Ab Initio Study on the Mechanism of the Reactions of the Nitrate Radical with Haloalkenes:  1,2-Dichloroethene, 1,1-Dichloroethene, Trichloroethene, and Tetrachloroethene

Abstract

A general mechanism for the reactions of the NO 3 radical with 1,2-dichloroethene, 1,1-dichloroethene, trichloroethene, and tetrachloroethene is proposed from ab initio DFT calculations. The calculated mechanism shows three main parallel reaction pathways. For the systems where the two carbon atoms are differently substituted, the study includes both the attacks with Markownikoff and contra-Markownikoff orientation. The first reaction pathway leads to the formation of an epoxide along with the NO 2 radical, the second one to the formation of carbonyl compounds, and the third one leads, through the cleavage of the C-C bond, to the formation of carbonyl compounds with a lower number of carbon atoms. The stationary points on the potential energy surface (PES) were found by means of density functional theory (DFT) calculations with the B3LYP functional and the 6-31G* basis set. For each stationary point, the geometry was optimized and the obtained conformation was characterized as an equilibrium structure (reactant, product, or intermediate) or transition state structure.

Evidence of A Possible Cycloaddition Channel in the Ethene + NO3 Reaction

“…At low pressures, and according to the mechanism proposed by Perez-Casany et al for the reaction of larger alkenes with NO 3 in an inert atmosphere, − cleavage of the cycle occurs (path D) to form NO and two aldehyde molecules (formaldehyde, in the case of ethene). In the absence of oxygen these products can only be obtained via the cyclic adduct.…”

“…The formation of the cyclic adduct may, in principle, also occur from the open adduct by the rotation of the nitrate group and a shortening of the O···C distance. In fact, to explain the observed formation of some simple aldehydes 10 Perez-Casany et al − propose such a path. The transition state which separates the two isomeric forms, however, has an appreciable energy.…”

“…In fact, even very high level quantum chemistry methods do not agree on the symmetry of the ground state of NO 3 . To our knowledge, the only theoretical studies on the reaction of alkenes with the nitrate radical are those performed by Pérez-Casany et al for ethene, propene, methyl substituted butene, and haloalkenes , using the ROHF and the CAS(SCF) methods in the first one, and density functional theory in the others. Their mechanism is compared with results of experimental studies at low pressures and temperatures in anaerobic conditions.…”

“…The CAS(SCF) calculations on ethene + NO 3 14 did not detect any structure between the reactants and the nitroalkyl adduct, and thus the 6.17 kcal/mol reported experimental activation barrier could not be reproduced. Later work by the same authors on other alkenes, − performed using the B3LYP density functional method, yielded a transition state whose reaction vector clearly corresponds to the formation of an O−C bond. Its energy, however, lies below the energy of the reactants, in disagreement with experimental results.…”

Isomeric Effects in the Gas-Phase Reactions of Dichloroethene, C2H2Cl2, with a Series of Cations

“…The difference in behavior of the isomers was explained by the existence of a barrier to the addition of OH to the chlorine-containing carbon of 1,1-C 2 H 2 Cl 2 . The mechanism of the reaction between polychloroethenes and neutral radicals has been described as an electrophilic attack at the less-substituted carbon atom of the alkene. − An ab initio study of the reactions of the NO 3 radical with polychlorinated ethenes predicted that for the 1,2-C 2 H 2 Cl 2 isomers one of the possible pathways leads to the production of acid chlorides, whereas the 1,1-C 2 H 2 Cl 2 isomer leads to the formation of aldehydes …”