Pre-Lab #5 Alkenes Preparation, Reactions and Properties

Pre-Lab: #5 Heading: Alkenes: Preparation, Reactions and Properties Experiment: # 5 Title: Dehydration of 2-methylcyclohexanol. Introduction: The purpose of this lab is to demonstrate the acid-catalyzed dehydration of cis-and trans2-methylcyclohexanol to form a mixture of 1-metyl and 3-methylcyclohexene. Samples of the products will be tested for unsaturation and another sample will be analyzed by Gas chromatography to determine the percentage composition of each product. The major product will indicate the more stable carbocation formed during the intermediate step. Overall Reaction: 2-methylcyclohexanol undergoes a dehydration reaction when reacted with conc. Sulfuric acid to give the two main products shown in the overall reaction below. OH H2SO4 CH3 cis-and trans- 2-methylcyclohexanol + + Heat CH3 1-methylcyclohexene H2O CH3 3-methylcyclohexene water 1 Mechanism: There are two mechanisms. The E1 reaction mechanism will be described for the formation of the two main products. (1) E1 reaction mechanism for the formation of 1-methlcyclohexene. OH H H O O H H + H2O H water CH3 H CH3 acid H (cis /ns) 2-methylcyclohexanol + H3O CH3 1-methylcyclohexene acid 2 (2) E1 reaction mechanism for the formation of 3-methylcycolhexene. H H H H H H OH O O H + H2O H H CH3 CH3 (cis / trans) 2-methylcyclohexanol water acid + H3 O+ CH3 3-methylcyclohexene acid Table of reagents: mol.wt. Vol. used (ml) 114.18 5 name of structure compound Cis-and trans-2methylCyclohexanol density (g/ml) 0.93 OH mass moles (g) used (theo.) 4.65 0.0407 phy. prop. 163166 oC CH3 Limiting Reagent: Cis-and trans-2-methylcyclohexanol. 3 Table of Products: Name of Compound Structure Mol.wt Theoretical yeild obtained (g) 2methyCyclohexanol OH 114.18 Actual Density Boiling color moles g/ml Point obtained (oC) 0.93 163166 Colorless 110 Colorless 0.801 104 Colorless 1.00 ~96 CH3 1methylCyclohexene 96.17 0.811 3.914 CH3 3methylCyclohexene 96.17 CH3 O water H 18 0.732 Colorless H Procedure: 1. Place a small magnetic stirbar and 5ml of the mixture of cis-and trans- 2methylcyclohexanol in a 25ml round bottomed flask, and add 3ml of 9M sulfuric acid. Gently swirl the flask to thoroughly mix the liquids. 2. Set up apparatus for fractional distillation as shown in the diagram on the next page. And attach the flask prepared in procedure (1). Use a 10ml graduated cylinder, cooled in an ice-water bath as the receiver. 3. Heat and stir the reaction mixture using a Thermowell heater and a magnetic stirrer. Adjust the voltage, to get a steady distillation rate. 4. Maintain the distillation temperature between 95-100oC, collect the distillate at a rate of about one drop, every one or two seconds, until 3-4ml of the organic olefin 4 mixture is collected. If the distillation rate becomes very slow, add 3ml of water to the reaction flask and resume the distillation. 5. Transfer the distillate to a small separatory funnel and wash the organic product successively with 10ml of water,10ml of 3N aqueous sodium hydroxide solution, and 10ml of saturated sodium chloride solution. 6. Drain the organic layer into a 25ml Erlenmeyer flask and add a small amount of anhydrous calcium chloride. After swirling for a few minutes, if the solution is not clear, add more of the drying agent swirl and wait until it clears. Decant the organic solution away from the CaCl2 using a Pasteur pipet, and transfer the solution to a tared vial. 7. Weigh the products and calculate the percentage yield. 8. Test the product mixture for unsaturation: Work at a hood. (A) The Bromine test: • Put 1-2ml of 0.1M bromine in dichloromethane in a test tube. • Add 1-2drops of the product under test to the test tube. • The rapid disappearance of the orange bromine color to give a colorless solution, is a positive test for unsaturation. • Repeat the above experiment to carry out two control test one cyclohexane, and another on cyclohexene. (B)The Baeyer Test with potassium permanganate. • Put 0.5ml of the product under test into a test tube. • Add 2-3 drops of potassium permanganate solution. • The formation of a heavy brown precipitate (MnO2) is a positive test for unsaturation. • Repeat the Baeyer test for control tests with cyclohexane and cyclohexene. 9. Analyze the products by gas chromatography to determine the percentage composition of each product in the mixture. Yield Data: Theoretical yield of the two organic products = moles of limiting reagent (cis-and trans2-methylcyclohexanol) * Mol.W of 1-methlcyclohexene or(3-methylcycolhexene) = 0.0407*96.17 = 3.914g Theoretical yield of water = 0.0407*18 = 0.733g 5 Flow chart of the distillate work-up OH CH3 1 cis-and trans- 2-methylcyclohexanol (unreacted reagent) CH3 CH3 2 3 3-methylcyclohexene 1-methylcyclohexene O HO S OH O H H O 5 4 water sulfuric acid wash with 10ml,water Aqueous layer 4 and 5 Organic layer 2, 3 and some 1 Also traces of 4 and 5 wash with 10ml, 3N aqueous sodium hydroxide Aqueous layer 4 and 5 Organic layer 2, 3 some 1 Traces of 5, 4 , OH-, and Na+ wash with 10ml, saturated sodium chloride Aqueous layer 4, 5, Na+ and OH- Dry organic products 2, 3, and some of the unreacted 1 Organic layer 1, 2, 3, and traces of 5 add some anhydrous calcium chloride to remove 5 6