In today experiment you will perform the reduction of 4- t -butylcyclohexanone t

Question

In today experiment you will perform the reduction of 4-t-butylcyclohexanone to synthesize a mixture of cis- and trans- 4-t-butylcyclohexanol. Answer the following questions regarding this reaction:

1.The reduction of 4-tert-butylcyclohexanone with sodium borohydride is a stereoselective reaction. What does this mean, in terms of the products that were isolated?

2. Why the reduction is done in methanol and not in water?

3. Why in the reaction a very big excess of NaBH4 will be used?

4.Compare and contrast the (A) reductive abilities of LiAlH4 with those of NaBH4, and, (B) compatibility of the two reducing agents with protic solvents (water, alcohols). Use as reference internet sources and your lecture manual. After the comparison, decide why NaBH4 was used in the reduction of 4-t-butylcyclohexanone (your experiment) rather than LiAlH4.

5.After the reduction the product mixture is quenched with cold dilute HCl. Why do we do that?

6.To isolate the reaction product you will perform an extraction. What solvent will be used for extraction and what layer is on the bottom? Circle the correct response:   Et2O,   H2O,   CH2Cl2, pentane.

7. What is the purpose of adding anhydrous Na2SO4 in step 8 of the procedure, before the filtration of the combined CH2Cl2 extracts?

8. Why do you expect cis- and trans-4-t-butylcyclohexanol to be separable by GC? Do you expect more cis- or trans-4-t-butylcyclohexanol?

9. Is the reaction thermodynamically or kinetically controlled?

10.Calculate the theoretical and actual % yield for the reduction of 50.0 mg 4-t-butylcyclohexanone (MW = 154.3 mg/mmol) with excess NaBH4 to obtain 30.0 mg of product (MW = 156.3 mg/mmol). Show your calculations with the proper units.

Explanation / Answer

1: The reduction of 4-tert-butylcyclohexanone with sodium borohydride is a stereoselective reaction means that the products formed are different in their stereochemistry. Hence the two products formed here are cis- and trans- 4-t-butylcyclohexanol which are diastereomers of eachother.

2: Since the product formed is an alcohol, methanol is a better solvent than water. Also methanol can be easily deparated during extraction.

3: An excess of NaBH4 is used due to the following two reasons. The first is unlike LiAlH4, NaBH4 is a milder reducing agent. The second is the order of the reaction with respect to NaBH4 is first order and hence the rate of the reaction increases with increase in the concentration of NaBH4.

4: NaBH4 is preferred over LiAlH4 due to the following reasons.

LiAlH4 is highly reactive and even reacts with polar protic solvents line water, ethanol, etc. to release H2 gas. Hence we can only perform the reduction in presence of a polar aprotic solvent in a highly anhydrous condition.

On the other hand NaBH4 is a milder reducing agent, eeasier to handle and the reaction can be easily performed in aquaous and alcoholic solution.

Also NaBH4 selectively reduces the carbonyl group without affecting other double bonds.

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