Theoretical study of methoxy group influence in the gas-phase elimination kinetics of methoxyalkyl chlorides (original) (raw)
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The Journal of Physical Chemistry, 1986
K gave rise to the following approximate rate constant for the unimolecular decomposition process: k3 = (5.5 f 2.0) X 10" cm3.mol-'.s-' for the unimolecular decomposition of C6Hs0 and the reaction of CH3 with C6H50, respectively. The relatively low values of the A factor and activation energy for the C6H50 decomposition reaction favor the mechanism that involves a bicyclic radical intermediate. Additionally, the modeling of CO yields in the very early stage of anisole decomposition at temperatures below 1200 kl (1.2 f 0.3) X 1OI6 exp(-33 10O/T) s-' The rate constant determined hereon for this important process to be useful for the interpretation of the complex C6H.5 is combustion chemistry.
Journal of Molecular Structure-theochem, 2009
The kinetics of the thermal decomposition of the title compounds in the gas phase have been studied at the B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), MPW91PW91/6-31G(d,p), MPW91PW91/6-31++G(d,p), PBEPBE/ 6-31G(d,p), and PBEPBE/6-31++G(d,p) levels of theory. These halide substrates produce the corresponding cyclohexadiene and hydrogen chloride. The DFT calculations suggest a non-synchronous four-membered cyclic transition state type of mechanism. The elongation and subsequent polarization of the C-Cl bond, in the direction of C d+ . . .Cl dÀ , is rate determining step in these elimination reactions. Differences in reactivity in these substrates are discussed in terms of the transition state structure and electron distribution.
Journal of Physical …
The gas-phase elimination of kinetics 4-chlorobutan-2-one, 5-chloropentan-2-one, and 4-chloro-1-phenylbutan-1-one has been studied using electronic structure methods: B3LYP/6-31G(d,p), B3LYP/6-31RRG(d,p), MPW91PW91/6-31G(d,p), MPW91PW91/6-31RRG(d,p), PBEPBE/6-31G(d,p), PBEPBE /6-31RRG(d,p), and MP2/6-31RRG(d,p). The abovementioned substrates produce hydrogen chloride and the corresponding unsaturated ketone. Calculation results of 4-chlorobutan-2-one suggest a non-synchronous four-membered cyclic transition state (TS) type of mechanism. However, in the case of 5-chloropentan-2-one and 4-chloro-1-phenylbutan-1-one, the carbonyl group assists anchimerically through a polar five-membered cyclic TS mechanism. The polarization of the C-Cl bond, in the sense of C dR . . .Cl dS , is a rate-determining step in these elimination reactions. The significant increase in rates in the elimination of 5-chloropentan-2-one and 4-chloro-1-phenylbutan-1-one is attributed to neighboring group participation due to the oxygen of the carbonyl group assisting the C-Cl bond polarization in the TS.
Progress in Reaction Kinetics and Mechanism, 2012
The gas phase elimination kinetics of 2-chloroethyltrichlorosilane (1), 2-chloro- ethylethyldichlorosilane (2), 2-chloroethyldiethylchlorosilane (3), and 2-chloro- ethyltriethylsilane (4), have been studied at the ab initio B3LYP/6-311 + G**, B3PW91 /6-311 + G** and MPW1PW91/6-311 + G** levels of theory. The B3LYP/6-311 +G** method is in good agreement with the experimental data for the activation parameters. The calculated data demonstrate that, in the elimination mechanism of 2-chloroethylsilane and derivatives, the polarization of the C1-C13 bond in the sense C1δ+ C1-δ3 is a determining factor. Based on the optimized ground state geometries using the B3LYP/6-311 + G** method, the natural bond orbital analysis (NBO) of donor - acceptor (bonding-antibonding) interactions revealed that in accordance with the increase of activation energy, the HOMO-LUMO energy-gaps in the ground state structures increase. Moreover, the order of energy barriers obtained could be explained by the numbe...
