Question

Except for the Tollens test, basic aqueous conditions are generally avoided with sugars because they lead to fast isomerizations.

1. Under basic conditions, the proton alpha to the aldehyde (or ketone) carbonyl group is reversibly removed, and the resulting enolate ion is no longer asymmetric. Reprotonation can occur on either face of the enolate, giving either the original structure or its epimer. Because a mixture of epimer results, this process is called epimerization. Propose a mechanism for the base-catalyzed equilibration of glucose to a mixture of glucose and its C2 epimer, mannose.
2. Propose a mechanism for the isomerization of a ketose to an aldose, via the enediol intermediate, shown immediately above. Note that the enediol has twoprotons and removing one or the other gives two different enolate ions.

Short answer

In base catalyzed reaction, as D-glucose has epimeric carbon and it is chiral, thus, in presence of base, firstly, base abstracts proton from epimeric carbon which leads to the formation of carbanion. Enolate is formed which is planar from either side, that is, one side will lead to the formation of glucose and other side will lead to the formation of mannose. Then, after hydrolysis, we get our required products. D-glucose and D-mannose are C2 epimers.

D-glucose D-mannose

Base-catalyzed mechanism of conversion of D-glucose to its C2 epimer D-mannose

1. 
2. 

Step-by-step solution

Step-1. Explanation of part (a):

In base catalyzed reaction, as D-glucose has epimeric carbon and it is chiral, thus, in presence of base, firstly, base abstracts proton from epimeric carbon which leads to the formation of carbanion. Enolate is formed which is planar from either side, that is, one side will lead to the formation of glucose and other side will lead to the formation of mannose. Then, after hydrolysis, we get our required products. D-glucose and D-mannose are C2 epimers.

D-glucose D-mannose

Base-catalyzed mechanism of conversion of D-glucose to its C2 epimer D-mannose

Step-2. Explanation of part (b):

Ketose to aldose conversion occurs via enolate and ene-diol formation. Base abstracts acidic proton in first step from alpha carbon of ketose which leads to the formation of carbanion which is resonance stabilised. Then, formation of ene-diol intermediate occurs from which base again abstracts proton and forms enolate which after hydrolysis leads to generation of an aldose. All of these steps are reversible.

Mechanism of conversion of ketose to an aldose