CH 2020/2270/2290 Distillation: Separation and Purification of Organic Liquids (Adapted from Laboratory Manual to Accompany Organic Chemistry: A Short Course, H. Hart, L. E. Craine, D. J. Hart, and T.K. Vinod 12th ed. Houghton-Mifflin, Boston, 2007.)

Materials From the Chemicals Hood: From the Stockroom (Blue Bin): Hexanes ½” pea-sized stirbar Toluene Stirbar retriever 3-way adapter 100 mL round-bottom flask

Bent vacuum adapter Cork ring Tupperware of “saddles”

Thermometer

Condenser Thermometer adapter

Large metal clamp (2) Small metal clamps

(2) Blue Keck clips (2) Wire clips Fractionating column

General Principles In a sealed container partially filled with a liquid, some molecules escape from the liquid’s surface into the space above. In their random motion, molecules that have escaped to the vapor may strike the liquid surface again and stick to it. At equilibrium, the number of molecules that leave the liquid surface equals the number of vaporized molecules that strike the liquid surface and stick. The molecules in the vapor also strike the walls of the container and exert a pressure, which is called the vapor pressure of the liquid. If the temperature of the liquid is raised, more molecules escape to the vapor until equilibrium is once again established. The vapor pressure of a liquid, therefore, increases with increasing temperature.*

The Boiling Point The boiling point of a liquid is that temperature at which the vapor pressure of the liquid becomes equal to the pressure exerted by its surroundings. If the liquid is open to the atmosphere, the boiling point is the temperature at which the vapor pressure of the liquid becomes equal to the atmospheric pressure. The vapor pressure of a pure liquid rises steadily as the temperature is increased until the boiling point is reached.

A thermometer placed in the vapor of a boiling pure liquid registers that liquid’s boiling point. The temperature remains constant throughout the distillation of a pure liquid. This is 1 because at the boiling point, vapor and liquid are in equilibrium; if the phase composition of the vapor and liquid remains constant throughout the process, the temperature also remains constant. The boiling point (at a given pressure) is a characteristic property of a pure liquid, just as the melting point is a characteristic property of a pure crystalline solid.

A Mixture of Ideal Liquids When a mixture of two miscible liquids with different boiling points is heated, the vapor does not have the same composition as the liquid. Instead the vapor is richer in the more volatile component.

When a mixture of A and B were placed in a distillation apparatus and heated to its boiling point, the vapor would be much richer in A, the more volatile of the two components, than was the original liquid.

As the distillation proceeds, A is selectively removed from the liquid. The composition of the liquid changes gradually to 100% B. The boiling point of the liquid gradually rises to the boiling point of liquid B. Thus, in the simple distillation of a mixture, the first material to distill is rich in the more volatile or lower-boiling component, and the last material is rich in the less volatile or higher-boiling component.

Objectives In this experiment you will distill a binary liquid with and without a fractionating column and observe the effect of the fractionating column on the composition of the distillate over the course of the distillation.

* A sealed apparatus is not used in normal distillations because such an apparatus will explode from increased vapor pressure when heated.

Simple Distillation of a Binary Mixture You will first distill a two-component liquid-liquid solution in a simple distillation apparatus. You will need to check out this glassware from the stockroom and leave your group number/letter. Sand for sand baths is found in a large tupperware in the balance room. Assemble the apparatus (distilling flask, thermometer, condenser, adapters, and graduated cylinder as receiver) as illustrated in Figure 1, using a 100-mL round- bottomed flask. Be sure that the thermometer bulb is positioned just below the side arm so that it can measure the temperature of the vapor as that vapor passes out of the flask into the condenser and receiver.

Place 15 mL hexanes and 15 mL toluene in the flask, add a magnetic stir bar (stirring prevents “bumping” due to superheating), put the thermometer in place, and start water circulating through the condenser. Have the instructor check your apparatus at this point.

Ca Caution Always stop a distillation before the flask becomes completely dry. When the flask is dry, its temperature can rise sharply. Some organic substances, especially alkenes and ethers, may contain peroxide impurities that become concentrated and can explode at dryness.

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Figure 1. Distillation Apparatus

Label three sample vials (from the side shelf) as “Simple 2”, “Simple 5” and “Simple 7”. Heat the reaction flask in the sand bath on medium heat (~ 300 °C setting on your hot plate) and distill the volatile product into a graduated cylinder. In order to obtain differences in fractions resulting from differences in boiling points, it is important not to distill the products too quickly. This will be reflected in your technique grade for this experiment. When liquid begins to drip into the graduated cylinder, adjust the heat so that the drops come slowly and steadily at a rate of about one per second. Record, in your notebook, the temperature after you have collected 2, 4, 6, 8, 10, 12, and 14 mL of distillate.

Save, in the vial labeled “Simple 2,” the 2 mL portion of distillate between 2 mL and 4 mL total distillate. Save in the vial labeled “Simple 5” the 2 mL portion of distillate between 8 mL and 10 mL total distillate. Save in the vial labeled “Simple 7” the 2 mL portion of the distillate between 12 mL and 14 mL total distillate. Then stop the distillation and lower the lab jack. Remove the round-bottom flask only, and discard the distillate and residue (save your magnetic stir bar!) in the Liquid Organic Waste container in the hood, saving fractions “Simple 2,” “5,” and “7.”

Separation of a Binary Mixture by Using a Fractionating Column Use a modification (Figure 2, below) of the apparatus used previously to again distill a mixture of 15 mL of hexane and 15 mL of toluene. Remember to add your magnetic stir bar. Notice that there is a column positioned vertically between your round-bottom flask and the distillation head. The column has glass projections inside at the bottom to hold up the packing. We will “pack” the column with small “saddles.” Add the packing material to fill the column. 3 Label three sample vials (from the side shelf) as “Fractional 2”, “Fractional 5” and “Fractional 7”. Heat the reaction flask in the sand bath on medium heat (~ 300 °C setting on your hot plate) and distill the volatile product into a graduated cylinder. Once the liquids begins to boil, wrap the heated portion of your distillation apparatus with aluminum foil all the way up to the thermometer bulb.

Adjust the heat during the distillation so that the distillate drips slowly and steadily into the receiver (~1 drip/second). Record the temperature in your notebook after every 2 mL as the distillation proceeds and collect and save the second, fifth, and seventh fractions (in 2 mL portions as before) in the labeled vials for use in the experiment on gas-liquid chromatography. After you have collected the seventh fraction turn off the heat and lower the lab jack with the hot plate on it to allow your apparatus to cool. Discard the distillate and the residue in the round-bottom flask in the Liquid Organic Waste container, saving “Fractional 2,” “5,” and “7”.

Record the temperatures observed every 2 mL in your notebook as before. Plot both sets of temperatures (for simple and fractionating distillations) in a graph (see below) in your notebook. Clearly label each data set.

Figure 2. Distillation Apparatus with Fractionating Column 4

115 ˚C 110 ˚C

105 ˚C 100 ˚C 95 ˚C 90 ˚C

85 ˚C boilingpoint ˚C 80 ˚C 75 ˚C 70 ˚C 65 ˚C 60 ˚C 0 2 4 6 8 10 12 14 16 18 20 volume of distillate, mL

WASTE DISPOSAL Discard the unwanted distillate as well as the residue in the round-bottom flasks into the Flammable Organic Waste container in the hood. Do not discard the stir bar.

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