Question

(a) Give the products expected when (-)-erythrose reacts with HCN.

(b) What is the relationship between the products? How might they be separated?

(c) Are the products optically active? Explain.

Short answer

(a) (-)-erythrose:

(b) The relationship between the products is that they are diastereomers of each other. They could be separated by crystallization, distillation process.

(c) Yes, both the products are optically active.

Step-by-step solution

(a) Give the products expected when (-)-erythrose reacts with HCN.

(-)-erythrose on reaction with HCN generates two products which are diastereomers of each other.

Cyanide ion acts as a nucleophile in this reaction and attacks at the carbonyl carbon of the aldehyde which is planar in nature, thereby generating chiral center, hence the position of hydroxyl group can be left or at right in the fischer projection.

The relationship between the products:

The products formed are diastereomers which possess different physical properties. Separation of diastereomers can be done by various methods such as crystallization, distillation, or chromatography. HPLC is a method of chromatography which is employed to separate mixture of diastereomers as diastereomers have different degrees of attraction to a stationary phase.

(c) Explain the products are optically active or not.

Both the products obtained are optically active. Each product possess two chiral centers and no plane of symmetry. Optical activity of the molecule depends whether the molecule has plane of symmetry or axis of symmetry, if it possess plane of symmetry or axis of symmetry, then the molecule will be optically inactive.