CHEM C2507y Spring 2000

Collaborative Learning:

To complete all three parts of this experiment, your group must work efficiently and collaboratively. If your group has four members, one pair should start with fractional distillation and the other pair with thin-layer chromatography (TLC). If your group has three members, one pair should start with fractional distillation and the individual with TLC. While the fractional distillation is running (and the process takes a long time) and the TLC plates are set up, members in the group should take time to teach each other on the part that the others are not involved in the initial set-up. This is essential because you are responsible for all materials, not only the part you do. The TLC part will probably finishes before the fractional distillation. In this case, the pair or individual should move on to column chromatography while the other pair will finish up fractional distillation and join in to complete column chromatography.

The practical goal of a distillation is to condense and collect the vapor. One cycle of evaporation and condensation is called a theoretical plate. If the distillate (or sometimes called condensate) is then equilibrated with its own vapor, and that vapor condenses, the resulting liquid is even more concentrated in the lower boiling compound. This is accomplished by providing a large surface area between the pot, where the initial mixture is boiled, and the head, where the vapor is condensed. A fractionating column would provide such a large surface area. It is also important to heat the mixture in the pot slowly.

As a result, as one component is "distilling off," the temperature at the head remains equal to the boiling point of that component. The pot temperature is also constant. After the first component completely boils off, the pot temperature increases until the next compound begins to boil off. During this time, vapors do not reach the thermometer (situated at the head) and the temperature drops slightly. This can cause problems because if the pot overheats, the head of the thermometer will not indicate it. When a second compound begins to boil, its vapor will reach the head of the thermometer again, and it will shoot up to the temperature of those vapors.

Of course, the distillation would be worthless if all of the distillate was collected in one receiving flask. Hence, a fraction is collected at each constant head temperature, since this indicates one compound is boiling off. The first fraction, (or the mixture between fractions) usually just a few drops, is called the forerun. It contains any very volatile substances that were present in the sample, and is a kind of first rinse of the distillation glassware. It may be combined with the next fraction if analysis warrants.

A distillation is always stopped before the pot runs dry. Otherwise, overheating can occur, and occasionally explosive compounds are concentrated in the pot.

Fractional distillation is used when the boiling point differences of the compounds to be separated are not large enough to employ the simple distillation technique. Simple distillation can separate compounds cleanly if the difference in boiling points between the two compounds is greater than 70ƒ C. The two components which we will be separating (cyclohexane, bp 81.4ƒ C, and toluene, bp 110ƒ C) differ in boiling point by only about 30ƒ C.

Fractional distillation is performed by packing a column with a stainless steel sponge. As the vapor rises through the column, it undergoes a series of condensation and vaporization events on the surface of the packing material. This series of condensation and vaporization is what we called theoretical plates, and they are equivalent to several simple distillations. With each evaporation and condensation event within the column, the composition of the vapor is progressively enriched in the low-boiling point compound (cyclohexane as in our distillation). When the lower boiling point compound is removed (first fraction), it is collected and a second fraction occurs when the temperature increases (second fraction) and this contains some of the higher boiling point compound and residual amounts of the lower boiling point compound. When the temperature reaches the boiling point of the higher boiling compound (toluene as in our experiment), a third fraction is collected.

There are two major advantages of fractional distillation. It is much easier to purify large volumes of liquids by one fractional distillation than by several simple distillations, and the losses of material are much smaller. Losses in material usually occur in the forerun, and the residue left in the distillation pot. The conditions that are necessary for a good separation are usually large amounts of liquid continually returning through the column, thorough mixing of liquid and vapor, and a large active surface of contact between the liquid and vapor.

A typical laboratory fractionating column usually contains between 2 and 4 theoretical plates, while an industrial column can contain 20 or more theoretical plates. If the number of equivalent plates is known, and we know the column length, we can calculate the height equivalent to a theoretical plate (HETP). This is defined as the length of a column divided by the number of theoretical plates in a column. Suppose for example that a laboratory column had three theoretical plates, and a length of 21 cm. The HETP would be 7 cm. The lower the HETP, the more efficient the column.

