Quantum mechanical study on the kinetics and thermodynamics of O-alkyl S-methyl dithiocarbonates through α-and β-elimination pathways (original) (raw)
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Leonardo Journal of Sciences, 2017
The study of α- and β-elimination pathways through pyrolysis on the kinetics and thermodynamics of O-alkyl S-methyl Xanthates in which the alkyl groups are ethyl, n-propyl, i-propyl, n-butyl, i-butyl and t-butyl were carried out in the gas-phase using density functional theory (DFT) with B3LYP at 6-311++G** using Spartan. The mechanism proceeded through a six-centered cyclic transition state. The result obtained for the C-H, C-O and S-H bond lengthening character in a single step suggested that the reaction is a concerted and asynchronous. The energy of formation for the α–elimination pathways is lower compared to the β-elimination initiated from the hydrogen of the β-carbon. The activation parameters obtained for the β-elimination pathways ranged from ∆H* (161.34-133.87 kJ/mol), ∆S* (-31.18-25.65 J/mol/K), ∆G* (180.65-149.77 kJ/mol) Ea (166.49-136.82 kJ/mol), k =1.14·10-3-6.0·10-2S-2), A (3.06·1011-5.89·1011). In addition, α-elimination pathway activation parameters for ipropyl and...
2021
The gas-phase elimination reaction of O-isopropyl S-methyl dithiocarbonate was studied using density functional theory with a hybrid B3LYP correlation with large 6-31G*, 6-311+G** basis sets. Calculated values of the activation and thermodynamics parameters for the thermal breakdown were estimated at 623.0K at interval of 25K. The entropy change was ∆S = (-29.842) and (-28.48) J/mol/K; free energy change ∆G = 181.491 and 131.164kJ/mol and enthalpy change ∆Hreaction = 162.808 and 113.720kJ/mol; activation energy Ea = 167.988 and 118.897kJ/mol; Arrhenius factor A = 3.56x1011 and 4.20x1011 and rate constant k = 1.4 x 10-2 and 2.9 x 10-3, 4.45x10-1S-1 compared well with the experimental results at 623K ∆S(-29.842J/mol/K) ∆G= (181.491kJ/mol) , ∆H = (162.808 kJ/mol), Ea = (167.988 kJ/mol) , A = (3.56x1011) rate constant k = (1.4 x 10-2). The results showed the influence of electron donating group on the kinetics and thermodynamics parameters of xanthates. It affirms concertedness of the ...
Journal of Taibah University for Science, 2017
Theoretical studies were carried out to investigate the thermal decomposition of ethyl dithiocarbonates (xanthate) using Hartree-Fock at the HF/321-G* level and the density functional method with Becke 3 Lee Yang pair DFT/(B3LYP), 6-31G*, 6-31G**, 6-31+G*, MP2 and CCSD in the ab initio method of calculation using Spartan 10. Geometric parameters, such as the bond length, bond angles, dihedral angles, heat of formation, atomic charges and vibrational frequencies, were obtained. The data were used to calculate the thermodynamics parameters, change in entropy S, enthalpy change H, free energy G, pre-exponential factor A, rate k at 623 K, and variation of rate k with temperatures from 498-623 K at temperature intervals of 25 K. It was observed that the values obtained are in good agreement with the experimental values for the ab initio methods, and according to Arrhenius theory, the calculated rate k increases with increasing temperature.
A computational study of the thermolysis of β-hydroxy ketones in gas phase and in m -xylene solution
Journal of Physical Organic Chemistry, 2012
Theoretical calculations at the M05-2X/6-31+G(d) level of theory have been carried out in order to explore the nature of the mechanism of the thermal decomposition reactions of the b-hydroxy ketones, 4-hydroxy-2-butanone, 4-hydroxy-2-pentanone, and 4-hydroxy-2-methyl-2-pentanone in gas phase and in m-xylene solution. The mechanism proposed is a one-step process proceeding through a six-membered cyclic transition state. A reasonable agreement between experimental and calculated activation parameters and rate constants has been obtained, the tertiary : secondary : primary alcohol rate constant ratio being calculated, at T = 503.15 K, as 5.9:4.7:1.0 in m-xylene solution and 44.1:5.0:1.0 in the gas phase, compared with the experimental values, 3.7:1.3:1.0 and 13.5:3.2:1.0, respectively. The progress of the thermal decomposition reactions of b-hydroxy ketones has been followed by means of the Wiberg bond indices. The lengthening of the O 1 -C 2 bond with the initial migration of the H 6 atom from O 5 to O 1 can be seen as the driving force for the studied reactions. Calculated synchronicity values indicate that the mechanisms correspond to concerted and highly synchronous processes. The transition states are "advanced", nearer to the products than to the reactants.
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.
