KINETIC STUDIES ON THE EPOXIDATION OF 4-METHYL-4-OCTENE Valéria Dutra Ramos*, Daniel Derouet** & Leila Léa Yuan Visconte* *Instituto de Macromoléculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, 21945-97 - Rio de Janeiro, RJ - Brazil **Laboratoire de Chimie Organique Macromoléculaire (UMR du CNRS), Université du Maine, Avenue Olivier Messiaen 7285, Le Mans, France Abstract - The kinetics concerning the performic acid epoxidation of 4-methyl-4-octene, a model compound for 1,4-polyisoprene, was studied by the High Performance Liquid Chromatography (HPLC) technique. The samples were characterized by Nuclear Magnetic Resonance ( 13 C and 1 H-NMR) and Fourier Transform Infrared Spectroscopy (FTIR) analysis. This investigation allowed an insight on the influence of the different reaction parameters (temperature and concentration of hydrogen peroxide) on the conversion rate. The determining step of the epoxidation was the formation of performic acid. The activation enthalpy of the reaction was found to be H* = 87 kj/mol and the entropy of activation S* = -44 J/mol K. Key words: Kinetics, Model compound, Epoxidation, 1,4-Polyisoprene INTRODUCTION Polydienes constitute a family of polymers suitable to several applications. Among them, natural rubber is, probably, the most employed material, in reason of its cost and the mechanical properties it offers. The chemical modification of natural rubber, or of synthetic polydienes with similar structure, has been the object of several research works, including chemical modification, in such a way as to provide the products with the characteristics of sensitivity to light. Recent studies in this area are being directed particularly to the chemical modification of epoxidized polydienes. Actually, epoxides constitute a class of synthesis intermediates very reactive towards most of nucleophiles and they offer multiple possibilities of chemical modification for 1,4-polyisoprene. This reaction has been used in the fixation of naphtylacetic acid, stimulant of plants growth 1-3, of acrylic acid, for obtaining photocrosslinkable varnishes 4, of antioxidants 5 and of reticulation agents. 6 Reactions involving polymers are hindered by the simultaneous production of lateral products and also by restrictions due to the molecular size. To obtain more detailed CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2, São Pedro - SP. Anais 5861
information on the mechanism of certain reactions, low molecular weight compounds are submitted to the same reaction conditions in order to provide a better understanding of the system. This work is devoted to the kinetic study concerning the performic acid epoxidation of 4-methyl-4-octene, a model compound for 1,4-polyisoprene, by the High Performance Liquid Chromatography (HPLC) technique. The investigation allowed an insight on the influence of the different reaction parameters (temperature and concentration of hydrogen peroxide) on the conversion rate. EXPERIMENTAL Reagents Formic acid (98%) was obtained from Merck and hydrogen peroxide (35 wt %) from Reagen. Phenanthrene was obtained from Jansen Chimica (98-99%) and used as the internal standard. All reagents were used without further purification. Solvents used in High Performance Liquid Chromatography (HPLC) were chromatography solvents for HPLC. Deuterated chloroform was used in N.M.R analysis. Procedure The model compound (4-methyl-4-octene) was prepared through the reaction of Wittig- Schölkopf 7-9 between n-butylidenetriphenylphosphorane and 2-pentanone. This reaction requires the previous preparation of the joint-stock n-butylidenetriphenylphosphorane from sodium t-amylate and bromide of n-butyltriphenylphosphonium. The main spectroscopics characteristics of the model compound are: Boiling point (76 mmhg) = 141-142 C, Yield = 6%. 1 H-NMR (CDCl 3 ): δ =.85-1. (m, CH 3 -CH 2 -, 6H), 1.3-1.5 (m, CH 3 -CH 2 -CH 2 -, 4H), 1.58 (trans) and 1.68 (cis) (s, CH 3 -C=C-, 3H), 1.92-2.2 (m, CH 2 -C=C-, 4H), 5.1-5.2 (t, -C=CH-, 1H). 