In This Experiment:

1. To a dry and clean 50-mL round-bottomed flask (i.e., this is the distilling pot), add a small magnetic stir bar and 30 mL of a mixture of cyclohexane and toluene. Minimize the exposure of the mixture in air as the chemicals are hazardous. Use a piece of parafilm to cover the round-bottomed flask until you are ready to move on to the next step.
2. Proceed to assemble the rest of the apparatus for fractional distillation as in Figure 1 (on page 5) except the thermometer. First, pack the column by gently pulling stainless steel sponge through the column using a hooked copper wire. Do not push the stainless steel sponge because of the danger of breakage or cutting your fingers. The amount of stainless steel used is not critical, but it should not be packed very tightly because this can result in the vapor not passing through the column. Also, if one condenser given to you is larger than the other, use the large condenser for the fractionating column. The fractionating column must be kept vertically in space at all times. Your instructor must check the set-up of the apparatus before you move on to the next step.
3. Very lightly grease all joints. Make certain that the joint is tight, and use as many clamps as necessary to secure the entire apparatus.
4. Next, insert the thermometer into the neoprene adapter and glass straight tube. This is done by adding a drop of glycerol onto the bulb of the thermometer, and gently inserting the thermometer into its holder on top of the distilling head (three-way adapter) as shown in Figure 1. Your instructor will demonstrate the proper way to insert a thermometer. Adjust the thermometer so that the bulb is completely below the sidearm in the distilling head. This will result in an accurate temperature reading because the bulb of the thermometer will be totally immersed in the vapor.
5. Attach the water hoses at the proper locations of the water-cooled condenser. One hose should be connected to water supply while the other is left for drainage. Pay attention to where the "inbound" water hose goes. When the hoses are secured, slowly let the water flows and observe carefully the rate of flow of water. Do not turn the water to full blast as water may explode at the connection between the inbound hose and the water-cooled condenser. After testing, you may discontinue the water flow.
6. Check again to make sure that all parts of the fractional distillation apparatus are set up properly.

1. First, label four 10-mL graduated cylinders as forerun, number 1, number 2, and number 3. Use the forerun graduated cylinder initially to collect the forerun.
2. Heat the mixture in the round-bottomed flask gently with a heating mantle plugged directly into the Variac placed over a magnetic stirrer. Do not turn on the HEAT on the stirring hot plate. Use the Variac, which controls the voltage to the heating mantle. You will be responsible for determining the proper voltage setting and thus the proper rate of heating. HINT: Heat the mixture relatively strongly at first, but please keep an eye out on the mixture. As soon as the mixture is near its boiling point, adjust the Variac setting to maintain a good constant boiling without over-heating. Over-heating occurs when the bubbles are reaching into the neck of the flask, or the liquid is splashing up into the fractionating column. Discontinue heating immediately when over-heating occurs.
3. After bringing the distilling pot to a gentle boil and before any material emerging from the top of the fractionating column, a ring of condensing vapor rising in the flask and through the column should be visible. This is called a reflux ring. It may be necessary to adjust the heat so that the reflux ring rises at a constant rate to the top of the column. As the reflux ring rises to the top of the column, the temperature reading will shoot up and vapor will start to condense in the condenser. Collect the first few drops (no more than 1 mL) of distillate that come into the receiving flask (10-mL graduated cylinder) and change the receiving flask to another one. At this time, the thermometer reading should be near the boiling point of cyclohexane (81.4ƒ C). A pure fraction should be collected at a constant boiling temperature, usually within a range of 3-4ƒ C or so. The first few drops collected are the forerun that contains low boiling impurities and should be discarded. If, after you heat your distillation pot for a while and you notice no distillate, you may need to insulate your fractionating column and distillation pot with cotton and aluminum foil.
4. With the second receiving flask (since the first one is used to collect the forerun), begin collecting temperature vs. volume of distillate data. Refer to the distillation curve on page 3 of the theory section of this experiment for reference.

Adjust the heat so that the rate of distillation is about one drop collected every 1-2 seconds. Keep in mind that the temperature of the thermometer may drop after most of the cyclohexane has distilled over due to their being not enough vapor surrounding the thermometer. Change the receiving flask when the temperature drops. Label the flask that contains the pure cyclohexane as flask number 1 (containing the first fraction), and set it aside for analysis (in the next experiment). At this point, increase the voltage to the heating mantle to distill over the toluene. The first few drops of "toluene" actually contain residual cyclohexane. The receiving flask should therefore be labeled as flask number 2. Collect the second fraction and save it for analysis. You may wish to collect successive second fractions throughout the distillation to see how pure your distillate is at each stage of the fractional distillation.

