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LABORATORY 4 Fractional Distillation Concept Goals: Role of Fractionating Column and When Such a Column Is Necessary, Refraction of Light by Molecules

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LABORATORY 4 Fractional Distillation Concept Goals: Role of Fractionating Column and When Such a Column Is Necessary, Refraction of Light by Molecules Organic Chemistry: Techniques and Transformations LABORATORY 4 Fractional Distillation Concept goals: Role of fractionating column and when such a column is necessary, refraction of light by molecules. Operational goal: Develop the skill to perform a distillation effectively, use refractive indices to determine the identity of compounds. Introduction This laboratory provides practice in separating a homogeneous mixture, or solution, into its pure components. This separation uses a technique called fractional distillation. This separation is a physical method, which depends on differences in boiling points of the two or more components. Fractional distillation is a common practice in industry and in the laboratory. On a large scale it is used to refine petroleum into the “fractions” used for gasoline, heating oil, etc. In the laboratory it is employed whenever a mixture of liquids must be separated. By noting the boiling temperature of the collected fractions and other physical properties, fractional distillation can also be used to identify the components in a solution of unknown composition. You will be assigned an unknown solution containing two pure, liquid components whose boiling points differ by at least 20 °C. Be sure to record the name or number of this fractional distillation unknown in both your laboratory notebook and in your final report. After fractionally distilling the unknown mixture, the data you collect will be graphed using a microcomputer with appropriate software, e.g., EXCEL™. You will use the data you collect to plot a graph of Head Temperature in °C vs. Volume of Distillate collected in mL. Head temperature is the boiling point of the liquid that condenses at the top of the fractionating column and enters the side arm of the condenser as a liquid. In a mixture of two components, the low boiling component will rise to the top of the column first and eventually be displaced by the higher boiling component. If the fractionating column provides good separation, the low boiling component will come off the column at a steady, or constant, temperature. Then the head temperature will rise abruptly, producing a “break” in the curve, and the second, higher-boiling component will come off the column at a constant temperature. Each period of steady, constant boiling produces a nearly horizontal “plateau” in the graph. Projecting this plateau to the Head Temperature axis of the graph gives a good approximation of the boiling point of that particular component. Thus, you can identify both the low boiling and high boiling components in your mixture. The position of the “break” between low and high boiling points can tell you the approximate composition (relative amounts of each component) of the original mixture. You will also be asked to determine the refractive indices of the fractions you collect. These data will help you to better identify the components in your unknown mixture. Organic Chemistry: Techniques and Transformations Carefully follow your particular instructor’s requirements for preparing a final report of your work in the laboratory. Ask your instructor if you have any doubt about his/her requirements for the report. Reading and Pre-Lab Assignments Read the following sections in The Organic Chem Lab Survival Manual: A Student’s Guide to Techniques by James W. Zubrick, John Wiley & Sons, Inc. • Jointware and Clamps and Clamping. These chapters will familiarize you with the equipment to be used in this experiment, and its care. We have clear glass joints that do not need to be lubricated. We always use a drop of glycerol on the • rubber part of the thermometer adapter to make it easier and safer to insert and adjust the thermometer. • Simple Distillation. See the apparatus set-up • Fractional Distillation. See the apparatus set-up • Theory of Distillation. This is not easy reading, however, you should be able to understand the figures demonstrating the theory. • Refractometry. You will be determining the refractive index of your collected fractions. Before you come to the laboratory, do the Pre-Lab assignments for this laboratory as assigned by your instructor. Prepare your laboratory notebook as required by your instructor. Procedure Read the reading assignments in the Reading section above. Notice that fractional distillation is just a modification of simple distillation. Read both assignments carefully. Assemble and setup the fractional distillation apparatus as illustrated in the two figures with the following modifications. It will not be necessary to use an ice-water bath or the receiving flask. Instead, replace the receiving flask with a 