Epoxide-Nucleophile interactions: Acid-catalyzed reaction of ethylene oxide with water

Abstract

We have studied the acid-catalyzed ring-opening interaction of ethylene oxide with water, a nucleophile. Ethylene oxide was selected as an initial model for the metabolically produced epoxides that may be the ultimate carcinogenic forms of some of the polynuclear aromatic hydrocarbons and are believed to undergo ring-opening interactions with a nucleophilic site (or sites) on DNA. We used an ab initio scf procedure (GAUSSIAN 70) to compute structures, energies, and charge distributions for the reacting system at several possible intermediate stages and in the final state, ethylene glycol. The initial protonation (due to the acidic conditions) facilitates ring opening both by weakening the C—O bonds and also by changing the charges on the carbons from negative to positive. For this gaseous phase interaction, a cis mode of attack (the water approaching from the oxygen side of the C—C bond) was found to be preferred over a trans attack. This may be due to internal hydrogen bonding, which also appears to play an important role at some other stages of the reaction.