Question

The correct order of name of four aldoses with configuration given below respectively is:

A.L-erythrose, L-threose, D-erythrose, D-threose
B.D-erythrose, D-threose, L-erythrose, L-threose
C.L-erythrose, L-threose, L-erythrose, D-threose
D.D-threose, D-erythrose, L-threose, L-erythrose

Answer 1

Hint: Threose and Erythrose are aldotetrose in which if two same groups are placed on the same side of chiral carbon atom then it is erythrose while if two identical groups are placed on different sides of chiral carbon it is named as threose.
The Carbohydrate assigned D-configuration if \[ - OH\] group at last chiral carbon lies towards right hand side while if \[ - OH\] group at last chiral carbon lies towards left hand side it is assigned to be L-configuration.

Complete step by step answer:
1.

In this carbohydrate two same groups are placed on the same side of the chiral carbon atom hence it is called erythrose and \[ - OH\] group at last chiral carbon lies towards the right hand side hence it is named as D-erythrose.
2.

 In this carbohydrate two same groups are placed on different sides of the chiral carbon atom hence it is called threose and \[ - OH\] group at last chiral carbon lies towards the right hand side hence it is named as D-threose.
3.

In this carbohydrate two same groups are placed on the same side of the chiral carbon atom hence it is called erythrose and \[ - OH\] group at last chiral carbon lies towards the left hand side hence it is named as L-erythrose.

4.

 In this carbohydrate two same groups are placed on different sides of the chiral carbon atom hence it is called threose and \[ - OH\] group at last chiral carbon lies towards the left hand side hence it is named as L-threose.
Therefore, the correct option is B.

Note:
D- and L- nomenclature of carbohydrates was given by Fischer who introduced this system. D- and L- have no relation with optical rotation that is dextrorotatory and laevorotatory. The algebraic signs ‘+’ and ‘-‘within the brackets after the letter D and L refers to the sign of optical rotation i.e. dextrorotatory and laevorotatory respectively.