Chem143 Lab 6 Martin Case Corinne Sadlowski Dehydration of Cyclohexanol http en wikipedia org wiki Elimination reaction http en wikipedia org wiki Distillation Write up for this lab to be completed on 10 27 and 10 29 Reagents and Equipment Cyclohexanol 10 mL Phosphoric acid 2 mL Anhydrous magnesium sulfate 100 200 mg Distillation apparatus Beaker Small Erlenmeyer flask Boiling chips Sand bath Cotton wool Disposable pipettes Purpose This is the first of two labs that will conduct a two step synthesis of adipic acid a precursor in the industrial synthesis of Nylon In the first lab you will prepare cyclohexene by the dehydration of cyclohexanol and in the next lab you will oxidize cyclohexene to adipic acid Scheme 1 Scheme 1 Synthesis of adipic acid OH H2O cyclohexanol O HO2C cyclohexene CO2H adipic acid The dehydration of an alcohol to an alkene is an example of an elimination reaction A small molecule in this case water is lost from the substrate and a new product is formed The elimination can be acid catalyzed or base catalyzed We will use acid The mechanism for the reaction is shown in Scheme 2 Scheme 2 Acid catalyzed E1 elimination of cyclohexanol OH H OH2 H2O H 1 Chem143 Lab 6 Martin Case Corinne Sadlowski Although the mechanism implies that we only need a catalytic amount of acid these reactions are usually conducted in an excess of a strong acid such as H2SO4 or H3PO4 Fortunately it is easy to separate the volatile product from the highly acidic reaction mixture by distillation In the mechanism above all the steps are in equilibrium Removing the product by distillation decreases its concentration in the reaction mixture and the reaction responds by moving the equilibrium further to the product side Le Ch telier s principle Manipulation of the equilibrium by product removal is a common trick in synthesis For example if a reaction can be done in a solvent in which the product is insoluble precipitation of the product removes it from the equilibrium and the reaction is driven further to the product side It is likely that some water or phosphoric acid will co distil with the product so our workup includes a drying step Anhydrous magnesium sulfate MgSO4 draws water out of organic liquids by forming a hydrated crystal MgSO4 6H2O The product will be analyzed by two methods First does its boiling point in the distillation agree with the literature boiling point for cyclohexene Second gas chromatography GC will give a good estimate of product purity Experimental Procedure 1 Accurately weigh approximately 10 mL of cyclohexanol into a 50 mL round bottom flask Add about 2 mL of 85 H3PO4 and swirl the mixture gently to mix the two layers Add two boiling chips 2 Fit the flask with a fractionating column a distillation adapter and a condenser Attach a thermometer to the distillation adapter to measure the temperature of the distillate Attach a delivery tube and a collection flask The pre weighed collection flask can be immersed in ice water in a beaker to minimize evaporation of the product 5 8 4 6 3 7 9 2 1 10 11 1 sand bath and heater 2 reaction flask 3 fractionating column 4 distillation adapter 5 thermometer 6 condenser 7 water in 8 water out 9 delivery tube 10 collection flask 11 ice water 3 Heat the reaction flask using a sand bath Begin by heating at a gentle reflux for about 5 minutes then increase the temperature to distil the product Distil the reaction mixture until only about 1 mL remains Record the temperature as the distillation proceeds 4 Re weigh your collection flask with the product and calculate a crude yield 5 Transfer the distillate to a separatory funnel and wash twice with 5 10 mL water Cyclohexene is the top layer 2 Chem143 Lab 6 Martin Case Corinne Sadlowski 6 Drain the organic layer into an Erlenmeyer flask and add a little anhydrous MgSO4 drying agent Let the mixture stand for 5 minutes If the drying agent forms clumps and there are no fines visible when you swirl the flask add a little more anhydrous MgSO4 7 Filter off the drying agent using a cotton wool plug in a pipette and collect the product in a pre weighed sample vial 8 Re weigh your product and calculate a yield Keep the product in your drawer for the next experiment 9 Take a GC of your product mixture Write up This lab uses distillation to isolate a product so your pre lab should include the boiling points of all the reactants and expected products Comment on the range of temperatures that you observed during the distillation What temperature should you use for the observed boiling point of your product How does this compare with the literature value Comment on your crude and purified yields Use the peak areas in the GC chromatogram to report the yield as a percent conversion of cyclohexanol to cyclohexene Waste disposal The acidic residue after distillation should be carefully poured into the appropriately marked acid waste container Wear gloves when handling concentrated acids The aqueous washings can be disposed of down the sink The fractionating columns and distillation apparatus should be thoroughly rinsed with water Pipettes should be disposed of as broken glass Safety Your pre lab should include a comprehensive safety assessment of all the solvents and chemicals used in this lab Phosphoric acid is corrosive The sand baths can get extremely hot and will remain so after they have been turned off 3