Theory and mechanism Epoxidation is a reaction of an alkene with a peroxycarboxylic acid (also called peracid) to produce an epoxide product, generally performed in inert solvents, such as dichloromethane. The epoxide product is a cyclic ether in which the ring contains three atoms. The alkene gains an oxygen from the peracid in a syn fashion. In this experiment, R-(-)-carvone is reacting with MCPBA, a peracid, to produce the epoxide product. Balanced equation: C10H14O + C7H5ClO3 –> C10H14O2 + C7H5ClO2
Since the rate of reaction depends on the electronic density of the double bond, and R-(-)-carvone has two double bonds, the reaction is regioselective. The more electron-riched double bond will react faster. Mechanism: Potassium permanganate is a strong oxidant and reacts readily with double bonds. In this experiment it is used to visually identify the production in the previous reaction, since it changes color from purple to yellow-brown if reacted with alkene under aqueous conditions. Balanced equation: C10H14O2 + KMnO4 + 2H2O –> C10H16O4
Mechanism: 2 – Table of reagents . |Compound |Mol Mass (g/mol) | |Dissolution of MCPBA in CH2Cl2 |solution was clear, MCPBA dissolved | |Dissolution of R-(-)-carvone in the solution |solution stayed clear | |1. 5 h after reaction |solution was cloudy white |
Part B: Syn dihydroxylation and visualization using KMnO4 (on a TLC plate) |Important steps |Observations | |TLC after 15 min, dipped in KMnO4 solution |All spots turned yellow-brown | |TLC after 1 h, dipped in KMnO4 solution |All spots turned yellow-brown | |TLC after 1. h, dipped in KMnO4 solution |The starting material was almost consumed; | | |All the spots are yellow-brown | |Evaporate the solvent to concentrate the product |The liquid obtained was transparent yellow | 5 – Results – 6 – TLC plate Table of Rf’s Compound |Rf Value | |R-(-)-carvone | | |MCPBA | | 7 – Calculations Calculation of Rf’s For the Rf of Benzil Rf = (distance travelled by the compound from the origin)/(distance travelled by the solvent form the origin) = = 8 – Flow diagram 9 – Discussion
Since in this part of the experiment, only a liquid product was obtained, the purity of the product can only be shown by TLC plate analysis. In part A, the TLC plate obtained after 1. 5 hours of the reaction showed that the reaction was not quite complete, however, another TLC was performed after 2 hours of the reaction, and the result showed the experiment was complete (no starting material remained). In part B, the TLC plates were dipped in the purple KMnO4 solution and all the spots on the plates turned yellow-brown, showing that the product contains double bond.
This cannot be a proof that the reaction was not completed or the product was not pure, since the regioselectivity of the reaction favors the electron-rich double bond. The other double may left unreacted in the reaction with MCPBA, and could be reacting with MnO4-, causing the change in color. The final product obtained was a yellow liquid. A more desirable product should be transparent orange. The use of improper equipment constitutes a source of error. In the isolation part of the experiment, the product solution was heated in a water bath to evaporate the solvent.
The container of solution was a beaker, the large opening may cause the water steam to cumulate on the inner walls and cause the dilution of liquid. As an improvement, an Erlenmeyer flask with smaller opening should be used instead. 10 – Questions -Addition of Na2SO3 solution: allows SO3 ion reacting with the carboxylic acid byproduct to produce Cl ion, which will react with Na ion to form salt, and then be filtered out of the solution; -Addition of Na2CO3 solution: due o the low solubility of CO3 ion in water, it will form CO2 gas and leave the solution. Na ion will remain in the aqueous phase. -Addition of Brine (saturated NaCl): because brine is very saturated, when added to a solution, it will dissolve and become less saturated, thus dries the water. a) Mechanism: b) a trans-2-chlorocyclopentanol would be obtained, because Cl- performed backside attack reaction which changes the stereosturcture of the C-Cl bond formed.