The Dehydration of Cyclohexanol

Transcription

1 The Dehydration of Cyclohexanol Objective: The overall goal of this laboratory is two-fold. The first goal is for you to perform an elimination reaction like the ones we have been discussing in class. The second is to begin to introduce you to the skills that will help you in the synthesis of organic compounds. In this laboratory you will convert an alcohol into an alkene, determine the percent yield and characterize your product using IR and qualitative tests. Introduction: In this experiment an alkene (cyclohexene) will be prepared by dehydration of an alcohol (cyclohexanol) using an acid catalyst such as phosphoric acid. This is one of the most common methods of preparing alkenes. The mechanism of this reaction most likely involves a carbocation since this is a secondary alcohol. The OH group is protonated in by the acid converting it to water, making it a good leaving group. Once the water leaves, a carbocation is generated. The carbocation now has a variety of fates that await it: 1. It can react with water to form the starting material again.

2 2. Lose a proton to form the desired product 3. React with cyclohexanol to form dicyclohexyl ether. The dicyclohexyl ether is a side-product and will probably distill with the desired product. In this case, a second distillation is carried out. Safety 1. Phosphoric acid (85%) can cause severe burns. Wash all spills on the skin with cold water for 15 min. 2. Bromine causes severe burns. Keep bromine away from your skin. Do not breathe the vapors. Use only in the FUME HOOD. 3. Cyclohexanol can be irritating to the respiratory system and skin. Do not breathe vapors and prevent contact with skin. Procedure: Into a 10ml round bottom flask, add ~6g of cyclohexanol. Add about 3 ml of 85% phosphoric acid. Add 2-3 boiling chips (make sure they are not acid sensitive). Set up the distillation apparatus as shown in the example in the lab. Heat the round bottom flask slowly. The reaction will begin. Heating too much will cause the reaction to turn brown and your starting material will begin to distill as well. You will collect the

3 distillate from the side the port on the distillation set up. Use a screw cap vial to collect the distillate. When about ~7 ml of distillate have been collected, discontinue the distillation. The mixture will separate into two layers. The crude product is contaminated with water, unreacted alcohol, phosphoric acid and some side products. The lower layer is the aqueous layer. Remove this from the tube using a pipette Add water (~ 3 ml) to the liquid in the tube, cap the tube and shake to allow thorough mixing of the liquids. This process is called washing the organic layer with water. Allow the layers to separate. Cyclohexene will separate out from the water. Remove the aqueous layer like above. Now wash the cyclohexene with 10% sodium carbonate solution (~ 3 ml). Allow the layers to separate and remove the aqueous layer. Repeat the washing once again with water (~ 3 ml). Transfer the cyclohexene into a clean dry 5mL vial and add a spatula full of sodium sulfate. Stopper your vial, swirl the mixture and allow it to stand. If the sodium sulfate is not free flowing add another spatula tip full of sodium sulfate. Allow the solution to stand over the drying agent for about 5 minutes then, by gravity filtration using cotton, filter the cyclohexene into a dried pre-weighed test-tube. Immediately cap the tube and discard the drying agent and cotton wool into the waste containers which are in the fume hood. Record the weight of your cyclohexene and determine the percent yield. Infrared Spectroscopy In this experiment an alkene has been prepared from an alcohol. Functional group absorptions can be used to assist in the identification of reactants and products. Perform and IR analysis of both your product and the starting material. Determine if the IR is consistent with your product being formed. Can you determine if there are any impurities? Chemical Tests The presence of an alkene functional group and be indicated by reaction the compound with bromine in dichloromethane. The bromine solution is reddish orange in color and when added to a sample of an alkene, the bromine reacts with the alkene and forms a nearly colorless solution. To a sample of your product, slowly add some of the bromine reagent. Note what happens to the color. If the color goes away, you can infer that your compound has a double bond. Try the same test on the starting material.

4 Pre-lab Detemine the molecular weights of the starting materials and the product. Find the density and boiling points of the starting material and product. Calculate the moles of each compound. In your notebook, make a table of reagents as shown below: Reagent Molecular Weight Amount mmol used H 3 PO 4 Please write up a brief procedure for your pre-lab as well. Please also predict the products of the following reactions for your pre-lab assignment. OH CH 3 H 3 PO 4 Cl CH 3 NaOEt EtOH Post Lab Calculate the percent yield of cyclohexene and using the IR and bromine test as evidence, do you think you made your desired product?

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