The researchers found that aldehyde oxidation by air occurs easily and at high yields on water at ambient pressure and 40°C. Water-insoluble aliphatic (branched and linear) and aromatic aldehydes float on top of the water. Stirring creates aqueous suspensions in which the aldehydes react (on water) with air to form the corresponding carboxylic acids. In small-scale laboratory trials, yield percentages reached the upper 80s for some compounds. “We can get higher yields if we use higher temperatures, O2 instead of air, and wait longer times. We can get well over 90%,” says Vigalok (Figure 1).
Figure 1. Greener result -- Arkadi Vigalok
“The reaction was especially straightforward for the preparation of solid carboxylic acids, such as benzoic acid, which precipitated from the reaction mixture and could be isolated by simple filtration. Importantly, very low amounts of byproducts (mainly the corresponding formate) were observed in the ‘on water’ reaction compared with the same reaction performed in bulk aldehyde [a common industrial approach for the oxidation]. No oxidation was observed under the same conditions when the reaction was carried out in methanol or dichloromethane instead of water,” the researchers note. For aldehydes and carboxylic acids that are soluble in water, competition can occur between on-water and in-water reactions, leading to mixtures of products. (Fuller details appear online in Angewandte Chemie International Edition -- http://www3.interscience.wiley.com/journal/26737/home.)
“Aldehyde oxidation is an interesting entry point to other reactions on water,” says Vigalok. Indeed, the researchers now are working on “using aldehyde oxidation in combination with another important process, for example, co-oxidation of organic molecules or radical addition reactions, under green on-water conditions.” They also are exploring “developing sequences of organic reactions (cascade reactions) that are accelerated by using water.”
Tel Aviv University has patented the oxidation of aldehydes on water.