Chem143 Lab 7 Martin Case Corinne Sadlowski Synthesis of Adipic Acid http en wikipedia org wiki Adipic acid http en wikipedia org wiki Potassium permanganate Write up for this lab to be completed on 11 02 and 11 04 Reagents and Equipment Cyclohexene from Lab 6 Potassium permanganate 6 0 g per gram of cyclohexene Concentrated H2SO4 1 mL Sodium bisulfite Erlenmeyer flasks Water bath Separatory funnel Disposable pipettes Purpose This is the second of two labs that will conduct a two step synthesis of adipic acid In the first lab you preparep cyclohexene by the dehydration of cyclohexanol and in this lab you will oxidize cyclohexene to adipic acid Scheme 1 Scheme 1 Synthesis of adipic acid OH H2O cyclohexanol O cyclohexene HO2C CO2H adipic acid The oxidation of an alkene is an example of an addition reaction Oxygen atoms in the oxidizing agent add to the carbon carbon double bond As a result the ring opens and the dicarboxylic acid is formed Several oxidants have been shown to accomplish this transformation efficiently and the reagent of choice for this lab is potassium permanganate KMnO4 The mechanism for the reaction is shown in Scheme 2 Notice that this reaction produces the syn 1 2 diol and that three equivalents of cyclohexene consume 4 equivalents of KMnO4 Under the conditions that we will use another two equivalents of KMnO4 are consumed in the oxidation of the diol to the dialdehyde and another two in the oxidation to the diacid Overall the conversion of 3 moles of cyclohexene to 3 moles of adipic acid consumes 8 moles of KMnO4 1 Chem143 Lab 7 Martin Case Corinne Sadlowski Scheme 2 Oxidation of cyclohexene using KMnO4 O MnO4 O Mn O O 3 MnO3 H2O MnO4 OH H2O MnO3 OH 2 MnO2 O OH MnO4 OH MnO4 HO2C CO2H O The acid is actually produced as a water soluble potassium salt The first step of the work up is to wash the aqueous reaction mixture with an organic solvent to remove any non acidic byproducts or intermediates We then acidify the aqueous layer to protonate the acid Adipic acid should precipitate at this stage Keeping your aqueous volume to a minimum is critical because the solubility of adipic acid in water is 1 8 by weight at room temperature Experimental Procedure 1 Prepare a water bath at 50 60 C by heating a beaker of water with a burner 2 For each gram of cyclohexene mix 6 g of potassium permanganate with 40 mL water in an Erlenmeyer flask Add the cyclohexene and mix by swirling The flask should get warm Keep mixing until the warmth subsides 3 Place the flask in the water bath and maintain the temperature at 50 60 C by occasional heating Let the reaction go for 45 min noting any changes 4 Check for the presence of excess permanganate by withdrawing a drop from the reaction mixture and touching it on a paper towel If you see a purple ring around a brown center manganese dioxide permanganate is still present If permanganate is still present add approximately 1 mL of methanol and heat and stir for several minutes Repeat until permanganate is no longer present 5 Filter the hot mixture using a Buchner funnel into a clean filter flask and rinse the solids with a few mL hot water don t use too much water 6 Transfer the filtrate to a separatory funnel and extract the aqueous phase with two 25 mL ethyl acetate washes ethyl acetate is the top layer After the washes transfer the aqueous layer to an Erlenmeyer flask 7 Carefully acidify the aqueous mixture with concentrated H2SO4 The solution should be acidified to pH 2 according to pH paper 8 Cool the flask in an ice bath to initiate crystallization of the product 9 When crystallization is complete collect the crystals by vacuum filtration 10 Weigh your product and calculate a yield 2 Chem143 Lab 7 Martin Case Corinne Sadlowski Write up Calculate a maximum theoretical yield and comment on your yield There is no analysis of the product from this lab we know from the workup that any product has to be an acid However this would not be acceptable in a research lab What analytical methods would you suggest for confirming the identity of the product Why do we use concentrated H2SO4 for the acidification and not for example concentrated HCl Why is methanol added to the excess permanganate what is the product of this reaction and why does it not interfere with the isolation of the adipic acid Waste disposal The first organic washes are flammable and should be disposed of in the appropriate labeled waste container The aqueous waste is acidic and therefore corrosive After it has been neutralized with excess sodium hydrogen carbonate it is harmless and can go down the sink Manganese dioxide should be disposed of as solid chemical waste and contaminated glassware can be cleaned with a dilute solution of sodium bisulfite Safety Your pre lab should include a comprehensive safety assessment of all the solvents and chemicals used in this lab Potassium permanganate and concentrated H2SO4 are extremely corrosive and any splashes of either of these materials should be washed down immediately with copious amounts of water 3