Question

treatment of benzaldehyde with hcn gives a mixture of two isomers which cannot be separated even by careful fractional distillation. Why?

Answer 1

This is because the isomers produced are optical isomers.

The treatment of benzaldehyde with HCN gives a mixture of two optical isomers. Almost all physical properties like boiling point are the same for such isomers. They also have identical chemical properties. Therefore, there is no property on the basis of which the two isomers can be separated even by careful fractional distillation.