Q52E - Organic Chemistry (Mcmurry)

Question

When crystals of pure $\alpha$ -glucose are dissolved in water, isomerization occurs slowly to produce  $\beta$-glucose. Propose a mechanism for isomerization.

Step-by-Step Solution

Answer

Step 1: Mutarotation

The Mutarotation is differ from specific rotation, due to the change in equilibrium between alpha and beta anomeric forms, in the aqueous solution.

Only those carbohydrates show mutarotation which is hemiacetal or hemiketal. 

Step 2: Mechanism for conversion of alpha glucose to beta glucose

In the first step lone pair of oxygen of hydroxyl get delocalized and the bond between carbon and oxygen gets a break. Further, the rotation of the bond is observed as shown in the following mechanism. In the next step negatively charge oxygen attacks the carbon and further rearrangement.