DFT Calculations of Triethyl and Trimethyl Orthoacetate Elimination Kinetics in the Gas Phase (original) (raw)
Related papers
Journal of Physical Chemistry A, 2010
The reaction paths for the gas-phase molecular elimination of triethyl and trimethyl orthoesters were examined at B3LYP/6-31G(d,p), B3LYP/6-31G++(d,p), B3PW91/6-31G(d,p), B3PW91++G(d,p), MPW1PW91/6-31G(d,p), and MPW1PW91/6-31++G(d,p) levels of theory. The thermal decomposition of ethyl and methyl orthoesters involves similar transition state configurations in a four-membered ring arrangement. Products formed are ethanol and the corresponding unsaturated ketal for ethyl orthoesters, while in methyl orthoesters are methanol and the corresponding unsaturated ketal. Calculated thermodynamic and kinetic parameters from B3LYP calculations were found to be in good agreement with the experimental values. The calculated data imply the polarization of the C 3 -O 4 , in the direction C 3 δ+ · · · O 4 δ-
Journal of Physical Organic Chemistry, 2008
Triethyl orthoacetate and triethyl orthopropionate were pyrolyzed in a static system over the temperature range of 291-351-C and pressure range of 80-170 Torr. The elimination reactions of these orthoesters in seasoned vessels are homogeneous, unimolecular, and follow a first-order rate law. The reaction products are ethanol, ethylene and the corresponding ethyl ester. The Arrhenius expressions of these eliminations were found as follow: for triethyl orthoacetate, log k 1 (s S1 ) ¼ (13.76 W 0.09) S (187.6 W 1.1) kJ mol S1 (2.303 RT) S1 (r ¼ 0.9993), and for triethyl orthopropionate, log k 1 (s S1 ) ¼ (13.63 W 0.07) S (193.3 W 1.8) kJ mol S1 (2.303 RT) S1 (r ¼ 0.9992). A reasonable mechanism of these elimination is to consider that the C-OCH 2 CH 3 bond, as C dR . . . dS OCH 2 CH 3 in the TS, is the rate-determining step. The nucleophilicity of the oxygen atom of OCH 2 CH 3 may abstract the hydrogen of the adjacent C-H bond for a four-membered cyclic structure to give the corresponding unsaturated ketal. The unstable ketal intermediate decomposes, in a six-membered cyclic transition state, into ethylene and the corresponding ethyl ester.
Journal of Physical Chemistry A, 2008
The gas-phase elimination kinetics of the title compounds have been examined over the temperature range of 310-369°C and pressure range of 50-130 Torr. The reactions, in seasoned vessels, are homogeneous, unimolecular, and follow a first-order rate law. The products are methanol and the corresponding methyl ketene acetal. The rate coefficients are expressed by the Arrhenius equation: for trimethyl orthoacetate, log k 1 (s -1 ) ) [(13.58 ( 0.10) -(194.7 ( 1.2) (kJ mol -1 )](2.303RT) -1 r ) 0.9998; and for trimethyl orthobutyrate, log k 1 (s -1 ) ) [(13.97 ( 0.37) -(195.3 ( 1.6) (kJ mol -1 )](2.303RT) -1 r ) 0.9997. These reactions are believed to proceed through a polar concerted four-membered cyclic transition state type of mechanism.
Combustion and Flame, 2011
A reaction mechanism of ethyl methyl ether (EME), methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE) for pyrolysis and oxidation have been constructed using the same method applied to diethyl ether (DEE) in our recent work [1]. The mechanism, comprising of 1051 reactions involving 215 species, was tested against the experimental data obtained using shock tubes with good agreement. It was found that the uni-molecular elimination reaction has a larger influence on the pyrolysis and oxidation of MTBE and ETBE compared to EME and DEE at high temperatures. The energy barrier height between reactants and transition states of molecular elimination reactions calculated by high level ab initio MO methods has revealed the difference in reactivity among the four ethers. It is also shown that ETBE or MTBE inhibit the reactivity of an equi-molar 2% mixture of hydrogen and oxygen, whereas EME and DEE do not inhibit reactivity.