Journal of Computational Methods in Sciences and Engineering, 2012
The mechanisms of the gas-phase elimination kinetics of 1-chloro-3-methylbut-2-ene and 3-chloro-3-methylbut-1ene and their interconversion have been examined at MP2 and DFT levels of theory. These halide substrates yield isoprene and hydrogen chloride. The results MPW1PW91 calculations agree with the experimental kinetic parameters showing the elimination reaction occurs at greater rate for 1-chloro-3-methylbut-2-ene than that for the 3-chloro-3-methylbut-1-ene isomer. The mechanism for the molecular elimination of 1-chloro-3-methylbut-2-ene suggests proceeding through an uncommon sixmembered cyclic transition state for alkyl halides in the gas phase, while 3-chloro-3-methylbut-1-ene eliminates through the usual four-membered cyclic transition state. The elongation and subsequent polarization of the C-Cl bond, in the direction of C δ+ . . . Cl δ− , is rate determining step of these reactions. The isomerization of 1-chloro-3-methylbut-2-ene and 3-chloro-3methylbut-1-ene was additionally studied. The 1-chloro-3-methylbut-2-ene converts to 3-chloro-3-methylbut-1-ene easier than the reverse reaction. This means that 1-chloro-3-methylbut-2-ene was found thermodynamically more stable than 3-chloro-3methylbut-1-ene.
The Journal of Physical Chemistry A, 1997
The reaction mechanism for the decomposition of 2-chloropropionic acid in the gas phase to form hydrogen chloride, carbon monoxide, and acetaldehyde has been theoretically characterized. Analytical gradients have been used by means of AM1 and PM3 semiempirical procedures and ab initio methods at HF and DFT (BLYP) levels with the 6-31G** basis set. The correlation effects were also included by using the perturbational approach at the MP2 level with the 6-31G** and 6-31++G** basis sets and the variational approach at the CISD/6-31G** level and by means of MCSCF wave functions with a (6,6) complete active space and the 6-31G** basis set. The global potential energy surface has been studied, and the stationary points were localized and characterized. The geometries, electronic structure, and transition vector associated with the transition structures have been analyzed and the dependence of these properties upon theoretical methods is discussed. The present study points out, in agreement with the experimental data, that the decomposition process occurs through a two-step mechanism involving the formation of the R-propiolactone intermediate. The transition structure associated with the first step can be described as a five-membered ring with participation of leaving chloride and hydrogen, assisted by the carbonyl oxygen of the carboxyl group. The second transition structure, controlling the R-propiolactone decomposition step, yields the formation of CO and CH 3 CHO molecules. The rate constants and the Arrhenius preexponential factors for the different interconversion steps have been calculated in terms of the transition state theory. The comparison of experimental and theoretical values for these parameters allows us to prove the validity of theoretical methods. The results suggest that the process must be considered as essentially irreversible, the first step being the rate-determining step. From a computational point of view, the inclusion of the correlation energy at the MP2/6-31G** level is necessary to obtain an accurate calculation of the kinetic parameters. Abstract published in AdVance ACS Abstracts, February 15, 1997.
Several mechanisms in the elimination kinetics of ?-chlorocarboxylic acids in the gas phase
Journal of Physical Organic Chemistry, 1995
The kinetics of the gas-phase pyrolysis of o-chlorocarboxylic acids were examined in a seasoned static reaction vessel and in the presence of at least twice the amount of the free radical inhibitor cyclohexene or toluene. In conformity with the available experimental data on rate determination, these reactions proved to be unimolecular and obeyed a first-order rate law. The presence of the primary chlorine leaving group in Cl(CH,).COOH (n = 1-4) showed a change in mechanism from intramolecular displacement of the CI leaving group by the acidic hydrogen of the COOH to anchimeric assistance of the carbonyl COOH to the C-CI bond polarization in the transition state. This mechanistic consideration is nearly the same for the series of 2-, 3-, and 4-chlorobutyric acids. The chlorine atom at the 2-position of acetic, propionic and butyric acids is dehydrochlorinated through a prevailing reaction path involving a polar five-membered cyclic transition state.
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.