Kinetics and mechanism of elimination of ethyl acetate in the gas phase: A theoretical study (original) (raw)
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Effects of polar .beta.-substituents in the gas-phase pyrolysis of ethyl acetate esters
The Journal of Physical Chemistry, 1980
The rates of the gas phase pyrolysis of six P-substituted ethyl acetates were studied in a static system over the temperature range 319-450 "C and the pressure range 63-207 mmHg. In seasoned vessels the reactions are homogeneous, follow a first-order rate law, and are unimolecular. The temperature dependence of the rate constants is given by the following Arrhenius equations for the compounds indicated: 2-(dimethy1amino)ethyl acetate, log k(s-l) = (13.90 f 0.30)-(220.4 f 3.8) kJ.mol-' (2.303RT)-'; 2-methoxyethyl acetate, log k(s-') = (12.04 f 0.24)-(203.7 f 2.9) kJemol-l (2.303RT)-'; 2-(methy1thio)ethyl acetate, log k(s-') = (11.27 f 0.39)-(179.0 f 4.6) kJ-mol-' (2.303RT)-'; 2-chloroethyl acetate, log k(s-') = (12.14 f 0.66)-(202.0 f 8.4) kJ.mol-l (2.303RT)-'; 2-fluoroethyl acetate, log k(s-') = (12.68 f 0.60)-(211.2 f 7.1) kJ.mol-' (2.303RT)-'; 2-cyanoethyl acetate, log k (d) = (11.51 f 0.13)-(171.9 f 1.7) kJ.mol-l (2.303R7'-'. The effect of substituents in the gas-phase elimination of @-substituted ethyl acetates may be grouped in three types. The linear correlation of several-I electron-withdrawing groups along strong u bonds is presented and discussed. A small amount of anchimeric assistance is proposed in the pyrolysis of the 2-(methy1thio)ethyl acetate. The experimental data are consistent with the transition state where the C,-0 bond polarization is the rate-determining process.
Theoretical Study of the Kinetics of the Pyrolytic Elimination Reaction of Ethyl Chloride
E-Journal of Chemistry, 2010
The products of the gas-phase elimination of ethyl chloride are hydrogen chloride and ethane. Using AM1, MNDO and PM3 Hamiltonians of quantum mechanical computer code called MOPAC, a procedure for the kinetics and computation of the Arrhenius parameters for the pyrolytic elimination reaction of ethyl chloride was devised in order to evaluate the predictive ability of the three Hamiltonians used. The first-order rate coefficient for the three Hamiltonians are 1.15x10-8s-1, 4.55x10-15s-1and 5.36x10-4s-1for AM1, MNDO and PM3 respectively. The results obtained showed that the rate constant for the computed Arrhenius parameters compare well with the experimental values in the literature, thus showing that the computational procedure adequately simulates experimental results; also the semi-empirical AMI calculation has the best predictive ability with experiment followed by PM3 while MNDO has the least.
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.
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.
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.
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.
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-
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.
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.