Monday, 11 January 2016
Nuclear magnetic resonance techniques were used to study the kinetics and products of the reaction of a variety of epoxides with various amines under varying pH conditions. In agreement with a previous finding, the amine-epoxide reactions were found to be water-catalyzed, and not directly dependent on the pH of the reaction environment. At pH values higher than the pKa of the particular amine, the amine-epoxide reactions were extremely efficient, outcompeting hydrolysis reactions even for conditions where water was the solvent and the amine was a relatively low concentration solute. This finding was rationalized by measurements that showed that the relative nucleophilic strength of amines relative to water was on the order of 1000, while the nucleophilic strength of protonated amines (which are predominant when pH < pKa) was negligible. The epoxide carbon substitution environment was found to have a large effect on the measured rate constants (more substituted epoxides had slower rate constants), while the amine carbon substitution environment had a much more limited effect. While the amine-epoxide rate constants are large enough such that amine-epoxide reactions may be kinetically feasible for isoprene-derived epoxides and high amine secondary organic aerosol (SOA) concentrations, most atmospheric amines are expected to be present in protonated form on the largely acidic SOA found in the atmosphere and thus are expected to be largely unreactive towards epoxides.
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