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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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. Be certain to use the SCATTER PLOT option so that you have independent variables for both axes. Plot the Head Temperature (Y-axis) vs. Volume of Distillate (X-axis) and use the graph to determine the boiling points of the low-boiling and high-boiling components in the mixture. Before printing your graph be certain that it has a meaningful title and clearly labeled and appropriately scaled axes.

Compare your measurements with the list below. The list gives the published values for the boiling points and refractive indices of the known compounds that were used to make the fractional distillation mixtures. Decide which of the known compounds in the list have boiling points that most accurately agree with the boiling points of the two unknown components of your mixture. The refractive index determinations of your low and high boiling fractions should refine your choices. Follow your instructor’s guidelines for a laboratory report and be certain that your report includes (1) the letter or number of your unknown mixture, (2) your measured boiling points and refractive indices of your unknown components, and (3) the names, structures, literature boiling points, and refractive indices of the known compounds that most clearly agree with your measured boiling points.

Table 2.1 – Fractional Distillation Knowns

20 Known Compounds Boiling Point, °C Refractive Index, n D Methyl acetate 56.9 1.3614 Acetone 58.1 1.3591 Methanol 64.7 1.3292 2-propanol 78.4 1.3770 1-propanol 97.2 1.3840 2-butanol 98 1.3970 2-methyl-1-propanol 108 1.3960 1-butanol 118 1.3990

Some Thoughts on the Graphical Presentation of Data Graphical communication of data is one of those important skills that scientists must master. In Experiment 2, Fractional Distillation, you collected data, which will help you to both identify the unknown liquids in your mixture and to see trends in the way a Organic Chemistry: Techniques and Transformations fractional distillation proceeds. Examination of these data in a table only reveals part of the story. A well drawn graph will help you and your audience to see the whole picture. An EXCEL plot of the data from a distillation experiment is reproduced below. The Excel plot wizard was used to construct the plot. A scatter plot with points connected by lines was chosen. Whenever you have variable x and y data, the scatter plot is the only proper choice. This graph was cut and pasted into a word processing document. You should be able to do this with your report.

experiment 2

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While the graph above does represent a plot of the data, it is badly flawed in several ways. A graph must always have a meaningful title. The x and y axes must be clearly labeled. In addition, the graph can be greatly improved by paying attention to matters of form as well as of substance. Double clicking on various parts of the graph will enable you to modify the form. In the plot below, the grey background was eliminated, the data range, was adjusted, and the major and minor ticks and the axes were improved. The graph was re-sized to emphasize the importance of the y axis.

Fractional Distilation of Binary Mixture BP-9

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Organic Chemistry: Techniques and Transformations

Results, Discussion and Conclusion Write your results, discussion of results and your conclusion. Complete any post-lab questions.

Abstract This part should be filled in after the completion of the experiment and analysis of all data. When submitting the report, the abstract should appear at the beginning of the report.

Report Adhere to the format required by your instructor and submit the report on time. Summary 1. Fractional distillation is a separation technique used to separate two or more liquids that have boiling points closer than 20oC. 2. There is a difference in temperature between the liquid in the round-bottomed flask and the vapor on top of the fractionating column (know which has a higher temperature, the vapor on top of the fractionating column or the liquid in the flask). 3. Refraction is the bending of light rays from the normal path when passing from one medium to another medium of different optical density. 4. The refractive index is the ratio of the sine of angle of incidence to the sine of angle of refraction; refractive index, being a ratio has no units. 5. The refractive index can only be determined for a pure compound Questions 1. Name one piece of apparatus that is not used in a simple distillation but is used in a fractional distillation. 2. What is the purpose of a condenser? How does a condenser operate? 3. What is a boiling chip? How does a boiling chip regulate boiling? When should a boiling chip be used? 4. Why should the bulb of the thermometer be placed below the side arm of the three-way adapter? 5. What type of glass is used in the laboratory? What property of this glass is useful for laboratory work? 6. The temperature in the pot (the round bottom flask) and the head (the three-way adapter) are different during a distillation of a binary mixture. Is this correct? Why? 7. Why did your lower boiling point substance “come off” the column prior to your higher boiling substance? Discuss this in terms of volatility and/or vapor pressure. 8. Define “azeotrope” and give an example of an azeotrope. 9. Sketch a graph showing the distillation of an azeotrope containing two pure substances. Plot the Head Temperature (Y-axis) vs. Volume of Distillate (X-axis). 10. In this digital age, the ability to read a scale is becoming a lost art. Our Abbé refractometers use just such an old fashioned scale. With a line of people behind you waiting to use the instrument, you will need to be able to quickly and accurately read your refractive index. What are the refractive indices, to four decimal places, of the three samples below? 1.390 1.400 1.370 1.380 1.360 Organic Chemistry: Techniques and Transformations

Temperature/ Composition phase diagrams are important aids in the understanding of the theory of distillation. Answer these questions after you have read Zubrick’s chapter on the theory of distillation. 11. Consider an 80:20 mixture of A and B. 140 130 120

(a) What would be the boiling point of this mixture? C

e 110 (b) What would be the composition of the vapors just r

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above the boiling liquid? e

m 90 (c) What would be the composition of the initial e

T distillate collected from a fractional distillation of 80 this mixture if the apparatus had a separation 70 efficiency of three theoretical plates? 60 (d) How many theoretical plates would be needed % A 0 10 20 30 40 50 60 70 80 90 100 in the distillation to be able to separate pure B B 100 90 80 70 60 50 40 30 20 10 0 from this mixture? Mole % A and B

12. Below are some real student data for Distillations of 40 mL of mixtures of cyclohexene (bp 81 °C) and toluene (bp 111 °C) performed in this laboratory in 1997. ABBY BRAD CINDY DOUG

(a) Which student performed a simple distillation of these substances? (b) Which student distilled these substances, but had the thermometer placed too high in the 3-way adapter? (c) Which student carried out a careful fractional distillation of a mixture of 30 mL cyclohexene and 10 ml toluene? (d) Which student carried out a careful fractional distillation of a mixture of 20 mL cyclohexane and 20 mL toluene?

13. One of the important lessons of this experiment (and the mp experiment) is that it is often necessary to collect multiple types of data before the identification of an unknown substance can be made with certainty. Consider the real case of Heather who carried out the fractional distillation experiment in 2003. Heather measured the boiling point of her third fraction to be between 97 and 98°C. She then took the refractive index and found it to be 1.3900. It was only when she ran the infrared spectrum of her sample (see appendix) and found a characteristic C-O-H stretching band at 1120 cm-1 that she identified her unknown. What was Heather’s high boiling component?