Theoretical study of the gas-phase decomposition of neutral ?-amino acid ethyl esters. Part 2?Elimination of ethyl picolinate and ethyl 1-methylpipecolinate (original) (raw)
Related papers
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.
MP2 study of the gas phase elimination mechanism of some neutral amino acids and their ethyl esters
Journal of Physical Organic Chemistry, 2008
The mechanisms of the gas phase elimination of N,N-dimethylglycine, picolinic acid, and N-phenylglycine and their ethyl esters have been examined at Mö ller-Plesset MP2/6-31G (d, p) level of theory. The ethyl esters of these 2-amino carboxylic acids produce the corresponding amino carboxylic acid and ethylene in a rate-determining step. However, the unstable intermediate amino carboxylic acid rapidly decarboxylate to give the corresponding amino compound. These calculations imply a concerted, semi-polar six-membered cyclic transition state type of mechanism for the ethyl esters, and a non-synchronous five-membered cyclic transition state for the amino acids decarboxylation. The present results support previous mechanistic consideration of the elimination of the above-mentioned compounds in the gas phase.
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.
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.
Computational Study of the Aminolysis of Esters. The Reaction of Methylformate with Ammonia
The Journal of Organic Chemistry, 2003
The aminolysis of esters is a basic organic reaction considered as a model for the interaction of carbonyl group with nucleophiles. In the present computational study the different possible mechanistic pathways of the reaction are reinvestigated by applying higher level electronic structure theory, examining the general base catalysis by the nucleophile, and a more comprehensive study the solvent effect. Both the ab initio QCISD/6-31(d,p) method and density functional theory at the B3LYP/6-31G(d) level were employed to calculate the reaction pathways for the simplest model aminolysis reaction between methylformate and ammonia. Solvent effects were assessed by the PCM method. The results show that in the case of noncatalyzed aminolysis the addition/elimination stepwise mechanism involving two transition states and the concerted mechanism have very similar activation energies. However, in the case of catalyzed aminolysis by a second ammonia molecule the stepwise mechanism has a distinctly lower activation energy. All transition states in the catalyzed aminolysis are 10-17 kcal/mol lower than those for the uncatalyzed process.
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.
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.
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.