J. Phys. Chem. 1989,93, 201-202
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Kinetics and Mechanism of Elimination of Primary Alkyl Methanesuifonates in the Gas Phase. Correlation of Alkyl Substituents' Gabriel Chuchani,* Sarah Pekerar, Rosa M. Domhguez, Alexandra Rotinov, and Ignacio Mar& Centro de Quimica, Instituto Venezolano de Investigaciones Cient@cas, Apartado 21827, Caracas 1020-A, Venezuela (Received: November 5, 1987)
The gas-phase elimination kinetics of eight primary alkyl methanesulfonates were studied in a seasoned, static reaction vessel over the temperature range of 280.6-350.2 O C and the pressure range of 32-178 Torr. The reactions are homogeneous and unimolecular, follow a first-order rate law, and are invariant to the presence of equal or excess amount of the radical chain suppressor cyclohexene and/or propene. The observed overall rate coefficients are represented by the following Arrhenius equations: For ethyl methanesulfonate, log kl(s-') = (12.18 f 0.12)-(171.7 1.3) kJ mol-' (2.303RT)-'; for n-propyl methanesulfonate, log k,(s-') = (12.36 0.28)-(171.6 f 3.3) kJ mol-' (2.303RT)-I; for n-butyl methanesulfonate, log kl(s-') = (12.16 0.20)-(168.7 2.3) kJ mol-' (2.303RT)-'; for n-pentyl methanesulfonate, log kl(s-') = (12.25 f 0.09)-(169.4 1.0) kJ mol-' (2.303RT)-'; for n-hexyl methanesulfonate, log kl(s-') = (12.21 f 0.19)-(168.9 3.3) kJ mol-' (2.303RT)-'; for 3-methyl-1-butylmethanesulfonate, log kl(s-') = (12.74 f 0.19)-( 174.7 f 2.2) kJ mol-' (2.303RT)-'; for 3-methyl-1-pentyl methanesulfonate, log k,(s-') = (12.28 0.17)-(167.9 f 1.9) kJ mol-' (2.303RT)-'; for 3,3-dimethyl-l-butylmethanesulfonate, log kl(s-') = (12.14 f 0.11)-(165.2 1.3) kJ mol-' (2.303RT)-'. The present data give good correlation lines when log k / k o are plotted against several steric parameters E, and and v values. Apparently, steric factors seem to be operating in these elimination reactions. The primary alkyl methanesulfonates are found to be faster in rates of olefin formation when compared to other primary organic esters. The reaction of this work is interpreted in terms of an intimate ion pair type of mechanism.
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Introduction Physical organic chemistry has described a considerable number of interesting reactions and phenomena with benzenesulfonates, tosylates, and brosylates as leaving groups in nucleophilic substitutions and elimination reactions in solution. However, the gas-phase homogeneous, molecular elimination kinetics of these type of compounds have not been reported as yet, presumably owing to lack of volatility. They generally are heavy oils or solids. If the aryl part of the sulfonates is replaced by the small methyl group, Le., methanesulfonates, it is possible to expect, from the corresponding low molecular weight, interesting mechanisms and phenomena in gas-phase processes. The literature just cites the "abnormal" pyrolytic 1,3-elimination of 2-adamantyl methanesulfonate with various mechanistic conclutions on a qualitative basis? With this background, the present work aimed at studying the pyrolysis kinetics of several primary alkyl methanesulfonates in the gas phase. The probable mechanism and the correlations of alkyl substituents, if any, are also to be considered. Experimental Section The alkyl methanesulfonates were prepared by method A, methanesulfonyl chloride was mixed with the sodium salt of the alcohol; and method B, methanesulfonyl chloride was added to the alcohol in triethylamine and benzene. The methanesulfonate products, as listed in Table I (Tables I-XI1 are available as supplementary material; see paragraph at end of paper regarding supplementary material), were distilled several times, and the fraction over 98.1% purity (gas-liquid chromatography) was used. The products ethylene, propylene, 1-butene, cis- and tram-2-butene were bought from Matheson, while 1-pentene, cis-2-pentene, 3-methyl- 1-butene, 2-methyl-l-butene, 2,3-dimethyl-1-butene, and 2-methyl-2-butene were from K&K Laboratory. 1-Hexene, trans-2-hexene, trans-2-pentene, 3-methyl-l-pentene, cis-2methyl-2-pentene, trans-2-methyl-2-pentene, 3,3-dimethyl-1butene, and 2,3-dimethyl-2-butene were acquired from Aldrich. The columns used for quantitative analyses of starting material and olefin products were 10% Dow-Corning 200/ 10Whromosorb W A W DMCS 80-100 mesh, Porapak Q 80-100 mesh, dinonyl (1) This paper is part of the work presented to the Academy of Physics, Mathematics, and Natural Sciences of Venezuela as requisite for G.C.'s incorporation as Corresponding Member. (2) Kaufmann, D.; De Meijere, A,; Luk, K.; Overton, K.; Stothers, J. B. Tetrahedron 1982, 38, 911.