13 C-NMR (CDCl 3 ): δ = 13.1 (trans) and 13.3 (cis) (CH 3 -CH 2 -CH 2 -C=C-), 13.3 (trans) and 13.5 (cis) (CH 3 -CH 2 - CH 2 -C(CH 3 )=C-), 15.2 (trans) and 22.8 (cis) (CH 3 -C=C-), 2.5 (trans) and 2.6 (cis) (CH 3 -CH 2 -CH 2 -C=C-), 22.5 (trans) and 22.7 (cis) (CH 3 -CH 2 -CH 2 -C(CH 3 )=C-), 22.7 (trans) and 29.4 (cis) (CH 2 -C=C-), 33.2 (cis) and 41.3 (trans) (CH 2 -C(CH 3 )=C-), 124. (trans) and 124.8 (cis) (-C=CH-), 134.5 (trans) and 134.7 (cis) (CH 3 -C=C-). FTIR: 1655 cm -1 : C=C, 146 cm -1 : -CH 3 (asym), 138 cm -1 : -CH 3 (sym). CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2, São Pedro - SP. Anais 5862
All the kinetics studies of the epoxidation of the model compound by performic acid were carried out starting from a " mother " solution containing the model compound (1.5869 x 1-2 moles), the internal standard (phenantrene = 2.781 x 1-3 moles) and the solvent (benzene = 25 ml). The reaction was carried out with different molar ratios of hydrogen peroxide (1 and 2 in relation to the model concentration) at room temperature, and a molar ratio [HCOOH]/[model] = 1, and at various temperatures for a molar ratio [H 2 O 2 ]/[model] = 1. The rate of reaction was then estimated at various regular time intervals and the reaction was interrupted when the entire model compound was consumed. Analysis 13 C and 1 H-NMR spectra of the products were taken at room temperature on a Bruker DPX 2 (5.32 and 2.13 MHz) spectrometer using deuterated chloroform as the solvent. Tetramethylsilane (TMS) was used as the internal reference and chemical shifts are given in parts per million (ppm). Infrared spectra were recorded using a ATI Mattson Genesis Fourier Transform spectrometer, Series FTIR TM. Liquid film samples were analyzed on NaCl plates. Analyses in HPLC were done with the help of a modular WATERS equipped with a duplicate detection system (UV and differential refractometry). The used device comprises two pumps MODEL 51, an injector U6K. The different module control, the acquirement and the treatment of data are assured with the help of a microcomputer NEC IV (operating system: Baseline 81 Waters). Analyses were achieved in isocratic mode. RESULTS AND DISCUSSION Identification of reaction products In this work, the epoxidation of the model compound by performic acid was carried out at room temperature with [H 2 O 2 ]/[model] = 2.. The molar ratio [HCOOH]/[model] = 1 was kept constant. The 1 H-NMR spectrum of the complex mixture obtained after 2 hours of reaction shows the presence of four main products. Among these, one comes from the epoxidation of the model compound: 4,5-epoxy-4-methyloctane 1 with a 58% yield. The three other products, 5-methyl-4-octanone 2, 4-methyl-4,5-octanediol 3 2-hydroxy-2-methyl-1-(n- CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2, São Pedro - SP. Anais 5863
propyl)pentyl formate 4, are the result of epoxide rearrangements with the respective yields of 1%, 2% and 2%. The epoxide 1 was characterized by chemical shifts at 2.7 ppm for proton of the oxirane cycle. The ketone 2 is identified in the reaction mixture by the presence of the triplet at 2.35-2.45 ppm (-CH 2 -CO-; 3J = 7.3 Hz) and the multiplet at 2.5-2.6 ppm (-(CH 3 )CH-CO-; 3J = 6.9 Hz), characteristics of protons in α and α'-position of the carbonyl function. The presence of diol 3 in the reaction mixture was confirmed by the chemical shifts of proton in α-position of the secondary alcohol function (CH-OH, triplet) at 3.4-3.55 ppm and of the two singlets at.96 and 1.28 ppm attributed to the methyl group bound to the α- carbon of the alcohol function (-C(CH 3 )-OH). The glycol ester 4 was characterized by the triplet at 4.9-5.1 ppm attributed to the proton in α-position of the ester function (CH-OCOH), by the singlet at 8.15 ppm attributed to the proton bound to the carbon of the carbonyl function (CH OCOH) and of the two singlets at.96 and 1.28 ppm attributed to the methyl group bound to the α-carbon of the alcohol function (-C(CH 3 )-OH). In 13 C-NMR spectrum, the chemical shifts characteristics of the products are due to the carbons of the double bond of the ketone function. The FTIR spectrum confirms the presence of the reaction products through the following absorption bands: 4,5-epoxy-4-methyloctane 1 : 1255 cm -1 (sym) and 889 cm -1 (asym) : C-O-C, 1467 cm -1 (asym) and 1381 cm -1 (sym) : C-H of -CH 3 and -CH 2 -. 