Question

The Wohl degradation, an alternative to the Ruff degradation, is nearly the reverse of the Kiliani-Fischer synthesis. The aldose carbonyl group is converted to the oxime, which is dehydrated by acetic anhydride to the nitrile (a cyanohydrin). Cyanohydrin formation is reversible, and a basic hydrolysis allows the cyanohydrin to lose HCN. Using the following sequence of reagents, give equations for the individual reaction in the Wohl degradation of D-arabinose to D-erythrose. Mechanisms are not required.

(1) hydroxylamine hydrochloride

(2) acetic anhydride

(3) OH^-,H₂O

Short answer

Wohl degradation of D-arabinose to D-erythrose

Step-by-step solution

Ruff degradation (shortening of the carbon chain)

Synthesis of new sugars and its structure can be known by a process known as Ruff degradation. The sugar chains are shortened in Ruff degradation. It is a two- step process which is given as below:

(1) oxidation of aldose to aldonoic acid by bromine water (Br₂/H₂O).

(2) aldonic acid formed is treated with hydrogen peroxide (H₂O₂) and ferric sulfate, Fe₂(SO₄)₃ that oxidises the carboxyl group to carbon dioxide (CO₂) and the resulting aldose is formed with one fewer carbon atom.

Kiliani- Fischer synthesis (lengthening of the carbon chain)

Adding one carbon atom to the aldehyde end of the aldose lengthens an aldose chain. The sugar chain is lengthened as a result of this process with a new carbon atom at C1 position and the aldehyde group (C1 position earlier) is now moved to C2 position. For determining the structure of existing sugars and also for synthesizing new sugars, Kiliani-Fischer synthesis is used.

The equation

Wohl degradation of D-arabinose to D-erythrose