Molecular decomposition of polyolefins: Kinetic parameters and geometry of the transition state

The experimental data on the molecular decomposition of olefins ( 1 -pentens) of various structures to two olefins in a gas phase were analyzed by means of the parabola intersection method. The enthalpies and kinetic parameters characterizing such decomposition were calculated for eighteen reactions. Decomposition of olefins representing two-centered concerted re action was found to be characterized by a very high classic potential barrier of thermoneutral reaction (197.4 kJ/mol). The kinetic parameters (activation energy and velocity constant) of fifteen reverse reactions of formation of olefins from two alkenes were calculated using the parabola-intersection method. The factors affect ing the activation energy of the reactions of olefin decomposition and formation are discussed. Quantum chemical calculations of the transition state energy and geometry for six reactions of olefin decomposition were performed.     

Geometric parameters of the transition state in radical reactions of antioxidants

An algorithm for calculating interatomic distances in a transition state of radical abstraction reactions through the enthalpy of a reaction is developed. The algorithm is based on a combination of quantum-chemical calculations with the calculation by the intersecting-parabolas method. Using this method and experimental data (enthalpies and activation energies of reactions), interatomic distances in a reaction center are calculated for the reactions of antioxidants with a symmetrical transition state of the N...H...N and O...H...O types and a nonsymmetrical transition state of the N...H...C, O...H...C, and O...H...N types. These distances are compared with other characteristics of the reactions and reactants. The influence of such factors as the enthalpy of the reaction, triplet repulsion in the transition state, electronegativity of atoms in the reaction center, and steric repulsion of reactants on the geometric parameters of the transition state is examined. The results obtained are used to calculate the increments characterizing the influence of various factors on interatomic distances of radical reactions considered.Translated from Kinetika i Kataliz, Vol. 45, No. 6, 2004, pp. 877–896.Original Russian Text Copyright © 2004 by Denisova, Denisov.     

Palladium catalyzed decarbonylation of aromatic acyl chlorides

A variety of palladium catalysts have been found to effectively promote the decarbonylation of aromatic acyl chlorides at elevated temperatures.     

On the rates of decarbonylation of hydroxyacetyl and other acyl radicals

The rate of decarbonylation of the hydroxyacetyl radical in acetonitrile is measured by effect modulated ESR spectroscopy and found to be log (K_D/s^?1) = (11.0 ± 1.5)–(2.3RT)^?1 (37 ± 8)kJ/mol. The result is shown to be reasonably well connected with a variety of decarbonylation data known for other acyl species by asimple Evans-Polanyi relation.     

Synthesis of cyclopropyl-iron σ complexes by decarbonylation of acyl complexes

Contrary to earlier reports [1,2] cyclopropyliron σ complexes can be conveniently prepared in fair to excellent yields by photodecarbonylation of the corresponding acyl complexes.     

The cis-hexatriene electrocyclization : Attempted calculation of the transition state geometry

A model has been developed to permit calculation of the energy changes associated with the electrocyclic reaction of cis-hexatriene to 1,3-cyclohexadiene. A novel method for solving the problems associated with calculations of the pi system energy and the energy of the forming single bond as the terminal carbons undergo rehybridization was developed. The result permits calculation of the total energy in terms of contributions from the forming sigma bond, the pi system, non-bonded interactions and bond energy changes resulting from rehybridization. The angle of rotation θ about the C₂C₃ and C₄C₅ bonds was used as an approximation to the reaction coordinate. Surprisingly the plot of energy vs θ showed no maximum for either the disrotatory or the conrotatory stereopath. Failure to produce a transition state was attributed to problems with assignment of a reasonable kcal equivalent to the Hückel gb, and an artificial device was proposed to solve the problem. The geometry of the disrotatory transition state achieved in this manner suggests that non-bonded interactions between hydrogens on the terminal carbon are not serious, but the dihedral angle between the p-orbitals at C₂C₃ (and C₄C₅) leads to a significant degree of isolation of C₂ and C₅ from the remainder of the pi system.     

A note on the transition state of radical addition reactions

An ab initio MO study of the reaction of F¹ atoms and CH^?₃ radicals with ethylene indicates that the transition state occurs later in the F¹ atom reaction and that there is considerable charge polarization in the alkene arising mainly from transfer of a β electron to fluorine.     

Kinetic parameters of radical reactions of 2-mercaptobenzothiazole with quinone imines

The chain reaction of N,N′-diphenyl-1,4-benzoquinone diimine with 2-mercaptobenzothiazole was studied by two methods developed earlier for the nonchain reaction of N-phenyl-1,4-benzoquinone monoimine with 2-mercaptobenzothiazole. In the methods used, the kinetic scheme of the reaction is simplified by creating conditions under which the rates of all stages except radical generation and decay can be neglected. One of the methods was updated. For the nonchain reaction of N-phenyl-1,4-benzoquinone monoimine with 2-mercaptobenzothiazole, both methods gave close results; for the chain reaction of N,N′-diphenyl-1,4-phenylenediamine with 2-mercaptobenzothiazole, the results differed by approximately one order of magnitude.     