5-methyl-4-octanone 2 : 1713 cm -1 : C=O, 146 cm -1 (asym) and 1379 cm -1 (sym) : CH 3 -CH-. 4-methyl-4,5-octanediol 3 : 345 cm -1 : (CH 3 -C-OH), 337 cm -1 : (CH-OH), 1465 cm -1 (asym) and 1379 cm -1 (sym) : CH 3 -CH-. 2-hydroxy-2-methyl -1-(n-propyl)pentyl formate 4 : 345 cm -1 : (CH 3 -C-OH), 173 cm -1 : C=O, 121-1163 cm -1 : -C(=O)-O, 146 cm -1 (asym) and 1379 cm -1 (sym) : CH 3 -CH-. Kinetics The performic acid used for the epoxidation of the 4-methyl-4-octene was generated "in situ" (equations 1 and 2). H-COOH + H2O2 k 1 k -1 H-COOOH + H2O O (1) H-COOOH + + H-COOH (2) 4-methyl-4-octene 4,5-epoxy-4-methyloctane 1 CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2, São Pedro - SP. Anais 5864
Taking the rate determining step as the production of the performic acid, the following rate law applies. d[1] = k 1 ([H 2 O 2 ] - [1]) [HCOOH] dt = k 1 ([H 2 O 2 ] - [1]) [HCOOH] (3) where the subscript denotes the initial concentration. In equation (3), the concentration of the formic acid is assumed to remain constant throughout the reaction and is approximated to [HCOOH] for the following reasons : (1) both the equilibrium constant for the performic acid formation (equation 1), k 1 and the dissociation constant of the formic acid, k 1,78 x 1-4 1, 11 are small; (2) formic acid is regenerated in the epoxidation process (equation 2). Integration of equation (3) yields Ln ([H 2 O 2 ] - [1]) = - k 1 [HCOOH] t + Ln [H 2 O 2 ]. (4) The analysis method used in the present study was the High Performance Liquid Chromatography (HPLC). The residual mass of the model compound and of the formed epoxide 1 were determined at regular intervals of time and the reaction was stopped when the model compound had been totally consumed. The reaction was done at room temperature with estequiometric amounts of model, formic acid and hydrogen peroxide, and the yield of residual model was 48.8%, after 25 hours of reaction, and the yield in epoxide 1 was only 36.9% (Fig. 1). This indicates the consumption of the epoxide by the secondary processes previously described, from which an improvement of the yield in epoxides with the use of an excess of hydrogen peroxide can be expected. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2, São Pedro - SP. Anais 5865
1 8 Yield (%) 6 4 % epoxides % residual model 2 % secondary products 5 1 15 Figure 1: Yields in residual model, epoxides and secondary products at 25 C versus reaction time. Influence of [H 2 O 2 ]/[model] molar ratio The epoxidation of 4-methyl-4-octene was carried out at different molar ratios 1 and 2, of hydrogen peroxide/model compound, at 25 C. The results (Fig. 2) show an increase in the reaction rate with increasing [H 2 O 2 ]/[model] molar ratios. The reaction becomes very slow when the molar ratio is 1 leading to an incomplete consumption of the model compound (conversion of 5%). However, 1% conversion is reached (Fig. 2.a) when an excess of hydrogen peroxide is used, but the yield in epoxides, under these conditions, is only of 58.5%. This result indicates the existence of secondary processes. As can be seen in Figure 2.b, the yield in epoxides is inferior when the molar ratio is 1. On the other hand, happen an increase of the yield in secondary products with increase [H 2 O 2 ]/[model] molar ratios (Fig. 2.c). CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2, São Pedro - SP. Anais 5866
