Question

A schematic analysis of the reaction of one enantiomer with racemic mixture is shown below
$\begin{array}{ccccc}d& +& dandl& \to & \left(d\right)-\left(d\right)\\ The(+)formof& & Aracemicmixtureof& & +\\ chiralmolecules& & othermoleculeswith& & \left(d\right)-\left(l\right)\\ & & 50\%\left(d\right)and50\%\left(l\right)& & \end{array}$

The products (d-d) and (d-l) are clearly neither identical nor enantiomers (non-superimposable mirror images) as the mirror image of (d-d) is (l-l) not (d-l). They are diastereomers, 'stereoisomers that are not mirror images'.

The formation of diastereomers allows the separation of enantiomers (called resolution) which is not easy as enantiomers have identical physical properties. One general procedure for separating enantiomers is to allow them to react with naturally occurring chiral molecule to form a pair of diastereomers. These can be separated easily as they have different physical properties. if the original chemical reaction can be reversed, the enantiomers can be isolated.

Which of the following is an example of diastereomers?

• A. Two gauche forms of butane.
• B. Products of bromination of cis-2 butene in the presence of $CC{l}_4$.
• C. Gauche and anti forms of butane.
• D. Both (A) and (C).

Answer 1

Answer: (C)

Gauche and anti forms of butane.

Two gauche forms are nonsuperimposable mirror images of each other and hence they are conformational enantiomers.

Gauche and anti forms of butane are examples of diastereomers.

They are non superimposable on each other and are not mirror images of each others.

Option C is correct.