A computational study of the structure, bonding, and thermochemical properties of primary Ozonides derived from substituted Phenol and Thiophenol
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Date
2013
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Abstract
Ozone (O3) is an allotrope of oxygen occurring naturally in the earth’s atmosphere. Its importance in environmental issues is well documented and cannot be overemphasized. Ozone reacts with carbon-carbon multiple bonds such as in alkenes via the ozonolysis process. The first step of ozonolysis involves the formation of an important but very unstable primary ozonide (POZ) which after formation, immediately rearranges to the more stable secondary ozonide (SOZ). Due to its instability, it is difficult to determine experimentally the structure, bonding and thermochemical properties of a POZ. Hence, theoretical methods are often used to compute the properties of this class of transient chemical species.
The results of density functional theory for the 1,3-cycloadditions of ozone to chlorophenol, dihydroxybenzene and hydroxythiophenol are presented here. Theoretical calculations for the chlorophenol, dihydroxybenzene and hydroxythiophenol ozonolysis yields reaction barriers of 56.8, 11.2 and 10.4 kcal/mol for the formation of the most stable POZs, respectively. The lowest energy POZs for chlorophenol, dihydroxybenzene and hydroxythiophenol are located at 19.1, 22.1 and 19.4 kcal/mol below the reactants, respectively. For the chlorophenol POZ, the calculated reaction enthalpies (ΔrH°) and Gibbs free energies (ΔrG°) are 20.0 and 6.8 kcal/mol below those of the reactants, respectively. For the dihydroxybenzene POZ, the calculated reaction enthalpies (ΔrH°) and Gibbs free energies (ΔrG°) are 23.1 and 9.9 kcal/mol below those of the reactants, respectively. For the hydroxythiophenol POZ, the calculated reaction enthalpies (ΔrH°) and Gibbs free energies (ΔrG°) are 20.3 and 6.9 kcal/mol below those of the reactants, respectively. Structurally, the carbon rings in the primary ozonides of chlorophenol, dihydroxybenzene and hydroxythiophenol are found to retain their planarity. This research determines that the most feasible pathway for the formation of the POZ of chlorophenol, dihydroxybenzene and hydroxythiophenol involve the addition of ozone to the 3,4-, 1,2- and 2,3-positions, respectively.
Description
A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science.
Keywords
Bonding, Thermochemical properties, Ozonides, Phenol, Thiophenol