Question

During a hydride reduction reaction where sodium borohydride reduces 3,3-dimethyl-2-butanone to 3,3-dimethyl-2-butanol,

1. What is the purpose of adding 6M HCl before performing the distillation process?

2. Why is the distellation discontinued as the distillation temperature reaches 80 degrees Celsius? Think about what is happening during distillation-what is happening to the composition of the distillate as the distillation temperature increases? Physical data is needed to support answer.

3. What are the possible contaminants left in the crude 3,3-dimethyl-2-butanol after the distillation?

Answer 1

During a hydride reduction reaction where sodium borohydride reduces 3,3-dimethyl-2-butanone to 3,3-dimethyl-2-butanol:

1). 6 M HCl is added dropwise before performing the distillation process in order to quench the excess NaBH₄. HCl will react with residual NaBH₄ to produce H₂ gas and a white precipitate of boric acid.

2). During the distillation the desired distillate is mainly methanol. The boiling point of the pure methanol is 64.7 degrees Celsius.
The distillate should collected that boils between 70-85 ^oC into the flask. As the temperature is raised above 80 ^oC all of the methanol vaporizes and the the composition of methanol in the distillate becomes 0.

3). The possible contaminants left in the crude 3,3-dimethyl-2-butanol after the distillation are solid boric acid waste and some liquid waste.