15-mL graduated centrifuge tube held in place with a clamp. In this way the volume of the distillate can be measured in milliliters and different fractions may be easily obtained by changing collecting tubes. Begin the assembly of your apparatus by clamping the heating mantle to the large ring stand about 5 cm from the bench top. Place the 100 mL round bottomed flask into the well of the heating mantle and clamp it in place. This step is particularly important in any heating operation. If it should be necessary to remove the heating source, the mantle can be dropped away from the flask without the whole apparatus falling apart! Lower the heating mantle and move it to the side. Add 20 ml of the binary liquid unknown assigned to you into the round-bottomed flask. (Now is a perfect time to record the number or letter of your unknown solution in your notebook. If you delay as Organic Chemistry: Techniques and Transformations long as 30 seconds, you may well have forgotten this important information.) Add 10-20 of the small boiling chips to the flask. Place the heating mantle back under the flask. Continue with the assembly by adding the fractionating column and securing it with a clamp. Place the three-way adapter on the top of the fractionating column. All glass components of your apparatus should be in a straight, vertical line, parallel to the ring stand rod. A Keck Clamp is used to hold the west condenser to the side arm of the three way adapter and the condenser is gently clamped in place with a clamp placed about 2/3 of the way down the condenser and attached to the small ring stand. This is the tricky part. Be certain that all joints fit well without leaks or strain on the joints or on the clamps. Use the last clamp on the small ring stand to position one of the 15 ml centrifuge tubes just below the exit opening of the condenser. Attach the rubber condenser tubing to the west condenser so that the water flows in the bottom and out the top. All lockers should now have a screw cap/ O-ring type of thermometer adapter. Place the thermometer adapter on the top of the three-way adapter and adjust the thermometer so that the entire bulb is below the sidearm of the adapter. (If you have one of the older adapters with a neoprene rubber cap with a hole in it, you will need to lubricate the thermometer. Place a small drop of glycerol (glycerine) on the rubber portion of the thermometer adapter and carefully insert the thermometer, keeping your hands as close together as possible. The glycerol provides an excellent glass-rubber lubrication and should enable you to adjust the height of the thermometer safely.) Consult with your instructor about any doubts or confusion you may have. Recheck all joints to insure a tight fit. When you are satisfied that your setup is correct, have it approved by your instructor or teaching assistant before you continue. Plug the powerstat into the outlet. Turn the powerstat on. Adjust the voltage to 45 to 50 volts. Distill the solution at a rate of no more than 5-10 drops of distillate per minute coming over into centrifuge tube. A slow equilibration is essential to a successful fractional distillation. For this reason, keep the voltage low at the beginning and increase it as necessary to maintain a slow, steady distillation. Record the head temperature of the distillate in °C for about every 0.5 mL of distillate. The first material to distill will be the low boiling component of your mixture. You should have a good idea of its identity by noting its distillation temperature. Collect about 4 to 5 mL of this material as your first fraction. Change centrifuge tubes and collect a second 5 mL fraction. During this second fraction you should see the temperature rise somewhat as a mixture of the two components distills. When the entire lower boiling component has distilled, the temperature will rise to the boiling point of the second component. You may need to adjust the voltage to 70 volts or higher in order to provide enough energy to distill the higher boiling material. When the temperature has stabilized at the boiling point of the higher boiling component, change sample tubes and collect a third fraction. This should be mostly the higher boiling component. After you have collected about 15 ml of total distillate, turn the power off, Organic Chemistry: Techniques and Transformations lower the heating mantle away from the flask, and allow the apparatus to cool before taking it apart. Determine the refractive index of the first and third fractions (the lower and higher boiling components of your binary mixture). Record your data directly into your notebook (it is good science to have a “no data on scraps of paper” rule). As a reminder, use only plastic pipettes in applying sample to the easily scratched prism of the refractometer. Clean the refractometer between samples by wiping with a Chem wipe. Use a computer with a program such as Excel™ to generate a fractional distillation curve for your data.
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