0022-3654/89/2093-0201$01.50/0
phthalate 20%-Gas Chromosorb Q 80-100 mesh, bis(methoxyethyl) adipate 20%-Chromosorb P A W DMCS 80-100 mesh, diisodecyl phthalate 5%-Chromosorb G A W DMCS 60-80 mesh, and Carbopack C-0.19% picric acid 80-100 mesh. The identities of substrates and products were further confirmed with a mass spectrometer and by infrared and nuclear magnetic resonance spectroscopies. The kinetics were determined in a static s y ~ t e mseasoned ,~ with allyl bromide and in the presence of a t least one-to-one ratio of the inhibitor propene and/or cyclohexene. The rate coefficients were calculated by pressure increase. The substrates were injected directly into the reaction vessel with a syringe through silicone rubber tubing. The temperature control was maintained within f0.2 OC with a calibrated platinum-platinum-1 3% rhodium thermocouple, and no temperature gradient was found in the reaction vessel. An important note is that after a few runs of the methanesulfonate, the silicone rubber septum was changed into another and the reaction vessel was again seasoned with allyl bromide in order to obtain reproducible k values.
Results Stoichiometry. It was expected that the gas-phase elimination of alkyl 2-substituted ethyl methanesulfonates would yield products according to eq 1 ( R = H , CH3, CH2CH3,CH2CH,CH3, CH2CH2CH2CH3, CH(CHs)2, CH(CHs)CH2CHS, C(CH3)j). HOSOzCHg (1) RCH2CH20S02CH3 RCH=CH2 -+
+
The stoichiometry of eq 1 was verified by the fact that the final pressure, Pf, in the reactor after 10 half-lives was nearly twice the initial pressure, Po(Table 11). It is interesting to note in Table I1 that at the lower temperature of pyrolysis of the substrates the final pressure tends to be somewhat smaller than 2 because of the low volatility of the methanesulfonic acid (bp 167'/10 mm) and/or a slight polymerization of the olefin product. The confirmation of the above stoichiometry (eq l ) , up to 60-80% reaction, was made by comparing the percentage decomposition of the methanesulfonate substrate from pressure measurements with those obtained from the chromatographic analyses of the corresponding olefin product (Table 111). (3) Truce, W. E.; Christensen, L. W. J . Org. Chem. 1968, 33, 2261. (4) Truce, W. E.; Vrencur, D. J. J. Org. Chem. 1970, 35, 1226. (5) Maccoll, A. Chem. Reu. 1969,69, 33.
