Effects of polar .beta.-substituents in the gas-phase pyrolysis of ethyl acetate esters (original) (raw)
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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.
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.
Homogeneous pyrolysis kinetics of ethyl 3-hydroxy-3-methylbutanoate in the gas phase
Reaction Kinetics & Catalysis Letters, 1996
The pyrolysis kinetics of ethyl 3-hydroxy-3-methylbutanoate have been examined over the temperature range of 286-330~ and pressure range of 29-108 Tort. In a seasoned vessel and m the presence of the free radical inhibitor cyclohexene or toluene the reaction is homogeneous, unimolecular and obeys a first-order rate law. The elimination products are mainly acetone and ethyl acetate, and very small amounts of ethyl 3-butenoate, acetic acid, ethylene and I~O. The rate coefficient is expressed by the following equation: log k~ (s l) = ( 12.39 • 0.46) -(174.5 • 5.2) kJ tool ~ (2.303KT) 1. The mechanism appears to proceed via a six-membered cyclic transition state, where polarization of the (CH3)C(OH)~+~CH2COOCH2CH3 bond is rate determining.
FUDMA JOURNAL OF SCIENCES
Kinetics and thermodynamics of gas-phase thermal decomposition of 1-phenylethyl acetate to vinyl benzene and acetic acid (ethanoic) were carried out using the density functional theory (DFT) method at B3LYP/6-31++G**. Geometric parameters obtained include atomic charge distribution, dihedral angles, bond lengths, and bond angle for the ground state reactant (GS), transition state (TS), and the product (PRD) while the thermodynamic parameters such as a change in entropy change (∆S), change in enthalpy (∆Hreaction) and free Gibbs energy were calculated at 623K with an interval of 25K. Kinetic parameters determined include activation energy (Ea), Pre-exponential Arrhenius factor (log A) and rate (k). Geometric results revealed that the decomposition reaction is through asynchronous cleavage of α-ether oxygen bonds and β-carbon-hydrogen in the six-membered cyclic transition state: C2-H1 and C5-O7 bond breaking occurred first while the C9-H1 bond formation process is lagging behind in a single step. The ∆S (5.867 J/mol/K); ∆Hreaction (38.45 kcal/mol), ∆G (39.69 kJ/mol), Ea (43.7 kcal/mol), log A (12.70) and k (6.1 x 10-2 S-1) compared well with the experimental available results in literature at 623K. The intrinsic reaction coordinate (IRC) on the TS structures shows that the reactant connects to the respective minima while the Wilberg bond index shows that the TS possesses 'an early' character closer to the reactant than the products. The theoretical calculation method can be used to study the thermodynamics, mechanism and kinetics of the thermal decomposition of acetates thus reducing the cost, laboratory experiments time and exposure to hazardous chemicals.
Journal of Physical Organic Chemistry, 2002
Theoretical studies of the thermolysis of two α-amino acid ethyl esters in the gas phase were carried out using ab initio theoretical methods, at the HF/6–31G(d) and the MP2/6–311 + G(2d,p)//MP2/6–31G(d) levels of theory. The reactions studied have two steps: the first one corresponds to the formation of ethylene and a neutral amino acid intermediate via a six-membered cyclic transition state, and the second is the rapid decarboxylation of this intermediate via a five-membered cyclic transition state. The progress of the first step of the reactions was followed by means of the Wiberg bond indices. The results indicate that the transition states have an intermediate character between reactants and products, and the calculated synchronicities show that the reactions are concerted and slightly asynchronous. The bond-breaking processes are more advanced than the bond-forming processes, indicating a bond deficiency in the transition states. The kinetic parameters calculated for both reactions agree very well with the available experimental results. Copyright © 2002 John Wiley & Sons, Ltd.
The Journal of Physical Chemistry, 1979
analogous to those shown in Figure 3 for the decomposition of the hydroxylamines formed in the reactions involving primary and secondary amines. This heuristic model offers explanations for both the unusually great importance of the R-loss route in the 0 + TMA reaction and the fact that the H20 loss route was not observed in the same reaction. The latter route would require the loss of two primary hydrogens in sequential steps, each of which involves a competition with a second pathway which is probably energetically favored. These six studies have begun to reveal details of the mechanism of 0 + amine reactions under essentially collision-free conditions following the formation of an energy-rich adduct. Recognizing that an excited amine N-oxide is the first intermediate in this reaction, we have shown in this study that amine N-oxides with 60-70 kcal/mol of internal energy decompose not only along the path of lowest free-energy increase, but to a very great extent by other routes which have not been observed before. Acknowledgment. The authors gratefully acknowledge the financial support of the National Science Foundation. References and Notes
International Journal of Chemical Kinetics, 2007
The rates of elimination of several ethyl esters of 2-oxo-carboxylic acid were determined in a seasoned static reaction vessel over the temperature range 350–430°C and pressure range 33–240 Torr. The reactions, in the presence of a free-radical inhibitor, are homogeneous, unimolecular, and follow a first-order rate law. The overall and partial rate coefficients are expressed by the Arrhenius equation.Ethyl glyoxalate Ethyl 2-oxo-propionate Ethyl 3-methyl-2-oxo-butyrate The mechanisms of these elimination reactions are described in terms of concerted cyclic transition state structures. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 268–275, 2007
Journal of Physical Organic Chemistry, 2003
Theoretical studies of the thermolysis in the gas phase of two α-amino acid ethyl esters, ethyl picolinate and ethyl 1-methylpipecolinate, were carried out using ab initio theoretical methods, at the MP2/6–31G(d) and MP2/6–311+G(2d,p) levels of theory. The reactions studied have two steps: the first corresponds to the formation of a neutral amino acid intermediate via a six-membered cyclic transition state, and the second is the rapid decarboxylation of this intermediate via a five-membered cyclic transition state. The progress of the first step of the reactions was followed by means of the Wiberg bond indices. The results indicate that the transition states have an intermediate character between reactants and products, and the calculated synchronicities show that the reactions are concerted and slightly asynchronous. The bond-breaking processes are more advanced than the bond-forming processes, indicating a bond deficiency in the transition states. The kinetic parameters calculated for both reactions agree very well with the available experimental results. Copyright © 2003 John Wiley & Sons, Ltd.
Molecular Physics, 2014
The kinetics and mechanisms of thermal decomposition of phenyl acetate and p-tolyl acetate in the gas phase were studied by means of electronic structure calculations using density functional theory methods: B3LYP/6-31GA is a stepwise process involving electrocyclic hydrogen shift to eliminate ketene through concerted six-membered cyclic transition-state structure, followed by tautomerisation of cyclohexadienone or by 4-methyl cyclohexadienone intermediate to give the corresponding phenol. Mechanism B is a one-step concerted [1,3] hydrogen shift through a four-membered cyclic transition-state geometry, to produce ketene and phenol or p-cresol. Theoretical calculations showed reasonable agreement with experimental activation parameters when using the Perdew, Burke and Ernserhof (PBE)functional, through the stepwise [1,5] hydrogenshift mechanism. For mechanism B, large deviation for the entropy of activation was observed. No experimental data were available for p-tolyl acetate; however, theoretical calculations showed similar results to phenyl acetate, thus supporting the stepwise mechanism for both phenyl acetate and p-tolyl acetate.
DFT Calculations of Triethyl and Trimethyl Orthoacetate Elimination Kinetics in the Gas Phase
Journal of Physical Chemistry A, 2009
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-