Question

In the multi - step conversion of an aldose into next higher aldose by Killani - Fischer synthesis , the reagent employed in the first step is :
A.${C_6}{H_5}NHN{H_2}$
B.$N{H_2}OH$
C.$B{r_2}/{H_2}O$
D.$NaCN/{H_2}O$

Answer 1

Hint: Killani - Fischer is a method of synthesising monosaccharides . In this method we have to increase the number of carbon atoms in the carbon chain . So the reagent used should be such that it increases the number of carbon atoms .

Complete step by step answer:
Aldose sugars are monosaccharides with carbon chain backbone which have a carbonyl group at the end carbon atom thus it is an aldehyde ,while hydroxyl groups are connected to the rest of the carbon atoms.
For example , glucose is an aldose sugar.
Kiliani - Fischer synthesis is named after the two chemists Hermann Emil Fischer and Heinrich Kiliani.
It is a method of synthesising monosaccharides .
A cyanohydrin is the first product formed from an aldehyde or aldose sugar which is hydrolysed so that the carbon chain of the aldose sugar is increased by one carbon atom, while the stereochemistry of all other chiral carbons is preserved.
The new product has both the stereochemistry , that is , it is a mixture of epimers.
For example, we get a mixture of D-glucose and D-mannose from D-arabinose.
Now ,we know that the carbon chain has to be increased in this process so cyanide ions can be useful in increasing the carbon chain by one carbon atom .That is why the first reagent used in the process is $NaCN/{H_2}O$ .

Therefore, option D is correct.

Note: The reason why we use $NaCN$ and not some other compound is that , after its reaction with a carbon chain the resulting nitrile can be easily hydrolysed with aqueous acid to give a carboxylic acid which in turn can undergo many further reactions .