科研之友
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摘要
The role of C-C bond-forming reactions such as aldol condensation in the degradation of organic matter in natural environments is receiving a renewed interest because naturally occurring ions, ammonium ions, NH4+, and carbonate ions, CO32-, have recently been reported to catalyze these reactions. While the catalysis of aldol condensation by OH- has been widely studied, the catalytic properties of carbonate ions, CO32-, have been little studied, especially under environmental conditions. This work presents a study of the catalysis of the aldol condensation of acetaldehyde in aqueous solutions of sodium carbonate (0.1-50 mM) at T = 295 +/- 2 K. By monitoring the absorbance of the main product, crotonaldehyde, instead of that of acetaldehyde, interferences from other reaction products and from side reactions, in particular a known Cannizzaro reaction, were avoided. The rate constant was found to be first order in acetaldehyde in the presence of both CO32- and OH-, suggesting that previous studies reporting a second order for this base-catalyzed reaction were flawed. Comparisons between the rate constants in carbonate solutions and in sodium hydroxide solutions ([NaOH] = 0.3-50 mM) showed that, among the three bases present in carbonate solutions, CO32-, HCO3, and OH-, OH- was the main catalyst for pH <= 11. CO32- became the main catalyst at higher pH, whereas the catalytic contribution of HCO3- was negligible over the range of conditions studied (pH 10.3-11.3). Carbonate-catalyzed condensation reactions could contribute significantly to the degradation of organic matter in hyperalkaline natural environments (pH >= 11) and be at the origin of the macromolecular matter found in these environments.
- 出版日期2010-11
