxide to produce substituted phenoxyethoxide ion, which reacts with phenol and gives phenoxyethanol. The transition-state complex will be written as [II]. However, ringopening reactions can be generally explained as both mechanisms operate in parallel, and then the reaction schema are as follows:, where sufix f, c, and i denote free, complex, and ionic. If k5 and k3 are rate constants of ratedetermining step in termolecular complex mechanism and in simple ionic mechanism, Apparent rate follows the second-order kinetic, -de/dt=k3ce, if Bf is very small, while it becomes apparently the third-order kinetic, -de/dt=kccpe, if Bi is negligible Experimentals Materials: Phenyl glycidyl ether was prepared as described in the previous paper7). theoretical values. The purities of tert. amines, substituted phenols and benzoic acids used were more than 99.5%. All solvents used were carefully dried and fractionated. Apparatus and Procedure: An example is as follows: 4.70g (50.0m mol) of phenol, 0.171g (1.00m mol) tri-n-butylamine, and 87.6g of xylene were added to 200ml roundbottled flask equipped with long condenser, thermometer and stirrer, and settled in m mol) of PGE, maintained at the same temperature, was quickly added to the flask to initiate the reaction. Reaction mixture was taken out through the condenser Volume 5-March 1963

3 Ishii, Sakai and Sugiyama: Base-catalyzed each 30 minutes, and residual phenol and/or epoxide therein were analysed by Piffer's methods8) and/or by Durbetaki's method9). Because the determination of epoxide is accurate and simple, and the relation -de/dt= -dp/dt holds in all cases studied here, so kinetical analysis has been done mainly by epoxide determinations. Results and Discussion Reaction of Phenol in Various Solvents: For all of reactions studied, the rates were accurately of the first-order in concentration of epoxide E. However in aprotic and nonpolar solvents, the rates were also effected with concentration of phenol P, while in protic or polar solvents they were unaffected with P. When the reactions of phenol with PGE are carried out in nitrobenzene using tri-n-butylamine as catalyst (it abbreviates -de/dt=-dp/dt=kbe=k3ce (5) NBu3] is clearly incomplete in high concentration of NBu3. Furthermore, curve A in Fig. 1 indicates that NBu3 does not work as catalyst for the reaction of epoxide with phenoxyethanol. tration of catalyst than that in nitrobenzene. -de/dt=-dp/dt=kape=kccpe (6) As ionic dissociation is difficult in xylene, Bi is negligibly small and P0=E0 in these experiments, equation (4-2) may be integrated: (E-1-E0-1)+K1ln(E0/E)=K1k5Ct=kcCt (7) K1, equilibrium constant of complex formation between phenol and NBu3 is too difficult to be measured in this system, but Fig. 2 shows that the second term in left equation is very small, and equation (7) will divert apparently to equation (6). Arrhenius plot was fit well to the reaction NBu3: Xylene system as shown in Fig. 4. These results show that reaction in non-polar solvent is complex in character. In NBu3: Xylene system the rate followed equation (6) (see Fig. 2), and if the reaction be complete ionic in xylene, straight line B in Fig. 3 would fall down in lower concen- Furthermore the reaction in xylene was anomalous. kc in xylene was not affected by the ratio of PO/Eo (see 1, 2, and 3 in Table

4 Ring-opening Reactions of Epoxides 1), but affected by the absolute value of P0 and E0 (see curve A in Fig. 5). Such results were not obtained about the rate constant kc Fig. 4 Effect of reaction temperature reactions of phenol. (P0=E0=0.500, C0=0.0125mol/kg) on the Table 2 Reactions in various solvents (E0=P0=0.500mol/kg, C0=0.0125mol/kg) Volume 5-March 1963

5 Ishii, Sakai and Sugiyama: Base-catalyzed plained as yet. The addition of small amount of high dielectric compounds to such low dielectric solvent effects very strongly on the rate, so dielectric effect and association of phenol around the reaction site may be an important factor. Table 2 indicates that the order of reaction using NBu3 as catalyst is affected by dielectric property of solvent, and that the borderline region between the second- and the third-order is in the solvent whose dielectric were autocatalytic and abnormally faster than that to be expected from its dielectric property, and the order of reactions could not be defined, but the values of k3 and k, in Table 2 were calculated by least square method. This phenomenon is interesting but can not be explained. In nitrobenzene, re- was used as catalyst, while they were of the second-order in the case of NBu3 catalyst. Therefore the effect of catalyst is clear. The second-order in lower alcohols, whose dielectric constants are rather small but those polarities are large, and their solvating and ionizing properties are powerful. Addition of water to lower alcohols did not so accelerate the reaction rate as addition of polar solvent to non-polar solvent did. When the reactions were carried out in mixed solvent of xylene and nitrobenzene were changed from the third-order kinetics to the second-order where the addition of nitrobenzene was more than 5 mole% to xylene, and the third-order rate constant in xylene was increased largely by the addition of 2.5 mole% of nitrobenzene (see Table 3). On the other hand, the reactions of benzoic acid with epoxide in the presence of tertiary amine followed the third-order kinetics in nitrobenzene, chlorobenzene, and xylene. Thus the order of reactions of epoxide is obviously controlled by the properties of catalysts, reactants, and solvents used. Effects of Substituents: Many reports have been published concerning the ratio of isomeric products in epoxide reaction1), but the effect of substituents on the kinetics of epoxide reactions has not been precisely discussed except that of D. R. Boyd and E. R. Marle3). In the previous paper5), we have found that the rates of polyadditions of ethylene oxide to substituted phenols using NaOH as catalyst were influenced by substituents, and the order of the rate of substituted phenols was as follows: p-ch3>(h)>p-br>p-n02. It was the inverse order to that obtained in the reaction of substituted phenols and epoxide in 98% aqueous ethanol using NaOH as catalyst2). We have suggested that the transition complex [I-1] might be formed and the basicity of substituted phenoxide

