JOURNAL OF PHYSICAL ORGANIC CHEMISTRY J. Phys. Org. Chem. 2003; 16: 839–848 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.665
Mechanism and structure–reactivity correlation in the homogeneous, unimolecular elimination kinetics of 2-substituted ethyl methylcarbonates in the gas phase Gabriel Chuchani,1* Edgar Marquez,2 Armando Herize,1 Rosa Marı´a Domı´nguez,1 Marı´a Tosta1 and Doris Brusco2 1
Centro de Quı´mica, Instituto Venezolano de Investigaciones Cientı´ficas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela Departamento de Quı´mica, Escuela de Ciencias, Universidad de Oriente, Cumana, Sucre, Venezuela
2
Received 10 February 2003; revised 3 April 2003; accepted 8 April 2003
ABSTRACT: The gas-phase elimination kinetics of 2-substituted ethyl methylcarbonates were determined in a static reaction system over the temperature range of 323–435 C and pressure range 28.5–242 Torr. The reactions are homogeneous, unimolecular and follow a first-order rate law. The kinetic and thermodynamic parameters are reported. The 2-substituents of the ethyl methylcarbonate (CH3OCOOCH2CH2Z, Z ¼ substituent) give an approximate linear correlation when using the Taft–Topsom method, log(kZ/kH) ¼ (0.57 0.19) þ (1.34 0.49) R (r ¼ 0.9256; SD ¼ 0.16) at 400 C. This result implies the elimination process to be sensitive to steric factors, while the electronic effect is unimportant. However, the resonance factor has the greatest influence for a favorable abstraction of the -hydrogen of the C —H bond by the oxygen carbonyl. Because is significant, a good correlation of the alkyl substituents of carbonates with Hancock’s steric parameters was obtained: log(kR/kH) versus Esc for CH3OCOOCH2CH2R at 400 C, R ¼ alkyl, ¼ 0.17 (r ¼ 0.9993, SD ¼ 0.01). An approximate straight line was obtained on plotting these data with the reported Hancock’s correlation of 2-alkyl ethylacetates. This result leads to evidence for the -hydrogen abstraction by the oxygen carbonyl and not by the alkoxy oxygen at the opposite side of the carbonate. The carbonate decompostion is best described in terms of a concerted six-membered cyclic transition state type of mechanism. Copyright # 2003 John Wiley & Sons, Ltd. KEYWORDS: gas phase; elimination; pyrolysis; kinetics; 2-substituted ethyl methyl carbonates; Taft–Topsom correlation
The kinetic and thermodynamic parameters for the gasphase elimination of 2-substituted ethyl N,N-dimethylcarbamates [(CH3)2NCOOCH2CH2Z, Z ¼ substituent]
were recently reported1 and the mechanism was described as in reaction (1). This process of decomposition was believed to be similar to the concerted six-membered cyclic transition state ascribed to the gas thermal decomposition of 2-substituted ethyl acetates2,3 [reaction (2)].
*Correspondence to: G. Chuchani, Centro de Quı´mica, Instituto Venezolano de Investigaciones Cientı´ficas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela. E-mail:
[email protected]
Several correlation methods for substituent effects for these series of 2-substituted ethylcarbamates gave random points with no meanning for mechanistic interpretation. However, plotting log(kZ/kCH3) against the Taft
INTRODUCTION
Copyright # 2003 John Wiley & Sons, Ltd.
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G. CHUCHANI ET AL.
original * values4 gave rise at the origin, *(CH3) ¼ 0.00, three good straight lines. This result implied that small alterations in the polarity of the transition state may be due to changes in electronic transmission at the reaction center. This means that a simultaneous effect may be operating during the process of elimination, especially with the electron-withdrawing and multiplebonded substituents at the 2-position of ethyl N,N-dimethylcarbamates. The Taft correlation was found to be as follows: * ¼ 1.94 0.30 at 360 C (r ¼ 0.9768, SD ¼ 0.0664) Electron-withdrawing groups: * ¼ 0.12 0.02 at 360 C (r ¼ 0.9364, SD ¼ 0.0571) Multiple bonded substituents: * ¼ 0.49 0.03 at 360 C (r ¼ 0.9907, SD ¼ 0.1036)
Alkyl groups:
However, the correlation analysis for 2-substituted ethylacetates [reaction (3)] was reported5 as described below: CH3 OCOOCH2 CH2 Z ! CH3 COOH þ CH2 -----CH2 Z ð3Þ Alkyl groups: Polar groups: Polar groups: Alkyl and polar:
Multiple bonds:
log(k/k0) ¼ Esc , ¼ 0.12 (r ¼ 0.916 at 400 C) log(k/k0) ¼ **, * ¼ 0.19 (r ¼ 0.961 at 400 C) log(k/k0) ¼ II, I ¼ 1.03 (r ¼ 0.960 at 400 C) log(k/k0) ¼ (0.45 0.01) (1.29 0.11)F (r ¼ 0.959 at 400 C) log(k/k0) ¼ (1.81 0.02) (0.38 0.03)F þ (7.34 0.17) R (r ¼ 0.984 at 400 C)
In view of the several good correlations obtained by the application of different structurereactivity relationship methods for 2-substituted carbamates and acetates, it was considered interest to examine the elimination kinetics of another type of organic ethyl ester. In this respect, the kinetic parameters and comparative rates of different substituents other than carbon at the acid side of an organic ethyl ester6 were correlated by using the Taft– Topsom method. An approximate straight line, as shown in Fig. 1, was obtained. The negative value of suggested a modest participation of the polarizability or steric effect of the substituent Z. According to the greatest absolute value of F, the field or electronic effect has the most important influence in the process of elimination. The use of the value R Copyright # 2003 John Wiley & Sons, Ltd.