When the thermometer reaches the boiling point of toluene (110ƒ C), change the receiving flask to flask number 3. Collect the pure toluene. Remember to keep an eye on the distilling pot. When the volume remaining in the pot runs down to only about 2-3 mL, shut off the Variac, and discontinue heating. Never let the distilling pot run dry. Record the volumes of the cyclohexane, toluene, and intermediate fraction collected.

Use the glass vials provided, label and place several mL of each sample into separate vials. Use parafilm for better sealing of a vial as organic liquids evaporate quickly. Your instructor will instruct you to place the labelled vials at a proper storage location in the laboratory.

• Use forceps or tongs to put the TLC plate in the developing chamber or iodine chamber. Do not lean the TLC plate against the filter paper. It will be difficult to measure the R_f values due to the solvent leaking from the sides.
• Make as small a spot as possible on the TLC plate. Large spots cause streaking.
• Mark the solvent front before it evaporates. This means you should have your pencil ready before you remove the plate.

1. Obtain a Pasteur pipette. Carefully push a small piece of cotton down to the narrow part. Clamp the pipette vertically with a thermometer clamp and add about 1/4 inch of sand (use weighing paper as a funnel). Add also about 2-1/2 inches of silica gel and then more sand. Load the column by adding your dry sample (mixture of ferrocene, acetylferrocene, and 1,1-diacetylferrocene directly from the vial) to the top of the column. Prepare two identical columns (i.e., ideally, each pair of members in the group should prepare one).
2. Begin eluting both columns by first adding 100% hexane via Pasteur pipette. This is in order to move the nonpolar compound down the column, and then gradually increase the solvent polarity to force the more polar compounds down the column. If necessary, push the solvent through the column by inserting a pipette bulb to the vertical column and then pump. HINT: After eluting with 100% hexane, increase the polarity of the solution by utilizing a 20:80 mixture of ethyl acetate and hexane, and finally work your way up to a 50:50 mixture of ethyl acetate to hexane. Do not let the column run dry at any time during the elution process until you are finished with it.
3. Notice the different colored bands and determine which band belongs to which compound and why.
4. To ensure a clean separation of the three compounds, you should completely elute each band before adding the increasingly polar solvent mixture. You should also collect the solvent between each band in a test tube different from the test tubes used to collect the separated compounds. Diluting the compounds with excess solvent will cause ill-defined spots during the final TLC assessment.
5. Collect 1 mL of sample per test tube in labeled tubes and run TLC to check the effectiveness of column separation with the best solvent mixture of hexane to ethyl acetate as determined in PART II.

This experiment contains short exercises that are meant to allow you to get familiar with fractional distillation, TLC, and column chromatography. By the end of Experiment 7, you should be able to compare and contrast all four types of chromatography as well as fractional distillation, which is our ultimate goal (see page 7 of the theory description handout for both Experiments 6 and 7). Therefore, the laboratory report for Experiment 6 will be concise, while the report for Experiment 7 will be comprehensive.

Hints for the Experiment 6 Laboratory Report:

Abstract – Keep it short.

Introduction – Recapture the theory of partition equilibrium in your own words. Apply partition equilibrium to explain how TLC and column chromatography can be used to separate ferrocene, acetylferrocene, and 1,1’-diacetylferrocene. Recapture also the theory of distillation in your own words. Explain how cyclohexane and toluene can be separated by fractional distillation.

Experimental Methods – Do not re-write the entire procedure. Instead, summarize key steps in few words and include all the details that would ensure successful separation (such as never let the solvent in a solvent chamber to run past the top of the TLC plate since in this case there is no way to locate the solvent front). Include your laboratory observations here.

Results –

PART I: Distillation curve

PART II: R_f values and solvent system evaluation

PART III: TLC assessment

Discussion – Evaluate the merits of this experiment. Distinguish the quantitative and qualitative aspects of this experiment. Discuss also the method to select the ideal solvent system to separate ferrocene, acetylferrocene, and 1,1’-diacetylferrocene. Recommend your ideal solvent system and provide the rationale.

References – See the theory description handout, page 1.