Epoxidation of (R)-carvone using hydrogen peroxide

Epoxidation of (R)-carvone using hydrogen peroxide

Pre-lab questions

1. Review:
   1. Sections 8.12 and 14.11 of Wade textbook (pages 393-395 and 693–696 of the 9^th edition).
   2. Procedure for liquid-liquid extraction (background document on BlackBoard).
2. Draw a reasonable resonance structure for the a,b-unsaturated ketone in carvone.
3. Calculate the moles of hydrogen peroxide present in 1.5 mL of a 35% aqueous H₂O₂ solution.
4. Give the characteristic IR peaks of the following functional groups:

 

1. 1. Conjugated ketone
   2. Isolated ketone
   3. Conjugated C=C bond

 

1. What is the purpose of using a saturated NaCl solution (brine) in the extraction process?

 

Introduction

(R)-(-)-Carvone is the major component of the spearmint essential oil (Figure 1).  It contains two alkene C=C bonds: one is located in conjugation with the ketone (A), making it an a,b-unsaturated ketone), and the other isolated on the side chain (B). The first bond is considered electron-deficient, while the other one is considered electron-rich. This difference in electron density allows us to react one alkene bond selectively in the presence of another depending on the reaction conditions.  So far, we have seen reactions in which an electron-rich alkene reacts with an electron-deficient peroxyacid such as m-chloroperoxybenzoic acid (MCPBA) to form an epoxide. In this experiment, the electron-deficient alkene will react through a nucleophilic epoxidation using hydrogen peroxide (H₂O₂). The mechanism for this transformation is shown in Figure 2. Sodium hydroxide (NaOH) is used to deprotonate H₂O₂ to make it a stronger nucleophile (equation 1). H₂O₂ then attacks the alkene (equation 2). This reaction is possible because the resulting intermediate is resonance stabilized. The third step in the mechanism results in formation of the epoxide (equation 3).

Figure 2: Proposed mechanism for alkaline epoxidation

 

In this experiment, you will carry out the reaction of (R)-carvone with alkaline (basic) H₂O₂. You will then use IR and NMR spectroscopy to identify and confirm the reaction product(s).

 

Experimental procedure:

Consult the SDS before starting the experiment. Both hydrogen peroxide and sodium hydroxide are corrosive. Handle these solutions carefully, avoid contact with skin.

Shift 1: Add 4.8 mmol of (R)-carvone to a 50 mL Erlenmeyer flask containing a magnetic stir bar, and dissolve in 8 mL of methanol. Make sure to clamp the flask. Cool the mixture to 0 °C (ice/water bath) for one minute, then add 1.5 mL of 35% H₂O₂. While still at 0 ºC and with stirring, slowly add 1 mL of 6 M aqueous NaOH solution, dropwise with a pipet, over a period of 1-2 minutes. Continue stirring the mixture at 0°C for 15 minutes, then remove from the ice bath and stir at room temperature for 20 minutes. Dissolve the reaction mixture in 15 mL of CH₂Cl₂.

 

Shift 2: Pour the reaction mixture into a separatory funnel and wash twice with water (2 x 10 mL). Wash the organic layer with saturated NaCl solution (10 mL), and separate the layers. Dry the organic layer with anhydrous MgSO_4. Begin with approximately 1 g, and add more as needed. Filter the mixture by gravity filtration into a 100 mL pre-weighed round bottom flask, using a small amount of CH₂Cl₂ to rinse the MgSO₄. Remove the dichloromethane using the rotary evaporator. Record the mass of your product. Characterize your product using IR, ¹H NMR and ¹³C NMR spectroscopy. The spectroscopic data for carvone will be provided for comparison.

 

Experiment adapted from Mak, K. K. W., Lai, Y. M., Siu, Y. -H. J. Chem. Educ. 2006, 83, 1058.

 

Notes on the lab manuscript:

This will be the regular manuscript, including all sections except for the Introduction. Answers to the following questions should be addressed in your Results and Discussion section, in addition to the regular components, as outlined in your syllabus.

1. Discuss the yield of the epoxidation reaction. Use proper chemical terms to explain sources of error.
2. Analyze and tabulate relevant IR, ¹H NMR and ¹³C NMR peaks of your product.
3. Compare the IR spectra of carvone (provided) and your product. Explain how the C=O stretch can be used to determine which alkene was epoxidized.
4. Compare the ¹H NMR and ¹³C NMR spectra of carvone (provided) and your product. Explain how the data can be used to determine which alkene was epoxidized?
5. What product would be obtained with replacement of H₂O₂ by MCPBA in the epoxidation reaction? Provide a rationalize for your answer.

 

GRADING SCHEME

Format/Title page                                                                                                                                                       2 points

Abstract, including reaction scheme                                                                                                 3 points

Reaction Scheme                                                                                                                                                        2 points

Experimental Section                                                                                                                                     4 points

Includes instrumentation

Results and Discussion                                                                                                                                             19 points

Includes data table(s)

Conclusions                                                                                                                                                                1 point

References                                                                                                                                                                  2 points

Appendix                                                                                                                                                                                3 points

Includes relevant calculations and spectra

Grammar                                                                                                                                                                                3 points

Feedback form                                                                                                                                                            1 point

Total                                                                                                                                                                           40 points

 

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