Sugar dilactones

The reaction probably proceeds via the cyclic forms of the sugars and lactones, as equilibria between the various cyclic forms and the acyclic form are presumably established rapidly under the strongly acidic conditions of these oxidations. Thus, D-galactose undergoes oxidation to (acyclic) galactaric acid, whereas similar reaction of D-mannose affords a dilactone (see preceding Chapter). 

Oxidation of aldoses yields aldonic acids (onic acids), uronic acids, and glycaric acids (sugar dicarboxylic acids, aldaric acids) (Fig. 36). Aldonic acids easily cyclize to the corresponding y-lactones. Glycaric acids may form dilactones. 

Alternatively, the reaction may proceed via the ring forms of the sugars and the lactones, but this refinement of the mechanism has not been clarified. Under the strongly acidic conditions of these oxidations, equilibria between the various ring and open-chain forms should be established quickly. In this connection, it should be noted that whereas galactose gives mucic acid (the open-chain form), mannonic acid gives a dilactone. 

---