Solvent effects on ozonolysis reaction intermediates (original) (raw)
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Effect of Solvation on Ozonolysis Reaction Intermediates and Transition States
Journal of Molecular Modeling, 2000
Electrostatic solvent effects on the ozonolysis of ethylene have been investigated using correlated ab initio and density functional approaches. We use a simple polarizable continuum model for the solvent. It allows us to evaluate the medium effect on both the electronic and nuclear structure of the chemical species involved in the reaction. The computations confirm that basically the reaction proceeds through the Criegee mechanism. However, formation of the van der Waals complexes ethylene/ozone and carbonyl oxide/formaldehyde also appears to play a role. All the calculated species are stabilized with respect to the reactants except the transition state corresponding to the primary ozonide formation. In general, electrostatic solvent effects are relatively small for activation barriers of single reaction steps and more substantial for the corresponding reaction energies. Moreover, the medium significantly modifies the structure of some species for which polarization effects are crucial.
J Am Chem Soc, 1970
The cleavage of the primary ozonides of a series of unsymmetrical olefins, RlCsH4CH=CHRz, has been studied. The proportions of the two ways of cleavage to yield (a) R1C6H4CH0 + R2C+HOO-and (b) R1C6H4C+-HOO-+ RzCHO have been found to be related to Hammett's u and p constants by the equation log [x/(lx)] = -AAGof/2.3RT + (pa -p&, providing the compounds involved have a common Rz substituent (x = proportion of cleavage following route a; AAGo* = Gibbs activation energy difference between assumed transition states of the primary ozonide, when R1 = H). An interpretation of the Hammett relationship and of the behavior of the R2 groups in terms of transition states concludes that the zwitterion, which is formed preferentially, is the one whose environment is better able to stabilize the positive charge by increasing the electron density in the vicinity of the potential zwitterionic carbocation (in the transition state) cia inductive and mesomeric effects. The relative efficiencies of the Rz groups in stabilizing zwitterions can be placed in the order COCH3 > CH3 > COOH > Ph > H > CHZOH > COOCH,. The behavior of the groups COCHB and COOH, which tend to stabilize the zwitterions RQHOO-despite the fact that they are electron-withdrawing groups, is explained by resonance stabilization.
Mechanisms of gas-phase and liquid-phase ozonolysis
Journal of the American Chemical Society, 1978
Generalized valence bond (GVB) and configuration interaction (CI) calculations using an extensive basis [double { plus polarization functions (DZd)] have been carried out on peroxymethylene (H2CO0, often referred to as carbonyl oxide or as the Criegee intermediate), dioxirane, and dioxymethylene (OCH2O). The a b initio thermochemical results are combined with existing thermochemical data to analyze possible modes of ozonolysis. The predicted heat of formation of peroxymethylene is 29.1 kcal, indicating that the dissociation of the primary ozonide to form peroxymethylene biradical and formaldehyde is 9 kcal endothermic. The ring state, dioxirane, is predicted to be 36 kcal below peroxymethylene with dioxymethylene lying 15 kcal above the ring state. Gas-phase experimental results are shown to be consistent with the predicted thermochemistry. In addition, solution-phase results on the stereospecificity of ozonolysis are shown to be consistent with a biradical intermediate.
Separation and Purification Technology, 2009
The ozonolysis of 2,4-xylidine (2,4-dimethyl-aniline) in acidic aqueous solution was investigated by determining the major reaction products and their evolution as a function of the reaction time and their dependence on the pH of the reaction system. 2,4-Dimethyl-nitrobenzene and 2,4-dimethyl-phenol were found to be primary reaction products; their formation might be explained by electron transfer and substitution reactions. 2,4-Dimethyl-phenol was further oxidized yielding 2,4-dimethyl-and/or 4,6dimethyl-resorcinol by electrophilic addition of HO • radicals. The best fitting phenomenological kinetic model and the good convergence of calculated and experimentally determined rate constants imply two additional competitive pathways of substrate oxidation: (i) electrophilic addition of HO • radicals and fast subsequent substitution would also yield the resorcinol derivatives. (ii) Substrate and isolated products are thought to be oxidized by hydrogen abstraction at the benzylic sites, but the corresponding products (alcohols, aldehydes, and carboxylic acids) could not be identified. Fe(II) was added to probe for the presence of H 2 O 2 , but had no or only a minor effect on the kinetics of the ozonolysis.
Formation of secondary ozonides in the gas-phase ozonolysis of simple alkenes
Tetrahedron Letters, 1996
Secondary propene ozonide and isobutene ozonide were formed in the gas-phase ozonolysis of ethene with added acetaldehyde and acetone, respectively. Combined with the formation of hydroperoxymethyl formate and methoxymethyl hydroperoxide in the ethene-ozone reaction system in the presence of HCOOH and CHsOH, respectively, formation of the secondary ozonides reveals a close similarity between the gas-phase and the liquid-phase ozonolysis of alkenes.
‘Reductive ozonolysis’ via a new fragmentation of carbonyl oxides
Tetrahedron, 2006
This account describes the development of methodologies for 'reductive' ozonolysis, the direct ozonolytic conversion of alkenes into carbonyl groups without the intermediacy of 1,2,4-trioxolanes (ozonides). Ozonolysis of alkenes in the presence of DMSO produces a mixture of aldehyde and ozonide. The combination of DMSO and Et 3 N results in improved yields of carbonyls but still leaves unacceptable levels of residual ozonides; similar results are obtained using secondary or tertiary amines in the absence of DMSO. The infl uence of amines is believed to result from conversion to the corresponding N-oxides; ozonolysis in the presence of amine N-oxides effi ciently suppresses ozonide formation, generating high yields of aldehydes. The reactions with amine oxides are hypothesized to involve an unprecedented trapping of carbonyl oxides to generate a zwitterionic adduct, which fragments to produce the desired carbonyl group, an amine, and 1 O 2 .
Stereochemistry of ozonide formation. Effects of complexing agents and rate of warm-up
J Am Chem Soc, 1978
Data concerning differences in cis-trans ozonide ratios, depending on (1) the presence or absence of complexing agents and (2) a fast or slow warm-up of the ozonolysis reaction mixture, are presented for five different cis-trans olefin pairs. Evidence is discussed which indicates that the carbonyl oxide intermediate is complexed at low temperatures and this either prevents equilibration of syn and anti carbonyl oxide stereoisomers or selectively favors the anti isomer through stronger complexation. The effects are greater the larger the olefinic double bond substituents. truns-1,2-DiisopropyIethylene gives cistrans ozonide ratios of greater than unity under slow warm-up conditions, which is contrary to predictions based on existing stereochemical mechanisms.