International Journal of Chemical Kinetics, 2016
Thermal decomposition kinetics of dicyclopentadiene-1,8-dione 7 implied an intramolecular competition between α,βand β,γ-double bond to assist the CO elimination. Experimental thermolysis of 7 in dioxane gave 3a,7a-dihydro-1H-inden-1-one (cisbicyclo[4.3.0]nona-2,4,7-triene-9-one), CO gas, and a very small amount of indanone. This result suggested β,γ-double bond favored the extrusion of CO gas. Calculations of several density functional theory (DFT) levels of theory and CBS-QB3 method were employed. Two mechanisms were considered: a one-step concerted pathway and a stepwise mechanism involving [1,3] and [1,5] hydrogen sigmatropic migrations. The CAM-B3LYP/6-31G(d,p) calculation reasonably agrees with the experimental kinetic parameters. The mechanism appears to be unimolecular in one step concerted through a five-membered cyclic transition state. Isomerization of product cis-bicyclo[4.3.0]nona-2,4,7-triene-9-one yielding 1-indanone is presented and described. Calculation from substrate 7 may explain in a similar way the mechanism of decomposition of compounds 1-6. The present work may well promote to the possibility of carrying out experimental research works on the thermal decarbonylation kinetics in a liquid solution and in the gas phase of β,γ-unsaturated aliphatic ketones.
International Journal of Quantum Chemistry, 2011
The kinetics and mechanisms of the gas-phase elimination reactions of neopentyl chloride and neopentyl bromide have been studied by means of electronic structure calculations using density functional methods: B3LYP/6-31G(d,p), B3LYP/ 6-31þþG(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31þþG(d,p), PBEPBE/6-31G(d,p), PBEPBE /6-31þþG(d,p). The reaction channels that account in products formation have a common first step involving a Wagner-Meerwein rearrangement. The migration of the halide from the terminal carbon to the more substituted carbon is followed by beta-elimination of HCl or HBr to give two olefins: the Sayzeff and Hoffmann products. Theoretical calculations demonstrated that these eliminations proceed through concerted asynchronous process. The transition state (TS) located for the rate-determining step shows the halide detached and bridging between the terminal carbon and the quaternary carbon, while the methyl group is also migrating in a concerted fashion. The TS is described as an intimate ion-pair with a large negative charge at the halide atom. The concerted migration of methyl group provides stabilization of the TS by delocalizing the electron density between the terminal carbon and the quaternary carbon. The B3LYP/6-31þþG(d,p) allows to obtain reasonable energies and enthalpies of activation. The nature of these reactions is examined in terms of geometrical parameters, electron distribution, and bond order analysis.
Theoretical Study of the Thermolysis Reaction of Ethyl β-Hydroxycarboxylates in the Gas Phase
The Journal of Physical Chemistry A, 2002
Theoretical studies on the thermolysis of three ethyl -hydroxycarboxylates in the gas phase were carried out using ab initio theoretical methods, at the MP2/6-31G(d) and MP2/6-311++G(2d,p)//MP2/6-31G(d) levels of theory. Two competitive reaction pathways for the decomposition process have been studied. The first pathway describes a two-step mechanism, with the formation in a first step of an aldehyde, or a ketone, and an enol intermediate, followed by the tautomerization of this intermediate to ethyl acetate. The second pathway is a one-step mechanism with formation of ethylene and a carboxylic acid. Both processes occur via sixmembered cyclic transition states. The thermolysis is favorable along the first pathway with the first step as the rate-limiting step for the global process. The progress of the principal reactions was followed by means of the Wiberg bond indices. The results indicate that the transition states are late, and the calculated synchronicities show that the reactions are concerted and highly synchronous. The bond-breaking processes are more advanced than the bond-forming ones indicating a bond deficiency in the transition states. The kinetic parameters calculated for the studied reactions agree very well with the available experimental results. A theoretical study on the kinetic deuterium isotope primary, and Rand -secondary, effects has also been carried out.
Theoretical study of the thermolysis reaction of ethyl beta-hydroxycarboxylates in the gas phase
Journal of Physical Chemistry A, 2002
Theoretical studies on the thermolysis of three ethyl -hydroxycarboxylates in the gas phase were carried out using ab initio theoretical methods, at the MP2/6-31G(d) and MP2/6-311++G(2d,p)//MP2/6-31G(d) levels of theory. Two competitive reaction pathways for the decomposition process have been studied. The first pathway describes a two-step mechanism, with the formation in a first step of an aldehyde, or a ketone, and an enol intermediate, followed by the tautomerization of this intermediate to ethyl acetate. The second pathway is a one-step mechanism with formation of ethylene and a carboxylic acid. Both processes occur via sixmembered cyclic transition states. The thermolysis is favorable along the first pathway with the first step as the rate-limiting step for the global process. The progress of the principal reactions was followed by means of the Wiberg bond indices. The results indicate that the transition states are late, and the calculated synchronicities show that the reactions are concerted and highly synchronous. The bond-breaking processes are more advanced than the bond-forming ones indicating a bond deficiency in the transition states. The kinetic parameters calculated for the studied reactions agree very well with the available experimental results. A theoretical study on the kinetic deuterium isotope primary, and Rand -secondary, effects has also been carried out.