Question

I completed a reduction of 4-tert-butylcyclohexanol with sodium borohydride, which was supposed to yield the cis- and trans- isomers of 4-tert-butylcyclohexanol. I followed every step exactly as the lab manual said, but yielded no product. How do I explain this?? What step in the process could have led to the loss of any product?

Answer 1

You are most definitely performing the reduction of 4-t-butyl cyclohexanone to a mixture of cis- and trans- 4-t-butyl cyclohexanol. How can you reduce 4-butyl cyclohexanol to 4-t-butyl cyclohexanol by using NaBH₄? Besides, NaBH₄ is a very specific reducing agent for aldehydes and ketones only.

There are two potential sources of error:

1) You used a lower amount of NaBH₄ than required; thus 4-t-butyl cyclohexanone wasn’t completely reduced to 4-t-butyl cyclohexanol. However, you should have had the starting material left after the extraction, washing and drying step. However, the fact that you had no material whatsoever leads me to believe that the reduction step was fine.

2) The second and possibly the greatest source of error stems from the extraction step. Extraction of the product is done using a mixture of diethyl ether and water in cold condition. It is common knowledge that the solubility of organic compounds in water increases with temperature. Possibly, you didn’t cool you reaction mixture before carrying out the extraction step. The high temperature caused the diethyl ether to volatilize and the product dissolved in water and was lost with water; hence you obtained no product.

Another source of error creeps in when you use a large volume of the extraction solvent (diethyl ether in this case) and perform the extraction only once. A single stage extraction is highly inefficient to extract the product from the aqueous reaction mixture and yields a minimum amount of the product. Hence, it is always recommended that multiple extractions using small volumes (say 5-10 mL) of the extraction solvent be effected in order to maximize the yield of the product.