Studies of acid/base reactivity in superheated and supercritical water.

摘要

Qualitative studies of deuterium exchange in amino acids in both acidic and basic superheated water (T {dollar}>{dollar} 250{dollar}spcirc{dollar}C) are reported. Exchange is found to involve ionization of a carbon-hydrogen bond in basic solution and protonation of the carbonyl group followed by rearrangement in acidic solution. Rapid hydrogen exchange ({dollar}<{dollar}10 minutes) accompanied by racemization of the chiral center is observed for the {dollar}alpha{dollar} hydrogen of all amino acids in both media. In basic media, deuteration of number of alkyl and aryl sites is observed with little decomposition of a number of amino acids while in acidic solution, rapid decomposition of all amino acids prevented observation of reactivity at sites less reactive than the hydrogen. An elementary scheme for the decomposition of amino acids in supercritical acid is proposed. The scheme involves decarboxylation followed by deamination and, in some cases, methyl migration. The possibility for production of unracemized deuterated amino acids is discussed and examples of potential candidates for such a reaction are suggested.; A quantitative method for the estimation of extremely weak acid and base dissociation constants based upon kinetic analysis of the rate of deuterium incorporation is reported. This method is applied to the reaction of hexane with hydrochloric acid in supercritical water at 400{dollar}spcirc{dollar}C. This analysis indicates that the base ionization constant for the methyl group of hexane is approximately {dollar}1times10sp{lcub}-30{rcub}{dollar} and that of the methylene groups in hexane is about 1.5 times greater than the methyl in accordance with expectations based upon the inductive effect of alkyl groups.