28. Carbohydrates

When D-tagatose is added to a basic aqueous solution, an equilibrium mixture of monosaccharides is obtained, two of which are aldohexoses and two of which are ketohexoses. Identify the aldohexoses and ketohexoses.

In glycolysis, why must glucose-6-phosphate isomerize to fructose-6-phosphate (Section 22.12 ) before the cleavage reaction with aldolase occurs?

Write the mechanism for the base-catalyzed conversion of D-fructose to D-glucose and D-mannose.

Ketohexoses form from aldohexoses. Suggest a mechanism for the acid-catalyzed version of this reaction. What type of reaction is this? [You’ve seen it before.]

Propose a mechanism for the formation of d-allose from d-glucose in a basic solution.

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

(b) Propose a mechanism for the isomerization of a ketose to an aldose, via the enediol intermediate, shown immediately above. Note that the enediol has two –OH protons, and removing one or the other gives two different enolate ions.

Predict the products obtained when d-galactose reacts with each reagent.

(b) NaOH, H₂O