[1] A typical rearrangement reaction is that between the aldose glyceraldehyde and the ketose dihydroxyacetone in a chemical equilibrium.
The equilibrium or the reactant to product ratio depends on concentration, solvent, pH and temperature.
At equilibrium the aldose and ketose form a mixture which in the case of the glyceraldehyde and dihydroxyacetone is also called glycerose.
If, for example, D-glucose (an Aldose) rearranges to D-fructose, the ketose, the stereochemical configuration is lost in the enol form.
In the chemical reaction the enol can be protonated from two faces, resulting in the backformation of glucose or the formation of the epimer D-mannose.