EXPERIMENT 2
DISTILLATION AND GAS CHROMATOGRAPHY
OBJECTIVES
In this experiment a simple and fractional distillation of mixtures of cyclohexane and toluene will be conducted. Gas chromatography will be used to analyze samples to determine the effectiveness of the distillation and if time allows explore the composition of various gasoline products.
The separation of organic compounds is one of the most important tasks of the organic chemist. Organic compounds seldom occur in pure form in nature or as products of a laboratory synthesis. The most commonly used method for purification of liquids is distillation, a process by which one liquid can be separated from another liquid, or a liquid from a nonvolatile solid.
For example when water is heated with a heating mantel in a simple distillation apparatus (see fig.1 in appendix), the vapor pressure of the liquid, or the tendency of molecules to escape from the surface, increases. This process continues until the vapor pressure becomes equal to the atmospheric pressure, at which point the liquid begins to boil. Addition of more heat will supply the heat of vaporization required for conversion of the liquid water to gas (steam), which rises in the apparatus, warms the distillation head and thermometer, and flows down the condenser. The cool walls of the condenser remove heat from the vapor and the vapor condenses to the liquid form. Distillation should be conducted slowly and steadily and at a rate such that the thermometer bulb always carries a drop of condensate and is bathed in a flow of vapor. Liquid and vapor are then in equilibrium, and the temperature recorded is the true boiling point. If excessive heat is applied to the walls of the distillation flask above the liquid level, the vapor can become superheated, the drop will disappear from the thermometer, the liquid-vapor equilibrium is upset, and the temperature of the vapor rises above the boiling point.
Consider the separation of cyclohexane (81°C) and toluene (111°C); boiling points at sea level. Using a simple distilling apparatus a mixture of these two miscible liquids starts to distil somewhat above the boiling point of cyclohexane and stops distilling somewhat below the boiling point of toluene. All fractions of the distillate are mixtures and little separation of the two compounds is achieved. If redistillation is repeated often enough, the two components of the mixture will eventually be separated. Fortunately this series of condensations and redistillations is done automatically in a fractionating column.
The fractioning column shown in fig 3 of the appendix contains a stainless steel or copper scouring sponge, which forms a porous packing for the equilibration of vapor and condensate. At the start of the distillation of a mixture of cyclohexane and toluene the mixture boils and the vapors condense in the lowest part of the fractionating column. The composition of this condensate is similar to that of the first fraction collected in a simple distillation – richer in cyclohexane than in toluene, but by no means pure cyclohexane. Then, as more cyclohexane and toluene boil, the temperature of these vapors is higher than that of the first portion of the mixture, because the portion contains less cyclohexane and more toluene. These hot vapors contact the liquid already in the fractionating column from the first part of the distillation and a heat transfer takes place, which causes the more volatile component (cyclohexane) to boil from that liquid. A succession of these condensations and redistillations occur throughout the column. The efficiency of a column is rated by the number of simple distillations that take place inside the column. After cyclohexane – toluene vapor has warmed the entire length of the column, the less volatile part condenses and trickles down over the surface of the packing, while fresh vapor from the flask forces its way through the descending condensate with attendant heat interchange. A number of equilibrations between ascending vapor and descending condensate take place throughout the column. The vapor that eventually passes into the receiver flask is highly enriched in the more volatile cyclohexane, whereas the condensate that continually drops back into the heated flask is depleted of the volatile component and enriched with the less volatile toluene. The packaging is used in the column to increase the vapor-liquid contact area. Since equilibration is fairly slow, slow distillation effects better separation.
A process similar to fractional distillation is used to process and produce gasoline from crude oil. Gas chromatography is a technique that can be use to analyze volatile organic liquids identify and determine purity of samples. See the appendix for more information on distillations and gas chromatography.
EXPERIMENTAL PROCEDURE
This experiment will be done in groups of 2-4 students as instructed by your TA.
Glassware and Equipment
Simple Distillation: 50 and 250mL round bottom flasks, 3-way connecting tube, West condenser, vacuum connecting tube, straight tube adaptor, 50mL graduated cylinder, two or three ring stands, 250mL heating mantel and voltage controller, 3-4 clamps and clamp holders, ring support, thermometer, thermometer holder, boiling chips, ice.
Fractional Distillation: (additional equipment needed): distilling condenser, steel wool.
Other Equipment: gas chromatograph.
Chemicals: 50mL of a toluene/cyclohexane mixture per distillation (each group will do two distillations).
Cautions: Toluene (2301), cyclohexane (2302), and gasoline (2301) are flammable and somewhat toxic. Keep away from sources of ignition and avoid contact. Use goggles at all times. Gloves are available for use. All distillations and manipulations of cyclohexane and toluene must be done in a fume hood. Review the safety information in the lab manual introduction with a focus on fume hoods and fire safety. To prevent fumes from escaping into the lab, all manipulations of cyclohexane, toluene, and gasoline must be done in a fume hood. In addition transport of these outside of a fume hood should only be done in stoppered containers. Also, any glassware used should be rinsed with acetone in a hood before washing in a sink.
1. Calibration of Thermometer
Test the 0°C point of your thermometer with a well-stirred mixture of crushed ice and distilled water. To check the 93°C point, measure the temperature of boiling water with the thermometer. Your TA will setup these baths for the class to use.
