Ultrasound in carbohydrate chemistry: sonophysical glucose oligomerisation and sonocatalysed sucrose oxidation.

Both aspects of the interest in using ultrasound are illustrated by our results in the field of carbohydrate chemistry. The course of the heterogeneous reaction of glucose with hydrophobic alcohols in acidic medium is directed towards the oligomerisation of glucose because of the wetting of the glucose suspension due to the efficiency of the sonophysical mixing. On the other hand, a sonocatalysis is observed during the course of the oxidation of primary hydroxyl groups in homogeneous aqueous medium by the NaOCl/TEMPO system.

Sucrose tricarboxylate by sonocatalysed TEMPO-mediated oxidation.

Oxidation of sucrose by the NaOCl/TEMPO system provided sucrose tricarboxylate without the addition of sodium bromide as co-catalyst when high-frequency (500 kHz) ultrasound was applied, in contrast to very limited conversion without sonication. In the presence of sodium bromide, sonication also caused acceleration of the oxidation. The rate increase due to sonication of the oxidant system prior to sucrose addition suggests that ultrasound acts at the level of the formation of the nitrosonium ion, the active oxidising species in the catalytic cycle.