Question

Using an ice bath, cool 10mL of saturated sodium bisulfite solution; then add, drop by drop, 5mL of 2-pentanone. Stir the solution until there is precipitation of a solid. Add 25 mL of ethanol to encourage precipitation; then filter the crystals and wash them with 5 ml of ethanol. The product is a bisulfite addition adduct. What makes the Bisulfite product soluble in 10% NaOH but insoluble in 10% HCl. What does ethanol do to the product in this reaction? Provide a mechanism.

Answer 1

Since the above product is a sodium salt, it gets dissolved in room temperature water (so it is advisable to use cold NaHSO3 solution in the beginning) and it is insoluble in organic solvent like ethanol. Therefore, when we add more ethanol, the dissolved bisulfite addition adduct thrown out from the water and more crystals formed. To eliminate the unreacted 5-pentanone, the crystals are washed with ethanol.

When the bisulfite adduct product reacts with dil.HCl it forms insoluble 2-hydroxypentane-2-sulfonic acid. When it reacts with dil.NaOH the bisulfite product gives 2-pentanone followed by the formation of disodium salt. Later, 2-pentanone was separated using organic solvents like diethyl ether. These reactions are used to purify 2-pentanone.