Figure 1. Taft-Topsom correlation for the pyrolysis of ZCOOCH2CH3:log(k/k0) ¼ (0.68 0.12) þ (2.57 0.12)F F (1.18 0.27)R (r ¼ 0.984, SD ¼ 0.119) at 400 C
confirms the interaction between the substituent Z and the incipient negative reaction center. The negative coefficient R implies a favorable effect for the abstraction of the -hydrogen of the ethyl ester by the oxygen carbonyl in the transition state. According to the linear correlation for ZCOOCH2CH3 (Fig. 1), and to the various correlations applied to aliphatic acetates and carbamates, it seemed of interest to examine esters of a more polar nature such as chloroformates or carbonates (Fig. 1). However, chloroformates are difficult to handle and unstable even at room temperature. For this reason, little research on the gas-phase elimination of these types of compounds has been reported. Consequently, the present work was aimed at examining the homogeneous molecular elimination of unsymmetrical 2-substituted ethyl methylcarbonates, CH3OCOOCH2CH2Z, Z ¼ substituent. It has been reported that esters of carbonic acids containing at least one -hydrogen atom at the alkyl side, when heated at high temperature, decompose to an alkene, alcohol (phenol) and CO2.7 O’Connor and Nace8 suggested that symmetrical carbonates undergo a firstorder rate law, give a negative entropy of activation and proceed through a unimolecular cyclic cis-elimination. In these sense, two mechanisms were proposed. Mechanism A:
Mechanism B:
J. Phys. Org. Chem. 2003; 16: 839–848
ELIMINATION KINETICS OF ETHYL METHYLCARBONATES
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A symmetrical carbonate without a -hydrogen atom, such as dimethyl carbonate, was reported to give dimethyl ether and CO2 gas when pyrolyzed between 209 and 232 C,9 and also up to 250 C10 [reaction (4)]. However, further investigation on dimethyl carbonate11 showed it to be extremely stable even up to 350 C. The stability of this compound was later confirmed at temperatures as high as 390 C.12
reason and in association with the pyrolytic elimination of 2-substituted ethyl N,N-dimethylcarbamates1 and acetates,2,3 in this work the kinetics of the homogeneous, unimolecular elimination of 2-substituted ethyl methylcarbonates were examined.
H3 C-----O-----Cð-----OÞ-----O-----CH3 ! H3 C-----O-----CH3 þ CO2 ð4Þ
The products of elimination of 2-substituted ethyl methylcarbonates in the gas phase are the corresponding alkene, metanol and carbon dioxide [reaction (5)].
In spite of the few studies on aliphatic carbonates in the gas phase, there is no evidennce for Mechanism B. Taylor13 presented several arguments in favor of Mechanism A when describing the correlation of aryl ethylcarbonate pyrolysis,14 ArOCOOCH2CH3, with ¼ 0.19 (Scheme 1). This result suggested that the electron-withdrawing substituents in the benzene ring increased the rate, aiding the C —O polarization, exactly the opposite of what part would be obtained if Mechanism B was involved, since electron withdrawal would make the ethereal part less nucleophilic, thereby lowering the reaction rate. This suggested that Mechanism A applies. Taylor and Thorne additionally pointed out that tertbutyl N,N-arylcarbamates15 gave a good Hammett correlation of ¼ 0.63. This value indicated a similar transition state of aryl ethylcarbonates, and that the abstraction of the -hydrogen to the carbonyl group is very important. Hence both elimination reactions must undergo pyrolysis via Mechanism A. Moreover, methyl alkylcarbonates were believed to be less reactive than phenyl alkylcarbonates. In this sense, 1-phenylethyl phenylcarbonate16 was found to react faster than 1-phenylethyl methylcarbonate,17 with k values of 22.2 103 and 6.98 103 at 600 K, respectively (Scheme 1). Finally, the relative rates of Et : i-Pr : t-Bu are 1 : 29.6 : 2934, which was higher than expected when compared with other esters for which consideration of steric acceleration was not ruled out. Even though these previous studies give to some extent reasonable arguments for Mechanism A, additional work may well be needed for a definite mechanistic elucidation of carbonate decomposition in the gas phase. For this
Scheme 1 Copyright # 2003 John Wiley & Sons, Ltd.
RESULTS AND DISCUSSION
CH3 OCOOCH2 CH2 Z ! ½CH3 OCOOH þ CH2 -----CH2 Z # CH3 OH þ CO2 ð5Þ The theoretical stoichiometry for the gas-phase pyrolysis of the substrates, as described in reaction (5), requires Pf/P0 ¼ 3.0, where Pf and P0 are the final and initial pressures, respectively. The average Pf/P0 values at four different temperatures and 10 half-lives were, within experimental error, nearly 3.0 (Table 1). The departure of Pf/P0 to
