Question

Which of the following on reaction with excess of $NaHS{O}_3$ in aqueous solution will give mixture of salts which can be separated in two fraction by fractional crystallization ?

• A.
• B.
• C.
• D.

Answer 1

The correct option is B

$NaHS{O}_3$ addition to carbonyl compound is an example of nucleophilic addition reaction.

Mechanism of addition of $NaHS{O}_3$ involves 2 steps.
Step-1:Nucleophilic attack on ​carbonyl carbon:


Step-2: Formation of an adduct


In given reaction we have two carbonyl group so $NaHS{O}_3$ get added to both carbonyl to form two chiral centre molecule.


In the above reaction, four stereoisomers, two pairs of enantiomers are formed. Each member of a pair of enantiomer is diastereomer of each member of other pair of enantiomer.
We know, enantiomers have identical physical and chemical properties and diastereomers have different physical and chemical properties.

Hence, fractional crystallisation would give two fraction, each containing racemic mixture of enantiomers.

All other reactant on reacting with $NaHS{O}_3$ gives only a single chiral centre. Hence, they can be separated in single fraction.