Chem 333L Experiment 8 Pre-lab – Dehydration of Cyclohexanol to Cyclohexene Objectives 1. Perform an acid catalyzed dehydration reaction to synthesize cyclohexene from cyclohexanol. 2. Purify the product by fractional distillation. 3. Determine identity of product by qualitative organic tests. 4. Determine the mass, percent yield, boiling point, and percent difference, of the product. Intro/Theory 1. Acid-Catalyzed Dehydration This is the process by which an alcohol can be converted to an alkene. Dehydration involves the elimination of a water molecule from adjacent carbon atoms. This often accomplished by heating the alcohol with phosphoric acid or sulfuric acid. a. Primary Alcohols These are the most difficult to dehydrate. Stronger acids and high temperatures (around 180 C) are required for a successful reaction. b. Secondary Alcohols The alcohol in this lab is a secondary alcohol. They are easier to dehydrate than primary alcohols and require lower temperatures. c. Tertiary Alcohols These undergo dehydration easier than their two alternatives. Often, room temperature is enough for the reaction to successfully occur. 2. β-Elimination Reactions β-Elimination reactions occur when molecules such as HCl, HBr, HI or H2O are eliminated from two adjacent carbons. When the molecule is water, the reaction is known as dehydration, as it is in this lab. There are two separate types of β-Elimination reactions known as E1, which is used in this lab, and E2 reactions. a. E1 Reaction This reaction is an elimination reaction where the leaving group first leaves to form a carbocation intermediate. The molecule is then deprotonated to form an alkene. The first step is the rate-determining step. This often competes with SN1 reactions. b. E2 Reaction In this reaction three steps occur simultaneously. The leaving group leaves, a base deprotonates the adjacent carbon, and a new pi bond is formed between the two carbons involved in the reaction. Like SN2, this reaction is a concerted reaction, meaning that the entire reaction occurs at once. A sterically hindered base can be used to cause an exclusive E2 reaction and major product. 3. Qualitative Test for Alkenes Alkenes undergo reactions known as “alkene addition” reactions. These can be used to detect the presence of an alkene. In this experiment, cyclohexene will be detected by the visible reaction with bromine. The if the reaction occurs, the brown color of bromine will turn clear as the bromine is added to the cyclohexene to form a dibromo product. MechanismProcedure 1. Add a boiling stone, 0.9 mL of water, 1.2 mL of 85% phosphoric acid to a 25mL round bottom flask followed by 6.2 mL of cyclohexanol. 2. Set up a fractional distillation apparatus. 3. Heat mixture gently on the heating mantle at a variac setting of 60. After a few minutes, increase the temperature by pushing sand against the flask and turning up the variac to begin the distillation. 4. Distill the mixture until the volume remaining in the flask is about 1 mL and little material is distilling. There will be two phases of the distillate, water and product. Record the boiling point of the solution. 5. Transfer the distillate into a separatory funnel and wash with 4 mL of saturated aqueous NaCl solution. 6. Separate the two layers into two separate test tubes. Discard the contents of the test tube with the lower aqueous solution. 7. Add a micro-scoop of anhydrous magnesium sulfate to the test tube containing the organic layer, swirl and let it settle to dry the solution. 8. Set the product aside in a capped vial until the next lab period. 9. In the next lab period, repeat steps 1-3. 10. Collect the product and determine the volume. Calculate the mass of the purified product from the density and volume of the product. 11. Remove the product using a pipette. Determine mass of the product. 12. Set up 4 clean small test tubes to perform the following qualitative tests: a. Add 1mL of cyclohexanol to two tubes b. Add 1mL of cyclohexene to other two tubes. 13. For one test tube from “a” and “b”, add 10 drops of the 1% kMNO4 solution. Record results for both tubes. 14. To the remaining two tubes, add 10 drops of bromine water solution. Record results. 15. Explain in conclusion if tests were positive or negative for the alkene product. ApparatusReagents Name, Structure, MW (g/mol) Melting C Boiling C Density g/mL Properties Cyclohexanol C6H11OH 100.16 20-22 161 0.962 Colorless, viscous liquid with faint camphor odor HYGROSCOPIC Phosphoric Acid H3PO4 97.99 21 158 1.685 Viscous, colorless, odorless, liquid solid at 70 C Sodium Chloride NaCl 58.44 801 1413 2.165 White Crystals HYGROSCOPIC Magnesium Sulfate MgSO4 1124 - 2.66 HYGROSCOPIC Drying agent, white powder Cyclohexene C6H10 82.15 -104 83 0.811 Colorless liquid Bromine Br2 79.904 -7.2 137.8 3.102 Brown liquid with strong smell TOXIC Potassium Permanganate KMnO4 158.09 270 dec - 2.7 Purple crystals, with a static chargeDisposal • Wash excess aqueous solutions down the drain with water. • Place organic solvents in organic liquid waste containers. • Dispose of gloves, paper towels, and broken glass in appropriate containers. • Consult TA if unsure. References Chemistry 333L&334L: Microscale Experiments in Organic Chemistry Organic Chemistry 10th