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LABORATORY 4
Fractional Distillation
Concept goals: refraction of light by molecules. Role of fractionating column and when such a column is necessary,
Operational goal indices to determine the identity of compounds: Develop the skill to perform a. distillation effectively, use refractive
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, liquids must be separated. etc By noting the boiling temperature of the collected fractions. In the laboratory it is employed whenever a mixture of and other physical properties, fractional distillation can al components in a solution of unknown composition. so be used to identify the
You whose boiling points differ by at least 20 °C. will be assigned an unknown solution containing Be sure to record the name or number of two pure, liquid components this fractional distillation unknown in both your laboratory notebook and in your final report. graphed using a microcomputer with appropriate software, e.g., EXCEL™. After fractionally distilling the unknown mixture, the data you collect will be
You will use the data you collect to plot a graph of Head Temperature in °C vs. Volume o condensesf Distillate collected in mL. at the top of the Head temperature is the boiling point of the liquid that 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. approximation of the boiling point of that particular component. Projecting this plateau to the Head Temperature axis of the graph gives a good Thus, you can identify both the low boili “break” between low and high boiling points can tell you the approximate compositionng and high boiling components in your mixture. The position of the (relative amounts of each component) of the original mixture. You will also be asked to determine th These data will help you to better identify the components in your unknown mixture.e refractive indices of the fractions you collect.
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 Guide to Techniques by James W. Zubrick, John Wiley The Organic Chem Lab Survival Manual: A Student’s & Sons, Inc.
- Jointware and Clamps and Clamping equipment to be used in this experiment, and its care.. These chapters will familiarize you with the We have clear glass
- joints that do not need to be lubricated.rubber part of the thermometer adapter to make it easier and safer to insert and^ We always use a drop of glycerol on the
- adjust the thermometer.Simple Distillation. See the apparatus set-up
- • Fractional Distillation. See the apparatus setTheory of Distillation. This is not easy reading,-up however, you should be able to
- understand the fRefractometry. You will be determining the refractive index of your collectedigures demonstrating the theory. fractions. Before you come to the laboratory, do the Pre assigned by your instructor. -Lab assignments for this laboratory as
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 Assemble and setup the fractional distillation apparatus as illustrated in the two figures both assignments carefully. with the following modifications. It will not be necessary to use an ice-water bath or the receiving flask. Instead, replace the receiv tube held in place with a clamp. In this way the volume of the distillate can being flask with a 15-mL graduated centrifuge measured collecting tubes. in milliliters and different fractions may be easily obtained by changing
Begin the assembly of your a stand about 5 cm from the bench top.pparatus by clamping the heating mantle to the large ring 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. mantle can be dropped away from the flask without the whole apparatus falling apart! If it should be necessary to remove the heating source, the
Lower the heating mantle and move it to the side. unknown assigned to you into the round-bottomed flask. ( Add 20 ml of the binary liquidNow is a perfect time to record the number or letter of your unknown solution in your notebook. If you delay as
lower the heating mantle away from the flask, and allow the apparatus to cool before taking it apart.
Determine the refractive index of the boiling components of your binary mixture). first and third fractions (the lower and higher 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 refractometer. only plastic Clean the refractometer between samples by wiping with a Chem wipe. pipettes in applying sample to the easily scratched prism of the
Use a computer with a program such as Excel™ to generate a fractional distillation curve for independent variables for both axes. your data. Be certain to use Plot the Head Temperature (Ythe SCATTER PLOT option (^) - soaxis) vs. Volume of that you have Distillate and high (^) - boiling components in the mi(X-axis) and use the graph to determine the boiling points of the lowxture. Before printing your graph be certain that it-boiling has a meaningful title and clearly labeled and appropriately scaled axes. Compare your measurements with the list below. the boiling points and refractive indices of the known compounds that were used to The list gives the published values for make the fractional distillation mixtures. list have boiling points that most accurately agree with the boiling points of the two Decide which of the known compounds in the unknown components of your mixture. and high boiling fractions should refine your choices. The refractive index determinations of your low 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
Known Compounds Methyl acetate Boiling Point, 56.9 °C Refractive Index, 1.3614 n (^20) D Acetone 58.1 1. Methanol 64.7 1. 2 - propanol 78.4 1. 12 - -propanolbutanol 97.2 98 1.38401. 2 - methyl- 1 - propanol 108 1. 1 - butanol 118 1.
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
fractional distillation proceeds. the story. A well drawn graph will help you and y Examination of these data in a table only reveals part ofour audience to see the whole picture. An EXCEL plot of the data from a distillation experiment is reproduced below. plot wizard was used to construct the plot. A scatter plot with points connected by lines The Excel was chosen. proper choice. This graph was cut and pasted into a word processing document. Whenever you have variable x and y data, the scatter plot is the only You should be able to do this with your report.
While the graph above does represent a plot ways. A graph must always have a meaningful title. The x and y axes must be clearly of the data, it is badly flawed in several 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. the data range, was adjusted, In the plot below, the grey background was eliminated,and the major and minor ticks and the axes were improved. The graph was re-sized to emphasize the importance of the y axis.
experiment 2
0
20
40
60
80
100
120
0 2 4 6 8 10 12 14 16
Series
Fractional Distilation of Binary Mixture BP-
5055
6065
7075
8085
9095
100105
110
(^0 1 2 3 4) Volume (^5 6) of (^7) Distillate, (^8 9 10) (mL) 11 12 13 14 15 16
Temperature/ Composition phase diagrams are important aids in the understanding the theory of distillation. Answer these questions after you have read Zubrick’s chapte ofr on the theory of distillation.
- Consider an 80:20 mixture of A and B. (a) What would be the boiling point of this mixture? (b) What would be the composition of the vapors just above the boiling liquid? (c) What would be the composition of the initial distillate collected from a fractional distillation of this mixture if the apparatus had a se efficiency of three theoretical plates?paration (d) How many theoretical plates would be needed in the distillation to be able to separate pure B from this mixture?
- Below are some real student data for Distillations of 40 mL of mixtures of cyclohexene ( ABBYbp 81 °C) and toluene (bp 111 °C) performed in this laboratory in 1997. BRAD CINDY DOUG
(a) (b) Which student performed a simple distillation of these substances?Which student distilled these substances, but had the thermometer placed too high
in the 3-way adapter?
(c) cyclohexene and 10 ml toluene? Which student carried out a careful fractional distillation of a mixture of 30 mL
(d) cyclohexane and 20 mL toluene? Which student carried out a careful fractional distillation of a mixture of 20 mL
(^13) often necessary to collect. One of the important lessons of this experiment (and the mp experiment) is that it is multiple types of data before the identification of an unknown substance can be made with certainty. Consider the out the fractional distillation experiment in 2003. Heather measured the boiling real case of Heather who ca point ofrried 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 identified her unknown. What was Heather’s high boiling component?-O-H stretching band at 1120 cm-^1 that she
% A B 100 0 10 90 80 (^20) 70 6 3 0 400 50 50 60 40 3 70 80 200 90 10 1000 Mole % A and B
6070
80
(^10090)
110
120130
140
Tem
perat
ure^ °
C
