Distillation
Reading
Williamson, Steam Chapter 6, p. 105-108 on the theory of steam distillation; p. 231-244 on how to record an IR spectrum. The details of the experiment on the microscale distillation and extraction appear on p. 110-111. Look at the short video illustrating the technique of steam distillation. Carrying out a steam distillation of an essential oil Separating and purifying the distilled oil by extraction Analyzing the purity of the product by TLC Recording an IR spectrum
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Important techniques in this experiment
General theory of steam distillation:
Steam distillation is a process that works for two immiscible liquids because Raoults Law does not hold. This means that the vapor pressure above them is the sum of the vapor pressures of the pure liquids, and is not related to the partial pressures based on a mole fraction of compound in the liquid phase. For this reason, the vapor pressure above a mixture of two immiscible liquids is greater than the vapor pressure of either liquid alone. Because of this, the vapor pressure equals the atmospheric pressure at a lower temperature than the boiling point of either liquid.
General parts of the experiment
Obtain 0.5 mL of either lemongrass oil, spearmint oil, or caraway oil. Carry out the steam distillation as illustrated in Fig. 6.3. Some specifics of the procedure are presented below. Isolate the product by extraction following the procedure on p. 111 After drying obtain a weight, and determine percent recovery. Take a TLC of the product you obtained and authentic material using 1:3 ethyl acetate-hexane as developing solvent. Take an IR spectrum of your product as described by your teaching assistant.
Some experimental specifics
Steam distillation. Place a sample (ca. 0.5 mL) of any one of the following essential oils in a 5 mL short necked flask: lemongrass oil, caraway oil, or spearmint oil. Add approximately 3 mL of water and a boiling chip. Assemble the steam distillation apparatus and have your TA inspect it before proceeding. Set the sand bath controller on high and perform the steam distillation quickly. Have a syringe with water handy. Inject water periodically (0.5 1 mL) through the septum when the volume in the distilling flask gets low. Note: Water must be present during the entire steam distillation process!
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The distillate will be cloudy. Droplets of an immiscible liquid should be observed. After collecting approximately 5-10 mL of distillate, stop the distillation at this point.
Separation of layers by dilution with a volatile solvent (MTBE) and extraction. The distillate normally consists of two layers: the volatile organic materials on the top and water on the bottom. However, in some cases, the distillate will appear cloudy (turbid) and only small droplets of a liquid will be evident. The organic layer can be separated from the aqueous layer by extraction. Follow the extraction procedure using the solvent, methyl tert-butyl ether, MTBE, in the book, Part 2, p. 111. Drying the organic extract. Dry the organic solution pellets with of CaCl2. There should be no noticeable traces of water (the presence of water is indicated by droplets of liquid or a turbid solution). Removal of the volatile solvent. Transfer the dried MTBE extract to a tared reaction tube. [You need to know the mass of the tube at this point in order to determine the amount of organic material you collected.] Evaporate the solvent by warming the tube in a beaker of warm water (see text). Use a boiling stick as a source of bubbles. Remove the last traces of solvent at reduced pressure by connecting the reaction tube to the water aspirator. Weigh your sample and determine the percent recovery. Thin Layer Chromatography (TLC) of the product. Dissolve the residual oil in a small volume of solvent (0.2-0.5 mL of CH2Cl2 or MTBE). Draw up a small amount of your sample solution into a spotting capillary and spot it on the TLC plate. For comparison you will also spot authentic samples available in the lab. A solution of 1:3 ethyl acetate-hexane is a good solvent for all compounds. After developing the plate, visualize the spots with a UV lamp and/or I2 vapors. Introduction to Infrared (IR) spectroscopy (Chapter 11) Infrared (IR) spectroscopy is an indispensable tool for determining the presence (or absence) of functional groups in an organic molecule. Virtually all common functional groups absorb infrared radiation and give rise to characteristic bands in the IR spectrum. For example, C-H bonds absorb at ~ 3000 cm-1, O-H at ~ 3200-3600 cm-1, and C=O at ~ 1700 cm-1. Thus, the appearance of a band at ~ 1700 cm-1 in the IR spectrum of a compound is diagnostic for the presence of a C=O containing functional group such as an aldehyde, ketone, or other classes of C=O compounds. Your TA will demonstrate the operation of the IR instrument. Two salt plates (NaCl disks) are used to "hold" the sample in the IR light beam. The disks are fragile and easily damaged. They are also easily damaged with water - avoid splashing water on them! Handle the plates like a CD disk - the moisture and oils from your fingers can contaminate the salt disks. Sodium chloride salt plates (disks) are stored in a dessicator
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and should remain there at all times when not in use. Directions are posted on or near the instrument. Spot a small amount of sample on a plate. Alternatively, spot a solution of the oil in CCl4 and allow the solvent to evaporate. Place a second salt plate on top of your sample. If the plates are fairly smooth you will observe the sample spreading to make a relatively thin film. Place the salt plate "cell" in the sample holder in the cell compartment and record your IR spectrum. After you have obtained your spectrum, carefully rinse the salt plates with a chlorinated solvent such as chloroform (ask your TA). Wipe the plates with a Kimwipe and return them to the dessicator.
Questions
1. In the steam distillation of an essential oil, the observed boiling point is below the boiling point of both the essential oil and water. Explain why this is so. 2. Why does the distillate in a steam distillation have a cloudy appearance?
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