Abiotic C?C bond formation under environmental conditions: Kinetics of the aldol condensation of acetaldehyde in water catalyzed by carbonate ions (CO32−)

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, NH⁺₄, and carbonate ions, CO₃^2?, 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, CO₃^2?, 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 CO₃^2? 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, CO₃^2?, HCO₃^?, and OH^?, OH^? was the main catalyst for pH ≤ 11. CO₃^2? 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 Wiley Periodicals, Inc. Int J Chem Kinet 42: 676–686, 2010