International Journal of Chemical Kinetics, 2007
The pyrolysis kinetics of 2-chloro-2-methylbutane and 2-chloro-2,3-dimethylbutane have been investigated, in a static system and seasoned vessel, over the pressure range of 50-280 torr and the temperature range of 260-320 "C. The reactions are homogeneous and unimolecular, follow a first-order law, and are invariable to the presence of a cyclohexene inhibitor. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for 2-chloro-2-methylbutane, log k1 (s-l) = (13.77 f 0.25)-(184.1 f 2.6) kJ-mol-' (2.303R7')-1; for 2-chloro-2,3-dimethylbutane, log k1 (9-I) = (13.33 f 0.18)-(175.3 f 1.9) kJ-mol-' (2.303RT)-'. The distribution of the olefin products from these reactions has been quantitatively determined and reported in details. The alkyl series ((CH3),C, (CH3)&H, CH3CH2, CH3, and H) in the tertiary halides, 2-chloro-2alkylpropanes, influence the rate of elimination by electronic effect. This is similar to those obtained with a-and P-alkyl-substituted ethyl chlorides. The plot of log k/ko vs. "*(R) gives a very good straight line with p* =-4.75, r = 0.994, and intercept = 0.048 at 300 "C. The previous and present results reveal that, if a reaction center at the transition state of an organic molecule is markedly polar, the +I inductive electron release of alkyl substituents may affect gas-phase elimination processes.
Kinetics and mechanism of elimination of ethyl acetate in the gas phase: A theoretical study
Using the PM3 semi-empirical quantum mechanical molecular orbital method, a procedure was devised to study the gas phase pyrolytic reaction of ethyl acetate in order to gain a deeper insight into both its kinetics and mechanism. By considering the involvement of formal charges and geometrical changes in the activation, a mechanism was proposed in which a pre-equilibrium of acidic proton transfer is followed by the rate limiting bond polarization of C-O bond in a cyclic transition state. The reactions involve a non-synchronous break in the β β β β-carbon-hydrogen and the α α α α-ether oxygen bonds through a six-centred transition state. The results obtained showed that the rate constant and the computed Arrhenius parameters compare well with the experimental values in the literature.
Theoretical study on the elimination kinetics in the gas phase of allyl methyl compounds
Monatshefte für Chemie - Chemical Monthly, 2018
The thermal decomposition kinetics of allyl methyl amine, allyl methyl ether, and allyl methyl sulfide in the gas phase has been studied theoretically using the M06-2x/aug-cc-pVTZ quantum chemical approach. The observed activation parameters are consistent with a concerted unimolecular mechanism involving a non-planar cyclic six-membered transition state. Based on the optimized ground state geometries, a natural bond orbital analysis of donor-acceptor interactions reveals that the stabilization energies corresponding to the electronic delocalization from the lone-pair (LP) non-bonding orbitals on the heteroatom to the neighboring r à C2ÀC3 antibonding orbitals decrease from allyl methyl amine to allyl methyl sulfide. This delocalization fairly explains the increase of occupancies of LP orbitals on the heteroatom from allyl methyl sulfide to allyl methyl amine. The results also suggest that the kinetics of the thermolysis of the studied compounds are dominated by LP ! r à electronic delocalization effects. Analysis of bond order, bond indices, and synchronicity parameters demonstrates that these reactions proceed through a concerted and slightly asynchronous mechanism.
Journal of Physical Organic Chemistry, 2003
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=substituent) give an approximate linear correlation when using the Taft–Topsom method, log(kZ/kH)=−(0.57±0.19)σα+(1.34±0.49)σR− (r=0.9256; SD=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 β-hydrogen of the Cβ—H bond by the oxygen carbonyl. Because ρα 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=alkyl, δ=−0.17 (r=0.9993, SD=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 β-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.
Journal of Physical Organic Chemistry, 2010
The gas-phase elimination of 1,1-dimethoxycyclohexane yielded 1-methoxy-1-cyclohexene and methanol. The kinetics were determined in a static system, with the vessels deactivated with allyl bromide, and in the presence of the free radical inhibitor cyclohexene. The working temperature was 310-360 -C and the pressure was 25-85 Torr. The reaction was found to be homogeneous, unimolecular, and follows a first-order rate law. The temperature dependence of the rate coefficients is given by the following Arrhenius equation: log k(s S1 ) ¼ [(13.82 W 0.07) -(193.9 W 1.0)(kJ mol S1 )](2.303RT) S1 ; r ¼ 0.9995. Theoretical calculations were carried out using density functional theory (DFT) functionals B3LYP, MPW1PW91, and PBE with the basis set 6-31G(d,p) and 6-31GRR(d,p). The calculated values for the energy of activation and enthalpy of activation are in reasonably good agreement with the experimental values using the PBE/6-31G (d,p) level of theory. Both experimental results and theoretical calculations suggest a molecular mechanism involving a concerted polar four-membered cyclic transition state. The transition state structure of methanol elimination from 1,1-dimethoxycyclohexane is characterized by a significantly elongated C-O bond, while the C b -H bond is stretched to a smaller extent, as compared to the reactant. The process can be described as moderately asynchronic with some charge separation in the TS.