Chemistry 261 Laboratory Experiment 6: Reduced Pressure Distillation of a Mixture of High Boiling Alcohols Reading from Zubrick, 10 th edition Jointware, pages 30-40 Sources of Heat, pages 131-139 Clamping, pages 140-149 (esp. 146-149) Distillation, pages 150-164 Introduction Distillation is a means of separating mixtures of liquid components, with a principal benefit being the separation of large quantities of materials. If one were to visit an oil refinery, it would be rather easy to spot the fractionating towers, since the separation of the crude oil mixture into more valuable individual components (or mixtures of similar cuts ) is an example of fractional distillation. We will carry out a fractional distillation next week, during which you will be more fully considering the idea of a theoretical plate no, not what you put imaginary food on when you are really hungry, but instead the establishing of an equilibrium between liquid and vapor phases when applied to distillation. According to Raoult s law, the total vapor pressure (P T ) of a 2 component homogenous liquid mixture at equilibrium takes the form P T = P A + P B = X A P o A + X B P o B Where X A is the mole fraction of component A, X B is the mole fraction of B, P o A is the vapor pressure of A when pure and P o B is the vapor pressure of B when pure. Thus, for example, if P o A = 2 P o B = 100 mm Hg, and both are at equal concentrations, i.e. X A = X B = 0.5, then P T = 0.5(2 P o B) + 0.5(P o B) = 0.5(100 mm Hg) + 0.5(50 mm Hg) = 75 mm Hg As P A = 50 mm Hg, P B = 25 mm Hg Most importantly, when the vapor has an opportunity to recondense, the newly formed liquid will be enriched in the lower boiling (higher initial vapor pressure) component: X A = P A /P T = 50 mm Hg/75 mm Hg = 2/3 X B = P B /P T = 25 mm Hg/75 mm Hg = 1/3 When revolatilized (2 nd theoretical plate) the vaper will be even more enriched in component A, owing to its increased mole fraction. More on this next week. 1
Hopefully it is clear to you that converting liquids to gases requires energy to overcome the intermolecular forces of interaction, so in order to carry out a successful distillation enough energy has to be available to allow condensed vapors to be revaporized. A surprising amount of heat is lost through the glassware leading to the distillation head, and I suggest you insulate any distillation you are attempting with glass wool wrapped in aluminum foil 1, or foam pipe insulation which has been slit along its long axis. Organic compounds are often sensitive to high heat, undergoing such unpleasantness as undesired elimination and rearrangement reactions. As a result, numerous techniques have been developed to allow the separation of high boiling liquids by the use of reduced pressure or vacuum distillation, where the system is sealed and air removed via mechanical pump (vacuum distillation) or the Venturi effect (reduced pressure distillation), so the vapor pressure meets the [reduced] external pressure requirement for boiling at a much lower temperature. In the following lab, you will conduct a vacuum distillation in order to separate 3 closely related unknown alcohols. These purified alcohols will be utilized later so take care to obtain a clean cut when you collect the fractions. Your mixture will contain n-butanol, cyclohexanol, and n-octanol. By looking up the normal boiling points for the alcohols and utilizing the boiling points you record, you will be able to estimate the reduced pressure of your distillation apparatus using a pressuretemperature nomograph. In addition to reporting the operating pressure of your distillation apparatus, you should report the relative ratios of the 3 alcohols 2. 1 If you have an unlimited laboratory budget, electrical heating tape is a welcome addition to the distillation apparatus, particularly for fractional distillations, which feature long paths with lots of surface area for liquid-vapor equilibrium 2 Since you should never distill to dryness, you can estimate the volume of the highest boiling fraction by difference after the first 2 fraction volumes have been obtained 2
Procedure 1: Heat source 2: Still pot 3: Still head 4: Thermometer/Boiling point temperature 5: Condenser 6: Cooling water in 7: Cooling water out 8: Distillate/receiving flask 9: Vacuum/gas inlet 10: Still receiver 11: Heat control 12: Stirrer speed control 13: Stirrer/heat plate 14: Heating (Oil/sand) bath 15: Stirrer bar/anti-bumping granules 16: Cooling bath. 3
You will find all the ground glass jointware necessary for assembling the distillation apparatus above under hoods C4-C6. Make certain you note the following modifications: As this is a reduced pressure distillation, you will need to connect the vacuum adapter (9) to the vacuum line with vacuum tubing. It is important to provide a trap set in ice to avoid pulling vapors into the pump and fouling the pump oil. A standard trap design uses a side arm flask with a 2 holed stopper as shown as figure 13.6 (p 99) in Zubrick, 10 ed. This design allows 1 line to connect to your distillation apparatus, 1 line to vacuum, and allows for a vacuum break with the judicious use of a short piece of straight glass tubing, a short section of vacuum tubing, and a screw clamp 3. Use the heating mantles you used last week for the hydrolysis of acetanilide reflux As we are conducting a vacuum distillation, you will need to grease the joints of your apparatus as you assemble (a little goes a long way here, and rotating the joints will help coat them completely). We will modify the distillation apparatus shown above by using a Claisen adapter. This allows the use of an ebulliator tube to provide a fine stream of air to aid in mixing and prevent bumping. This is shown as figure 19.8 in Zubrick We have O-ring based ground glass thermometer adaptors (red screw cap) which will allow a better seal and lower pressures and distillation temperatures compared to the ground glass adaptors with rubber cap typically found please use these for both thermometer and ebulliator tube Having to break the vacuum when a fraction has completely distilled to change a receiving flask adds a considerable amount of time to the proceedings. This problem has been solved by the invention of the cow, an udderly fantastic variation of the vacuum adapter (9/10 in the figure above) which allows you to rotate between fractions without breaking the vacuum. Notice that with 4 receiving flasks, one can be used for collecting a few drops between the main body of the fractions to lessen cross-contamination. Please use the 25 ml receiving flasks and plastic clips provided. 3 While they are not ideal, use the traps you are used to, but do make certain to seat them into ice and clamp them effectively 4