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E, = o , E:=. Figure 1. log kz/log kH against steric parameters in the pyrolysis of CH3S020CH2CH2Z at 320 'C. (- - -) Taft E, values (open circles; slope 6 = -0,248 f 0.059, intercept i = 0.040 f 0.033, and correlationcoefficient r = 0.990). (-) Hanckock I$ values (filled circles; slope 6 = -0.180 f 0.092, intercept i = 0.077 f 0.040, and correlation coefficient r = 0.973). Charton Y values (half-filled circles; slope 1c. = 0.560 -+ 0.068; intercept i = 0.051 -+ 0.090, and correlation coefficient r = 0.987). The lines were drawn by the least-squares procedure. (-e-)
The addition of an effective free radical suppressor, propene and/or cyclohexene, had no effect on the rate coefficients. This is described in Table IV. Moreover, no induction period was observed. The homogeneity of reaction 1 was examined by packing the reactor with glass tubing to give an approximately 6-fold increase in the surface area-to-volume ratio. The packed and unpacked vessels seasoned with allyl bromide had no effect on the rate coefficients. On the other hand, clean packed and unpacked Pyrex vessels gave a significant heterogeneous effect due to surface catalysis (Table V). The average k value in several substrates appears to be lower in the clean packed Pyrex vessels than in the clean normal Pyrex vessel. These results may possibly be rationalized in the sense that some of the heterogeneous effect is accompanied by some catalytic polymerization of the olefin in the presence of the methanesulfonic acid product. The analyses of the decomposition products of the primary methanesulfonates, at 60-80% reaction, in an unpacked seasoned vessel are given in Table VI. The olefin distribution has been found to change slightly or none at all at the pyrolysis temperature. Apparently, the terminal olefin products isomerize in the presence of the CH3S03Hproduced during decomposition. The rate coefficients from the kinetic elimination of these methanesulfonates were invariant with the changes of their initial pressure (Table VII), and the first-order plots from k , = (2.303/t) log P0/(2P0- P t ) are satisfactorily linear up to 6 6 8 0 % decomposition. The temperature dependence of these reactions, in seasoned vessels and in the presence of the inhibitor, is shown in Table VIII. The rate Coefficients are reproducible with a standard deviation not greater than 35% at a given temperature. Table IX shows the Arrhenius parameters obtained by using the least-squares procedure. The errors were estimated to 80% confidence coefficients.
Discussion Table X lists a reasonable number of substituent effects in the gas-phase elimination kinetics of primary alkyl methanesulfonates. In this way, it is possible to assess the influence of the alkyl groups in these elimination reactions. Plotting log k,, of alkyl substituents against well-known steric parameters6*' gave very good straight (6) Hansch, C.; Leo, A. Substituent Constants for Correlation Analysis in Chemistry and Biochemistry; Wiley: New York, 1979.
lines as described in Table XI. This means that branching of alkyl groups appears to enhance the pyrolysis process due too steric acceleration (Figure 1) and that steric effect is of paramount importance in these elimination processes. The ease by which the methanesulfonate leaving group is eliminated may be due to the greater stability of the negatively charged oxygen atoms and sulfur in the transition state. This high stabilization of the methanesulfonate anion is responsible for the faster elimination rate when compared to the pyrolysis of other organic esters (Table XII). Consequently, the greater the stability of the negatively charged leaving group, the more polar is the transition state and the faster is the elimination rate. The fact that ethyl chloroformate is greater in rate than methyl ethyl carbonate is due to the stronger electron-withdrawing effect of the chloro group than the methoxy of the carbonate, thus causing a greater stability of the anionic part of the leaving group in the transition state. In association with these results, the mechanism of primary alkyl methanesulfonates may be rationalized in terms of a tight intimate ion pair as described in eq 2.
"
L
CbS03H
+
RCH=CHp
(2)
Registry No. Ethyl methanesulfonate, 62-50-0; propyl methane-
sulfonate, 1912-31-8; butyl methanesulfonate, 1912-32-9; pentyl methanesulfonate, 6968-20-3; hexyl methanesulfonate, 16156-50-6; 3methyl-1-butyl methanesulfonate, 16156-55-1; 3-methyl-1-pentyl methanesulfonate, 72132-79-7; 3.3-dimethyl-1-butyl methanesulfonate, 69436-45-9.
Supplementary Material Available: Tables I-XI1 showing results of analysis and reaction kinetics (12 pages). Ordering information is given on any current masthead page. (7) Charton, M. J . Am. Chem. SOC.1975, 97, 1552. (8) Davis, W. H., Jr.; Pryor, W. A. J . Chem. Educ. 1976, 53, 285. (9) de Burgh Norfolk, S.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1976, 280. (10) Taylor, R. J . Chem. Soc., Perkin Trans. 2 1983, 291. (1 1) Johnson, R. L.; Stimson, V. R. Aust. J . Chem. 1976, 29, 1389.