6 Ring-opening Reactions of Epoxides molecule in [I-1] would be important as a rate determining factor in the polyaddition, and that such difference might be caused by the reaction conditions such as difference of reaction medium. Complex [I-1] is equivalent to complex [I], but it contains two substituents X1 and X2. Here we discuss further the reactions of PGE and substituted phenols using tri-nbutylamine as catalyst in nitrobenzene, xylene, and n-dodecene-1. It is more favourable in these cases than in complex [I-1] to discuss the results obtained, because only one substituent is contained in transition complex [I], and the results are more straightforward. The results are shown in Table 5. Table 5 Effects of substituents on the reactions of PGE and substituted phenols in some solvents. (C0=NBu3=0.0125mol/kg, E0=P0=0.500mol/kg) Fig. 7 Hammett's plots of the reactions of substituted benzoic acids with PGE. Fig. 6 is plot of rate constant vs. Ham- K1 K2 is controlling factor, and that free phenoxide ion would be true reacting species. However, is shorter in xylene than in nitrobenzene. Such accelerations in xylene would not be explained if "simple ionic mechanism" was operative. These results will suggest "termolecular complex mechanism". Accelerations in rate in n-dodecene-1 are seemingly larger than in xylene, but precise discussion should be avoided because these kinetics are not clear. The reactions of substituted benzoic acids with PGE using NMe3 as catalyst follow the third-order kinetics in xylene, in chlorobenzene, and in nitrobenzene10). The effects of substituents are summarized in Table 6. Hammett's plots shown in Fig. 7 indicate Volume 5-March 1963

7 Ishii, Sakai and Sugiyama: Base-catalyzed Table 7 Reactions of substituted benzoic acids and PGE using NMe3 as catalyst in xylene. Table 8 Reactions of substituted benzoic acids and PGE using NMe3 (P0=E0=0.500mol/kg, C0= P0)

8 Ring-opening Reactions of Epoxides Table 9 Rate constants of the reaction of Phenol and PGE using various catalysts. (P0=E0=0.500mol/kg, C0=0.0125mol/kg) the second-order kinetics, activities of catalysts are determined mainly by basicity of catalysts or inductive effect of alkyl groups in tertiary amines, but steric effects of alkyl groups are small. As the order of catalytic reaction in u-dodecene-1 or in xylene. i. e. NEt3>NBu3 and N-ethylmorpholine>N-ethyl-2, 6-dimethylmorpholine. This difference depends on the difference of steric requirement in xylene or in n-dodecene-1 from in in-butanol, that is, the transition state [I] of the reaction in the former solvent will be more crowded than the transition state [II] in the latter solvents. For all catalysts used, the plots of k3 VS. T-1 are linear in the reactions in n-butanol, and the activation energies are linear with log PZ as shown in Fig. 8, in which compensation effect is observed in tertiary amine: u-butanol system, in NBu: nitrobenzene sys- Table 10 Rate constants of reactions of substituted phenols with PGE Volume 5-March 1963

9 Ishii, Sakai and Sugiyama: Base-catalyzed Ring-opening Reactions of Epoxides fact suggests that the same mechanism will be operative in these systems. But all reactions in xylene do not fit to Arrhenius equation, and the rates of reactions using morpholine derivatives as catalyst are decreased with temperature, which can not be explained by simple ionic mechanism. The same phenomenon is reported by S. Komori et al.12) in the reaction of higher alcohol with The authors express their hearty gratitudes to the staffs of Sansuiso Oil & Fats The rate constants of reactions of substituted phenols with PGE in the presence of NEt3 or NBu3 as catalyst in xylene as well as in n-butanol as solvents are tabulated in Table 10. When substituted phenols are made react with PGE in the presence of NEt3 or NBu3 as catalyst, the ratio of rate constant k(net3)/k(nbu3) is always equal to 1.0, regardless of the position of substitution or the kind of catalyst. The results mean that there are on steric hindrances of substituted groups in phenol. While, when the reactions are carried out in xylene, the ratio k(net3/ k(nbu3) is always about 1.60 in the cases of p-substituted phenols and it means that some difference of the steric effects between NEt3 and NBu3 as catalyst exists. The ratio is much larger in the case of o-substituted phenols than in the case of p-substituted phenols and it shows that o-substitutions have larger steric hindrances than p-substitutions. These results also mean that the same reaction mechanism can not be applied to both the reactions in n-butanol and in xylene as solvent. Acknowledgement Industry Co. for their kind assistances in pursueing these studies. References 1) Parker, R. E., Isaacs, N. S., Chem. Rev., 59, 737 (1959). 2) Boyd, D. R., Marle, E. R., J. Chem. Soc., 1914, 105, 2117; 1919, ) Ishii, Y., Sekiguchi, J., Ito, S., Kogyokagaku Zashi, 62, 86 (1959). 4) Sakai, S., Ishii, Y., ibid., 61, 1473 (1958). 5) Sakai, S., Ishii, Y., ibid., 62, 413 (1959) 6) Patat, F., Makromol. Chem., 37, 1 (1960). 7) Sakai, S., Sugiyama, T., Ishii, Y., Kogyokagaku Zashi, in press. 8) Piffer, C. W., Wollish, E. G., Schmall, M., Anal. Chem., 25, 310 (1953). 9) Durbetaki, A. J., ibid., 28, 2000 (1956). 10) Sakai, S., Ishii, Y., Kogyokagaku Zashi, 64, 2159 (1961). 11) Jaffe, H. H., Chem. Rev., 53, 218 (1953). 12) Oshiro, Sumida, Komori, Kogyokagaku Zashi, 64, 2132 (1961).