Kinetics of the base-catalysed aldol condensation of acetaldehyde

Abstract

Previously unreported complications have been observed in the base-catalysed condensation of acetaldehyde in water. New experimental data are presented which firmly establish a reaction mechanism.

Abiotic CC bond formation under environmental conditions: Kinetics of the aldol condensation of acetaldehyde in water catalyzed by carbonate ions (CO3 2− )

“…Instead, the kinetic study in this work was based on the formation of crotonaldehyde at 222 nm, which was free from interferences from other products and allowed to monitor the reaction over 95% of conversion. By contrast, previous studies of this reaction catalyzed by OH − [20] used the absorbance of acetaldehyde at 277 nm and were thus limited to the first few percentages of conversion, at best. Moreover, monitoring a reaction product instead of acetaldehyde in the present work ensured that side reactions of acetaldehyde, in particular a known Cannizzaro reaction [20], which will be discussed below, did not impact the results.…”

“…By contrast, previous studies of this reaction catalyzed by OH − [20] used the absorbance of acetaldehyde at 277 nm and were thus limited to the first few percentages of conversion, at best. Moreover, monitoring a reaction product instead of acetaldehyde in the present work ensured that side reactions of acetaldehyde, in particular a known Cannizzaro reaction [20], which will be discussed below, did not impact the results. Earlier kinetic studies of this reaction catalyzed by bases used dilatometric techniques [16,21], which are also known to suffer from experimental artifacts (see the discussion in [20]).…”

“…Their pH was measured with a pH meter (VWR pH100) with a precision of ±0.1 units, calibrated with standard solutions of pH 4, 7, and 10. Following the recommendations of previous studies [20], none of the solutions studied in this work were buffered because this would have added other base species to the system and disturbed the kinetics to be studied (see also the Discussion section). As in our previous studies [3,4,6,8], the experiments were performed in glass vials containing 4 mL of solution, continuously stirred and protected from light.…”

“…Carbonate ions are present in atmospheric aerosols [9] and in hyperalkaline surface and spring waters in New Caledonia (pH 9.2-10.8) [10], the United States (pH 10) [11,12], Cyprus (pH 9.5-11.6) [13], and Jordan (pH 12.1-12.7) [14,15], at concentrations between 0.1 mM and 1 M. But, with a few exceptions [16], their catalytic properties have been studied only under conditions very different from environmental ones: in organic solvents [17,18] or at high temperature [19]. Moreover, as will be discussed in this work, most previous studies of the base-catalyzed aldol condensation [16,20], usually using OH − as catalyst, appeared to be flawed. In this work, we propose a systematic study of the aldol condensation of acetaldehyde in carbonate solutions 0.5-50 mM, completing our previous study in 0.1-1 M carbonate solutions [8] and avoiding the artifacts of previous studies.…”

The Aldol Condensation of Acetaldehyde: the Equilibrium Constant for the Reaction and the Rate Constant for the Hydroxide Catalyzed RetroAldol Reaction

“…-condensation is usually described as "irreversible" (6)(7)(8). For reasons of experimental practicality the reaction has been studied in rather concentrated solutions' and these studies have had to contend with the notorious (6,7,9) complexity of the acetaldehyde-water-hydroxide system. Nevertheless it is curious that the statement that the self condensation of acetaldehyde is "irreversible" is so widely accepted, since all the information needed to evaluate the equilibrium constant is available in the literature.…”