Kinetic versus Thermodynamic Control in the Dehydration of 2-Methylcyclopentanol: A Two-Part Laboratory Experiment Utilizing the Gignard Reaction and GC-MS

Abstract

was obtained from Eastman Kodak. Calcium chloride was obtained from J. T. Baker Chemical Co. (1313).GC-MS analyses were performed on a Hewlett Packard 5890 gas chromatograph coupled to a 5970 series mass selective detector. The column used was a Supelco 2-4026 15-m × 0.25-mm capillary column packed with SPBM-1 (0.25 µm). GC conditions were as follows: injector and detector temp = 200 °C; oven conditions: initial temp = 70 °C; initial time = 1 min, rate = 15°/min, final temp = 200 °C, final time = 1 min. The total time for each run was 10.67 min. ProcedureThe Grignard Reaction: Preparation of 2-Methyl-1-phenylcyclopentanol As with any Grignard reaction, care should be taken to assure the use of dry glassware and solvent. A magnetic stir bar, 0.47 g of magnesium turnings, and 15 mL of anhydrous ether are placed in a 100-mL, two-necked roundbottom flask fitted with a reflux condenser and an addition funnel containing 2.0 mL of bromobenzene and 5.0 mL of anhydrous ether. A drying tube filled with calcium chloride is connected to the top of the reflux condenser. The bromobenzene solution is then added dropwise at a rate of 1 drop per second with stirring. The appearance of a cloudy reaction mixture signals the initial reaction of magnesium with the halide. The reaction is refluxed after all the bromobenzene has been added. During this time, a solution of 2.0 mL of 2-methylcyclopentanone in 10 mL of anhydrous ether is placed in the addition funnel. After refluxing for 30 min, the heat source is removed and the ketone solution is added dropwise at a rate of 1 drop per second to the Grignard. The heat given off allows the reaction to continue refluxing on its own and a green solution results. After all the ketone solution is added, the heat source is used to continue the reflux for an additional 15 min. The mixture is then cooled in a warm water bath; additional cooling in an ice bath follows. An ether/ethanol solution is made by mixing 1.5 mL of ethanol and 10 mL of ether. While the reaction mixture is still in the ice bath, the ether/ethanol solution is placed in the addition funnel and added slowly (ca. 1 drop per second) to the reaction mixture. This causes formation of a precipitate of magnesium salts. The resulting solution is greenish yellow. Next, 20 mL of a 1 M sulfuric acid solution is placed in the addition funnel and added dropwise to the reaction mixture (still in the ice bath) to dissolve the magnesium salts. The stir bar is removed, and the contents of the flask Figure 1. Formation of the kinetically and thermodynamically favored alkenes 3 and 4 from 1. 1 2 3 4The concept of kinetic versus thermodynamic control is a common theme in organic chemistry. In the introductory organic chemistry classroom, it is often used to explain the products obtained in dehydration reactions, additions to conjugated olefins, and enolate formation. However, while the concept is firmly entrenched in the lecture curriculum, applications of kinetic versus thermodynamic control in the laboratory are less common (1-3)-though such e...

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