Effect of medium on the rate constant of decarbonylation of phenylacetyl radical

The rate constant k _CO of decarbonylation of phenylacetyl radicals generated by photolysis of dibenzyl ketone was measured by laser flash photolysis technique in six solvents in a wide temperature range. The pre-exponential factors A and activation energies E _a of decarbonylation were found for all solvents. The k _CO value decreases with an increase in the dielectric constant of the solvent, whereas an increase in the ability of the solvent for hydrogen bonding increases k _CO. The results of quantum-chemical calculations confirm the mutual compensation of the contributions of specific and nonspecific solvations to the activation energy of decarbonylation in alcohols.     

Kinetic and thermodynamic parameters for the equilibrium reactions of phenols with the dpph. radical

The kinetics and energetics of the reversible reaction of phenols with the dpph. radical have been studied; steric shielding of the divalent N by the o-NO2 in dpph. seems to be the main cause of the entropic barriers of this reaction.     

"Beta-cis-SAr effect" on decarbonylation from alpha,beta-unsaturated acyl and aroyl complexes

Lone pair of heteroatom located at the beta-cis position in alpha,beta-unsaturated acyl and aroyl group 10 metal complexes dramatically facilitated the stoichiometric and catalytic decarbonylation reactions.     

Limits on the transition state geometry for metal insertion into a carbon-hydrogen bond

Using chelating ligands to fix the position of the metal and CH bond, evidence is found which favors a triangular geometry for metal insertion.     

Thiol-catalyzed acyl radical cyclization of alkenals

Thiol-catalyzed direct generation of acyl radicals and their intramolecular addition to olefins of alkenals gave 2-substituted five- and six-membered cyclic ketones in reasonably good yields. The combination of odorless tert-dodecanthiol and AIBN or V-40 was the initiator of choice among surveyed radical generators for the cyclization of alkenals. Aldehydes having electron-deficient olefins cyclized more easily than those having unactivated olefins.     

Non quasi-stationary state pyrolysis. Kinetic parameters of neopentane pyrolysis mechanism

The study of neopentane pyrolysis during its induction period has allowed to determine, from the quantitative measurement of major primary products only, the rate constant of the limiting chain propagation step, as well as the kinetic parameters of initiation and termination steps.

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Kinetic evidence for a tetrameric transition state in the asymmetric autocatalytic alkylation of pyrimidyl aldehydes

Experimental kinetic data coupled with kinetic modeling implicates a tetrameric transition state in the Soai autocatalytic alkylation of pyrimidyl aldehydes. The kinetic model accurately predicts both the reaction rate and the amplification in enantiomeric excess observed in reactions carried out under a wide range of conditions. These studies reveal the Soai reaction to be an example of true autocatalytic efficiency in a template-directed "minimal system" for self-replication.     

Kinetic energy release and position of transition state during the intramolecular substitution of ionized 2-benzoyl pyridines

The loss of substituents X from molecular ions of ortho substituted 2-benzoyl pyridines has been investigated as a function of the dissociation energy of the C? X bond. Comparison of unimolecular and collisional induced decompositions of the resulting [M ? X]⁺ ions and reference ions arising from 3-hydroxypyrido[1,2-α]indole shows that cyclic fragment ions are formed in every case by an intramolecular substitution reaction with the exception of the parent compound (X = H), which gives rise to a mixture of [M ? H]⁺ ions with different structures. The heat of formation of the cyclic ion has been estimated experimentally and by calculation using thermochemical data, and from this value and the appearance energies, the activation energies of the reverse reactions have been evaluated for the different reaction systems. Measurement of the kinetic energy release during the substitution reactions shows that only part of the reverse activation energy is released as kinetic energy. The energy partitioning quotient varies from 0.37 to 0.08 depending on the dissociation energy of the C? X bond or the reaction enthalpy. A sudden change in the energy partitioning quotient is observed with increasing exothermicity of the reaction, paralleling the behaviour of similar reaction systems. These results are interpreted as a demonstration of the influence of the variation of transition state position on the energy partitioning quotient.     

Polar effect and geometry of the transition state in the reactions of ozone with C-H bonds of polar molecules

A large body of experimental data on the reactions of ozone with C-H bonds of polar molecules in the liquid and gas phases is analyzed in the framework of the intersecting parabolas model. The reactions are considered as the abstraction reaction O₃ + RH → HOOO^. + R^.. The contribution from the polar effect to the activation energy of such reactions is calculated. This contribution is ?6.8 kJ/mol for the reactions of ozone with aliphatic alcohols, and is ?8.1, ?11.7, ?6.8, and ?2.2 kJ/mol for the reactions of ozone with ketones, ethers, 1,3-dioxolanes, and 1,3-dioxanes, respectively. The contribution is insignificant in the reactions of ozone with aldehydes. The interatomic distances in the transition state of these reactions r ^#(C…H) and r ^#(O…H) and the angle between the C…H and O…H bonds are calculated. For the reactions in polar solvents, the contribution from solvation to the activation energy is calculated. In most of the systems considered, this contribution is insignificant (from ?1 to ?3 kJ/mol). The reactions involving ozone are compared to the reactions of peroxy radicals with the same classes of compounds.