Question

Write short notes on chain-shortening and chain-lengthening reactions for sugars.

Answer 1

Hint: Carbohydrates: Large complex organic compounds composed of three basic components that are carbon, oxygen and hydrogen atoms are known as carbohydrates. Sugar is a type of carbohydrate with a molecular formula $$C_6{H}_{12}{O}_6$$.

Complete answer:
Chain-shortening reactions of sugar:
Reducing the length of chain by carbon is termed as chain-shortening. The method for chain-shortening is known as Ruff Degradation which is explained as follows:
Step-1: The aldehyde group in the chain is oxidized to carboxylic group in the presence of bromine and water as follows:

Step-2: Decarboxylation (of the compound formed in step-1) takes place in the presence of hydrogen peroxide, calcium carbonate and iron (III) acetate as follows:

Hence the final compound has one carbon less than the reactant so chain-shortening of sugar molecule takes place.
Chain-lengthening reactions of sugar:
Increasing the length of chain of a compound by one carbon atom is termed as chain-lengthening. The Kiliani-Fischer synthesis is used to extend the carbon chain of a compound. Mechanism is for chain-lengthening reaction for sugar is as follows:
Step-1: Attack of cyanide ion on the aldehyde group of the compound takes place due to which formation of cyanohydrin group on the compound takes place as follows:

Step-2: Conversion of cyanohydrin group to the imine followed by hydrolysis in the acidic conditions takes place as follows:

Hence the final compound has one carbon more than the reactant, so chain-lengthening of sugar molecules takes place.

Note:
In Kiliani-Fischer synthesis, two epimers are formed as the product whereas in Ruff-degradation, only the conformation of the reactant is retained.
Epimer: The two isomers of a compound having different configurations about a single asymmetric carbon atom.