(a) Which of the D-aldopentoses will give optically active aldaric acids on oxidation with HNO₃ ?

(b) Which of the D-aldotetroses will give optically active aldaric acids on oxidation withHNO₃ ?

(c) Sugar X is known to be a D-aldohexose. On oxidation with HNO₃ , X gives an optically inactive aldaric acid. When X is degraded to an aldopentose, oxidation of the aldopentose gives an optically active aldaric acid. Determine the structure of X.

(d) Even though sugar X gives an optically inactive aldaric acid, the pentose formed by degradation gives an optically active aldaric acid. Does this finding contradict the principle that optically inactive reagents cannot form optically active products?

(e) Show what products results if the aldopentose formed from degradation of X is further degraded to an aldotetrose. DoesHNO₃ oxidize this aldotetrose to an optically active aldaric acid?

Nitric acid (HNO₃) is considered to be stronger oxidizing agent than bromine water since it can oxidize both aldehyde group and the terminal -CH₂OH group of an aldose to carboxylic acid (-COOH) groups. The resulting dicarboxylic acid formed is known as aldaric acid.

Meso compounds are defined as those which contain two or more asymmetric centres that are optically inactive. They consist of internal plane of symmetry such that one half of the molecule forms the mirror image of the other half. Since the optical activity due to one half is counterbalanced by the optical activity of the other half, so meso compounds are optically inactive.

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.

(a) D - arabinose and D - lyxose gives optically active aldaric acids on oxidation with HNO₃ .

D - arabinose

D - lyxose

(b) D - threose gives optically active aldaric acid on oxidation with HNO₃ .

D - threose

(c) Sugar X is D - galactose

D - galactose

D - galactose

(d) No, it does not contradict the principle since the aldaric acid formed from aldohexose is a meso compound.

(e)

D - lyxose D - threose aldaric acid

The aldaric acid formed is optically active.