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.

Answer 1

1) The reduction of 4- tert-butylcyclohexanone with sodium borohydride is a steroselective reaction thts mean 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 each other.

2) Methanol is a better solvent than water as the product formed is an alcohol . Apart from this methanol methanol also can be deparated easily during the extraction.

3) NABH4 is used in excess in the reaction because a) NABH4 is mild reducing agent unlike LiALH4 .

b) The order of the reaction with respect to NABH4 is first order due to which rate of reaction increases with increase in concentration (NABH4).

4) NABH4 is preferred in comparison of LiALH4 due to:

a) LiALH4 is highly reactive as it also reacts with polar protice solvents like ethanol, water etc. For the release of H2 gas. Hence , for the reduction we required a polar aprotic solvent also in highly anhydrous condition.

b) as we know NABH4 is mild reducing agent , also easy to handle and the reaction can be easily performed in both alcoholic and aqueous solution.

One more thing Of NABH4 is that only specifically reduces the carbonyl group and have no effect on the other double bonds .

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