Question

Aldohexoses A and B both undergo Ruff degradation to give aldopentose C. On treatment with warm nitric acid, aldopentose C gives an optically active aldaric acid. B also reacts with warm nitric acid to give an optically active aldaric acid, but A reacts to give an optically inactive aldaric acid. Aldopentose C is degraded to aldotetrose D, which gives optically active tartaric acid when it is treated with nitric acid. Aldotetrose D is degraded to (+)-glyceraldehyde. Deduce the structures of sugars A,B,C and D, and use Figure 23-3 to determine the correct names of these sugars.

Short answer

Ruff degradation is employed when carbohydrate chain needs to be shortened or degraded by a single carbon. Aqueous solution of bromine is used in first step which is used for oxidation of aldehyde to carboxylic acid and then in second step, ferric ion catalyzes oxidation reaction with hydrogen peroxide and bond cleavage between carbon-1 and carbon-2 occurs forming an aldehyde. When aldohexoses A and B undergo Ruff degradation, then resulting aldopentose C will have one less carbon than parent chain. A gives optically inactive aldaric acid whereas B and C gives optically active aldaric acid on warming with nitric acid. Oxidation of aldehyde and alcoholic group takes place on nitric acid addition. From this statement, structures of A, B and C are deduced.

D-galactose D-talose D-lyxose

(C)

Optically active D-galactose D-talose Optically inactive

(A) (B)

Step-by-step solution

Step-1.

Ruff degradation is employed when carbohydrate chain needs to be shortened or degraded by a single carbon. Aqueous solution of bromine is used in first step which is used for oxidation of aldehyde to carboxylic acid and then in second step, ferric ion catalyzes oxidation reaction with hydrogen peroxide and bond cleavage between carbon-1 and carbon-2 occurs forming an aldehyde. When aldohexoses A and B undergo Ruff degradation, then resulting aldopentose C will have one less carbon than parent chain. A gives optically inactive aldaric acid whereas B and C gives optically active aldaric acid on warming with nitric acid. Oxidation of aldehyde and alcoholic group takes place on nitric acid addition. From this statement, structures of A, B and C are deduced.

D-galactose D-talose D-lyxose

(C)

Optically active D-galactose D-talose Optically inactive

(A) (B)

Step-2

Aldopentose C is degraded to aldotetrose D, which when treated with nitric acid gives optically active tartaric acid which is not meso compound. Aldotetrose D is further degraded to (+)-glyceraldehyde. Structure of D can be deduced by carrying out Ruff degradation of C and on nitric acid addition, aldehyde and alcoholic group of D gets oxidised to carboxylic acid. (+)-glyceraldehyde gets formed on Ruff degradation of D as hydroxyl group is on right side of chiral carbon in glyceraldehyde.

D-lyxose D-threose

(C) (D)