Question

1.   

a. Sometimes crystallization does not occur, even after cooling the solution in the ice bath. Describe the probable cause of this problem and explain what steps might be taken to induce crystallization.

b. Write balanced equations for the reduction of camphor, assuming that each mol of NaBH4 provides four moles of hydride.

c. Assign R or S configurations to each stereogenic center of isoborneol and borneol. Give the relationship between these two stereoisomers.

d. Why is it important to use a large excess of sodium borohydride when doing a reduction in aqueous ethanol? (Hint: Consider what reaction might occur between water and sodium borohydride.)

Answer 1

A.Crystallisation may fail to occur due to the following reasons:

1.The solution is too dilute i.e.the amount of solvent taken is more than the required amount.In this case,the excess solvent should be boiled off to reduce its amount and then let the solution cool at room temperature or in ice bath.

2.The solution is supersaturated.In this case,scratch the inside of the test tube with a glass rod to initiate seeding.Scratching leads to the formation of smaller particles which acts as crystal nuclei.Also we can add a tiny seed crystal in the solution which will initiate crystal formation.