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The Reaction Paths of the Decomposition of Carbonates

In continuation of our previous work on carbonates, ab initio calculations at the MP2(full)/6-311+G(2d, p) level of theory have been employed to investigate the reaction paths for heterolysis of methyl aryl, methyl alkyl and methyl hydrogen carbonate esters that yields the methyl carbocation and the corresponding aryl/alkyl/hydrogen carbonate anion. The paths also include further decomposition of aryl, alkyl and hydrogen carbonate anions to free carbon dioxide and the corresponding phenoxide/alkoxide/hydroxide anion via heterolytic TS and a subsequent complex. The calculations have been conducted in both gas phase and neutral solution (IEFPCM). Energy relations as well as geometry parameters and bond orders in optimized structures on the reaction paths strongly suggest that the negative hyperconjugation is a dominant electronic effect in the aryl, alkyl and hydrogen carbonate anions that stabilizes them toward a carbocation, but causes their further decomposition as well. In addition, the energetics of model reactions of the aryl, alkyl and hydrogen carbonate anions with hydronium ion in PCM reveals that the negative hyperconjugation does not only decrease the basicity of the carboxylate oxygen atoms in these anions, but also increases the basicity of the aryloxy/alkoxy/hydroxyl oxygen which ultimately renders the decomposition of the Ar/R/H carbonate anion in acidic solution to carbon dioxide and phenol/alcohol/water dominant over forming the corresponding carbonic acid.

carbonate ; negative hyperconjugation ; MP2 calculations ; reaction path ; decomposition

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