1 A 1 B Yield in residual model (%) 8 6 4 2 [H 2O 2]/[model] = 1 [H 2O 2]/[model] = 2 Yield in epoxides (%) 8 6 4 2 [H 2O 2]/[model] = 2 [H 2O 2]/[model] = 1 5 1 15 5 1 15 1 C Yield in secondary products (%) 8 6 4 2 [H 2O 2]/[model] = 2 [H 2O 2]/[model] = 1 5 1 15 Figure 2 : Yields of the reaction products versus reaction time at various molar ratios [H 2 O 2 ]/[modelo] at 25 C. A : yield in residual model, B : yield in epoxides and C : yield in secondary products. [model] = 1.939 x 1-3 moles, [HCOOH] = 1.9231 x 1-3 moles. Equation (4) allows the confirmation of the reaction second order (Fig. 3). CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2, São Pedro - SP. Anais 5867
-5, -5,5 [H 2O 2]/[model] = 2 K 1 = 1.4534 x 1-3 l.mol -1.mn -1 ln ([H 2 O 2 ] - [1]) -6, [H 2O 2]/[model] = 1 K 1 = 9.8776 x 1-4 l.mol -1.mn -1-6,5-7, 1 2 3 4 Figure 3 : Plots of ln ([H 2 O 2 ] - [1]) versus reaction time at various molar ratios [H 2 O 2 ]/[model] at 25 C. [model] = 1.939 x 1-3 moles, [HCOOH] = 1.9231 x 1-3 moles. Influence of the temperature The epoxidation of 4-methyl-4-octene was carried out at 25, 35 and 45 C. The molar ratio [H 2 O 2 ]/[model] = 1 was kept constant. The results show that an increase in the reaction rate is found at higher temperatures (Fig. 4.a). The conversion of the model compound is 1% at 35 C, but the yield in epoxides is only 65.1%. This result indicates a partial consumption of the formed epoxides as a secondary process. The yield in epoxides increases up to a maximum value with the temperature (Fig. 4.b). However, happens a fall of the yield and it is fall becomes so more important when higher it goes to temperature. This again indicates the occurrence of secondary reactions. The influence of the temperature was also observed for the formation of the secondary products (Fig. 4.c) since an increase in the reaction rate with the temperature was noticed. These results indicate that the yields in reaction products are dependent of the temperature and, consequently dependent on the structure of the activated complex in the transition state. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2, São Pedro - SP. Anais 5868
1 A Yield in residual model (%) 8 6 4 2 35 C 25 C 45 C 5 1 15 1 8 45 C B Yield in epoxides (%) 6 4 35 C 25 C 2 5 1 15 1 C Yield in secondary products (%) 8 6 4 2 45 C 35 C 25 C 5 1 15 Figure 4 : Yields of the reaction products versus reaction time at various temperatures. A : yield in residual model, B : yield in epoxides and C : yield in secondary products. [model] = 1.939 x 1-3 moles, [HCOOH] = 2.1577 x 1-3 moles and [H 2 O 2 ] = 1.939 x 1-3 moles. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2, São Pedro - SP. Anais 5869
The use of the equation (4) allows the determination of the rate constant of epoxide formation at each used temperature (Fig. 5). -6, -6,5 T = 25 C K 1 = 9.8776 x 1-4 l.mol -1.mn -1 ln ([H 2 O 2 ] - [1]) -7, T = 35 C K 1 = 5.132 x 1-3 l.mol -1.mn -1-7,5-8, T = 45 C K 1 = 9.4328 x 1-3 l.mol -1.mn -1 1 2 3 4 Figure 5 : Plots of ln ([H 2 O 2 ] - [1]) versus reaction time at various temperatures. [model] = 1.939 x 1-3 moles, [HCOOH] = 2.1577 x 1-3 moles and [H 2 O 2 ] = 1.939 x 1-3 moles. Table 1 : Rate constants k 1 at various temperatures. Product T ( C) k 1 (l.mol -1.min -1 ) 25 9.8776.1-4 1 35 5.132.1-3 45 9.4328.1-3 Rate constants k 1 obtained from the plots of ln ([H 2 O 2 ] - [1]) vs time are shown in Table 1. The Arrhenius plot yields the enthalpy of activation of the reaction which was found to be H* = 87 kj/mol and the entropy of activation S* = -44 J/mol.K. The rate determining step of the epoxidation was found to be the formation of performic acid. ACKNOWLEDGEMENTS The authors are indebted to UMR du CNRS, Université du Maine, Le Mans - France where this research was carried out. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2, São Pedro - SP. Anais 5861
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