The role of CC 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, NH+4, 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.
2010. Vol. 42, no 11, 676-686 p.