2. Simple Distillation
Apparatus: Assemble the apparatus for simple distillation shown in Fig 1 in the appendix. Place the flask with the liquid to be distilled inside the heating mantel. One or two boiling stones are put in the flask to promote even boiling. Each ground joint is greased by putting a thin film of grease length wise around the male joint and pressing the joint firmly into the other. The air is eliminated and the joint will appear almost transparent. (Don’t use excess grease as it will contaminate the product.) Water enters the condenser at the lower entrance to the condenser jacket (closest to receiving flask. Because of the large heat capacity of water only a very small stream (3mm diameter) is needed; too much water pressure will cause the tubing to pop off. A heavy rubber band or clip can be used to hold the condenser to the distillation head. Note that the bulb of the thermometer is below the opening into the side arm of the distillation head. Collect the distillate in a 25 or 50 mL graduated cylinder.
Simple Distillation of Cyclohexane-Toluene Mixture: Place 35 mL mixture of an unknown mixture of cyclohexane and toluene (provided by TA) and 2-4 boiling chips in a dry 250mL round bottom flask and assemble the apparatus for simple distillation. After making sure all connections are tight, heat the flask strongly until boiling starts by setting voltage of the power supply at 70 V and then back off to 50 V as needed (these are approximate setting that will differ depending on the heating mantel – power supply). Adjust the heating mantel until the distillate drops at a regular rate of about 1 drop per second. Record both the temperature and the volume of distillate at regular intervals. Take a sample for GC analysis in a clean dry vial when 3 mL have distilled. When the temperature begins to increase more rapidly switch the graduated cylinder used for receiving the distillate with another dry/empty one. There may not be a clear change in temperature when you begin distilling toluene instead of cyclohexane. Why? Pour the distillate already collected into a clean, dry flask and stopper it. After about 33 mL of distillate are collected, discontinue the distillation. Never distill to dryness. Run a gas chromatogram (GC) on the first sample collected and if there is time run another GC on the second sample of distillate collected. Make any thermometer correction as necessary and plot boiling point vs. total volume of distillate collected as shown in figure 5 of the appendix. Graph using Excel or Graphical Analysis.
3. Fractional Distillation
Apparatus: Assemble the apparatus shown in Fig. 3 in the appendix. The fractionating column is loosely packed with stainless steel or copper sponge. The column should be perfectly vertical and it should be insulated with glass wool covered with aluminum foil with the shiny side in. However, in order to observe what is taking place within the column, insulation may be omitted for this experiment.
Fractional Distillation of a Cyclohexane-Toluene Mixture: Obtain 35 mL of an unknown toluene-cyclohexane mixture from your TA. Place the rest in a clean dry 250 mL round bottom flask. Attach the flask to the fractional distillation apparatus. Collect the distillate in an ice cooled 50 mL graduated cylinder. Quickly bring the mixture to a boil. As soon as boiling starts, turn the heating mantel to the smallest setting that boils the liquid. Heat the flask slowly at first. The upper edge of condensate will rise slowly through the column. The rise should be very gradual, in order that the column can acquire a uniform temperature gradient. Do not apply more heat until you are sure that the ring of condensate has stopped rising, then increase the heat gradually. In a properly conducted distillation, the vapor-condensate mixture reaches the top of the column only after several minutes.
Once distillation has commenced, it should continue steadily without any drop in temperature at a rate not greater then 1 mL in 1.5-2 minutes. Observe the flow and keep it stead by slight increase in heat as required. Record the temperature as each mL of distillate collects. Take a sample for GC analysis in a clean dry vial when 3 mL have distilled. When the temperature starts to increase quickly change containers and make more frequent temperature and volume readings. Pour all the distillate before the temperature rise into one Erlenmeyer flask and stopper it. Put the distillate collected during the rapid temperature rise in another flask and the distillate collected at the stable higher temperature in yet another flask. Stop the distillation when you have collected about 33 mL after a second constant temperature is reached. Plot a distillation curve as shown in figure 5 of the appendix. Run a gas chromatogram on the first low boiling fraction and give this product to your TA. Place all of the cyclohexane/toluene distillates into the bottle labeled “Cyclohexane/Toluene Distillates”. All other used chemical into the “Acetone Waste” bottle.
You will do a data analysis for this experiment that will include:
· Plots of volume versus temperature for both the simple and fractional distillation.
· Gas Chromatograms taken.
· Calculations of the percent composition of toluene and cyclohexane in the original mixture from the fractional distillation curve and the GC data.
· Write a paragraph interpreting the distillation plots and GC data. Compare the effectiveness of the two types of distillations.
4. Gas Chromatogram of Gasoline (if time allows)
Run a gas chromatogram on a sample of gasoline provided by your TA or one of interest to you. Follow the TA’s instructions. It is ok to work in groups of 2-4 and share data on this. Comment on the chromatogram(s) of gasoline in your lab notebook.
Pre-Lab Work
1. Read experiment and the appendix covering distillation and gas chromatography.
2. Draw pictures of a simple & fractional distillation & label all the components of each.
3. What is the difference between a simple and fractional distillation?
4. Briefly describe how a fractional distillation separates volatile organic liquids.
5. What is a gas chromatograph used for in this experiment?
6. Draw a schematic of a gas chromatograph and describe how it works
7. List three ways to prevent fumes from escaping into the lab room.
8. How is crude oil processed to make gasoline?
9. Risk Assessment: What are the safety hazards and precautions for this experiment?