Obtain 70 ml of the alcohol mixture. Using a 100 ml round bottom flask (RBF) as your still pot, transfer the mixture. Place the still pot in a heating mantle, and finish assembling the apparatus. With an ebulliator tube, no boiling chips or magnetic stir bar should be necessary Read Zubrick carefully. You should come into the lab knowing how you want to clamp your distillation apparatus. Give some thought how to keep this expensive suspended glassware safe, bearing in mind we do have scissorjacks at your disposal. It may of some spatial benefit to initially lay the glassware on its side, temporarily assembled, in order to get a sense of relative heights for the actual assembly. 4 o If you build the distillation apparatus still pot, Claisen adapter, still head, condenser, and leave the condenser suspended with no support, you will lose all of the points for the lab, whether it falls and shatters or not Keep in mind that in order to effectively fill a condenser, the cold water inlet line is the lower connection, the upper connection the outlet line. Making certain the condenser inlet and outlet are pointing up will ensure complete filling. Check to make certain your joints are all well seated. You are now at a point where you can begin the distillation. The key question is what do I do first? Zubrick does a good job of stressing the appropriate order of events, and suffice it to say here that a routine method for cleaning a distillation apparatus (or our very cool Rotovap ) is to heat up your solvent until it starts to bubble, then close the vacuum line so don t do this unless you want your unseparated mixture to go shooting through your setup I suggest setting up your lab book so you can record distillation temperatures every 2 ml. You will also want to clearly report boiling range and volumes associated with each fraction 4 Given a large enough scissor jack, a useful adaptation is to place the heating mantle on the partially raised scissor jack, which allows one to adjust the height of the still pot when assembling, as well as lowering the hot plate from the still pot (clamped at the neck) when the distillation is complete 5
Choose one of the receiving flasks to act as the waste receiver and collect the first ml or so of distillate before rotating into position for the first fraction. When the first fraction has finished distilling, rotate back to the waste receiving flask and continue heating. Repeat for the 2 nd and 3 rd fractions o o o Ask yourself the question Should I collect the 2 nd fraction as soon as it begins distilling over if the whole purpose of a distillation is purification? As the 3 rd fraction nears completion, repeat to yourself the mantra Never, EVER distill to dryness, especially in a vacuum distillation As mentioned in footnote 1, you only need to distill the 3 rd fraction long enough to obtain a stable distillation temperature. Estimate the relative proportion of fraction 3 by difference Place the fractions in small bottles provided for long term storage, label appropriately, and submit. Clean out the distillation apparatus prior to disassembly, using acetone as your cleaning solvent. If you would like to try the whoosh technique by boiling the acetone before placing under vacuum, feel free. In this case we are trying to promote exaggerated bumping, so open the clamp attached to the ebulliator tube to allow exaggerated entry of air Pre-lab Questions These must be completed prior to your entering the lab, and of course your lab book should be completely prepared for the experiment. Also, be prepared to answer a question concerning your approach to the day s experiment. 1. Where should the thermometer bulb be placed in the distillation apparatus? 2. At what point should you begin taking distillation temperature readings? 3. What is the principal reason for carrying out a vacuum distillation? 4. What is the purity of component A in a 2 component system (A + B) after 4 theoretical plates if initial X A = 0.10, P A = 200.0 mm Hg, X B = 0.90, and P B = 50.0 mm Hg 6
Time Management Distillations require a fair amount of observation observation that can border on tedium. This is why any good organic chemistry lab experience affords the opportunity to multi-task. While you will obviously need to keep an eye on the process, there is no reason why you cannot finish any remaining work on the previous lab(s) while collecting fractions. Set the distillation up first, then worry about finishing off any other work to be done Since you have worked diligently to this point, I will offer an opportunity for 5 extra credit points if you undertake the separation of the active components of excederin by column chromatography separate post Post-Lab Using the normal boiling points for the alcohols separated, together with the boiling points you recorded, find a pressure-temperature nomograph, and deduce the pressure your distillation was occurring at. A very nice interactive version is provided by one of the nation s major chemical providers Sigma-Aldrich and may be found at http://www.sigmaaldrich.com/chemistry/solvents/learning-center/nomograph.html Report your finding on a printed nomograph. A nice version may be found at http://www.tciamerica.com/useful-info/pressure-temperature-e.pdf Post-Lab Questions 1. Cite 2 factors that would cause the pressure inside your system to fluctuate 2. Decane (C 10 H 22 ) is a linear hydrocarbon which boils normally at 174 o C. A vacuum distillation using a mechanical pump produced a distillation temperature of 24 o C. What was the pressure inside the distillation apparatus? 3. What is the boiling point of hexanol? What is the boiling point of cyclohexanol? What accounts for the difference? 7