0.2 Carvone reduction  (Page 2/2)

*All of these reagents (not just NaBH4) reduce aldehydes to 1ºROH and ketones to 2ºROH. Transformations given here are some of the more common uses for these reagents.

Prelab: reduction of carvone

(Total 10 Points)

On my honor, in preparing this report, I know that I am free to use references and consult with others. However, I cannot copy from other students’work or misrepresent my own data.

…………………………………………………………….. (signature)

Print Name: _________________________________________

1. Which reagent (LiAlH4/NaBH4) would be useful for the following: (4 point)

(a) Reduction of a ketone to an alcohol in protic solvent?

(b) Reactivity depends on pH?

(c) Reduction of an ester group?

(d) Reduction of an aldehyde group in the presence of an amide group?

2. Write the major product from the following reactions. (6 points)

Grading

• Isolation and analysis of the reduction product.
• Analysis by Thin Layer Chromatography (TLC).
• Write-up in your Lab Notebook.
• Completion of Pre-Lab and Report Questions.
• TA Points.

Materials required

Safety

Wear gloves all the time, especially when you use sodium borohydride solution. Keep safety glasses on all the time to avoid some unwanted accident.

Experimental procedure

1) Place 0.5 mL of (-)-carvone in a 5 mL conical vial with a stirring vane.

2) Add 1.5 mL of the sodium borohydride solution drop wise. As the sodium borohydride begins to react with the carvone, the yellowish solution will evolve some heat and effervescence (bubbling) may be observed.

3) Place the mixture on a stirring plate and let it sit with gentle stirring for 15 minutes.

4) At the end of 15 minutes, very carefully add 3M HCl drop wise until the solution is acidic to litmus paper. There will be quite a bit of bubbling.

5) Once the reaction has subsided, add 1-2 mL of methylene chloride. Shake the vial gently, and then let it sit for a few minutes until two distinct layers form. Remember that methylene chloride is denser than water. Since the by-products are salts, they will be dissolved in the aqueous layer. (You can add more methylene chloride first, then water if the layers are not separating. Do not increase the total volume too much.)

6) Insert a small cotton plug into the tip of a Pasteur pipette. Then carefully add approximately 100 mg of Na2SO4 to the pipette. (If you add more than 100 mg, no harm - weigh it roughly) Use a Pasteur pipette to take up the methylene chloride layer from the 5 mL vial and transfer this liquid into the filter pipette. Hold the pipette over a clean, tared 3 mL conical vial with a boiling chip and let the methylene chloride drip into this vial through the filter pipette.

You may need to use a rubber bulb to force the methylene chloride solution through the Na2SO4 plug.

7) Use thin layer chromatography (TLC) to determine the purity of your product. Remember that you should see multiple spots resulting from the complex mixture of diastereomers and other products formed. Your teaching assistant will provide a sample of (-)-carveol to spot next to your sample as a control. Stain the resulting TLC plate with p-anisaldehyde. Although these TLC results may not be very clean, you should see three main spots between 0.4 and 0.6 Rf. Each of these spots represents one of the possible products described in the introduction to this lab. Note that the diastereomers produced by the reaction have different Rf values, unlike enantiomers. There will be other spots near the top and bottom of the plate, but the three spots in the center should be larger and darker. (Important Notes: 1. CH2Cl2 is denser than water. 2. MeOH is soluble in water. 3. Sometimes layers do not want to separate, force them as described.)

8) Remove the methylene chloride and methanol left in the solution by heating the mixture at 70C until it stops boiling. You may be able to identify carveol in the mixture by smelling it and comparing it to the pure carveol sample that your teaching assistant will provide. Describe the smell (odor) of your product.

9) Determine the boiling point of your product. Do you expect a sharp boiling point range?

CAUTION

1. NaBH4 is very reactive.

2. H2 (bubbles) tends to explode. So, do not cap the system.

Waste disposal

Dispose organic substances in the proper container.

Approximate lab time: 1.75-2.25 hours

Report QuestionsReduction of Carvone

(Total 30 Points)

On my honor, in preparing this report, I know that I am free to use references and consult with others. However, I cannot copy from other students’work or misrepresent my own data.

…………………………………………………………….. (signature)

Print Name: _________________________________________

1) What are the two products of the sodium borohydride reduction of 2-tertbutylcyclohexanone? Will one be favored and if so, why? (2+2 = 4 points)

2) Explain why the two diastereomers of carveol would have different Rf values. (2 points)

3) If instead of HCl, the reaction was quenched with DCl, where would the D be located? Draw the product. (2 points)

4) Give the structure of all the products of the following reactions. Indicate proper stereochemistry where appropriate. (18 points)

5) Explain the stereochemistry of the following product distributions observed. (4 points)