engleski

Inverse Hyperconjugation in Organic Carbonate Anions

Solvolysis rates of the series of benzhydryl aryl and alkyl carbonates in 80% aqueous ethanol revealed that aryl carbonates solvolyze at least one order of magnitude faster than the corresponding alkyl carbonates, indicating additional electronic stabilization in the former nucleofuges. This rate ratio be rationalized if existence of negative hyperconjugation in the carbonate anion is considered, which enables transfer of the negative charge from carboxylate moiety to the aromatic ring. Ab initio calculations at the MP2(full)/6-311+G(2d, p) level of theory in gas phase and in solution (IEPCM) have been employed to investigate the reaction paths for heterolysis of the model esters and further decomposition of the generated aryl, alkyl and hydrogen carbonate anions to free carbon dioxide and the corresponding phenoxide/alkoxide/hydroxide anion via heterolytic TS and a subsequent complex. Energy relations, geometry parameters and bond orders in optimized structures, as well as NBO analysis 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. The calculations also reveal that due to negative hyperconjugation, the basicity of the carboxylate oxygen atoms are decreased, and the oxygen atom on alkyl/aryl group is the most basic site in carbonate anion. Therefore, decomposition of the protonated carboante anions (monoeasters of carbonic acid and its monoesters) in acidic and neutral media may proceed according to presented mechanism.

negative hyperconjugation; bicarbonate; organic carbonate

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