Please Note: As indicated in your initial laboratory lectures, the first three lab reports will not follow the investigative or preparative report formats described in the introductory pages of this lab manual



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Please Note:

As indicated in your initial laboratory lectures, the first three lab reports will not follow the investigative or preparative report formats described in the introductory pages of this lab manual. For these labs, you will only turn in what is requested in the individual lab. Pre-lab questions must be completed before lab.

Experiment 1
Fractional Distillation and Gas Chromatography
General Safety Considerations



  1. The liquids are flammable! No flames in lab!

  2. Study glassware for cracks before beginning. If you find any, show the damaged glassware to your instructor.

  3. Make sure that the joints are flush in the distilling apparatus and that the vacuum duct is unobstructed.

  4. Secure H2O hoses with wire. Keep the water pressure moderate to low.

  5. Do not discard distillate or pot residue (exception: the spent cloves can go in the trash) in sinks, garbage cans or general waste containers (large red cans in waste hood). Used cyclohexane and toluene should be poured into the appropriately labeled containers in the dispensing hoods. These liquids will be used again next year.

  6. Never distill to dryness.

  7. Unless instructed otherwise, heat conservatively.

  8. Build your still several inches off the top of the bench.

  9. Do not plug your powermite directly into a "normal" socket. It must be plugged into a "powermite" (also called variac, step control).

  10. Cyclohexane, toluene, and methylene chloride are irritants and toxic. Methylene chloride is particularly irritating when it is trapped under a ring or a glove. Avoid contact with these substances by wearing gloves and goggles. Keep these compounds in the hood as much as possible. In the event of a significant spill, consult your instructor. If you suspect you have gotten any of these substances on your skin , flush the exposed area with cold water for fifteen minutes. Occasionally, an organic compound will seep through the nitrite gloves. Remember, gloves are just a primary barrier. If you feel any itching or burning under your gloves, take the gloves off and flush the exposed area for fifteen minutes with cold water.




Chemistry 211-212

Investigative Experiments


Name

TA Name:

Experiment # 1

Lab Day:

Unknown #

Section 1 (Pre-lab)

(25 points)

Section 2 (Results)

(56 points)

Section 3 (Post-lab questions)

(70 points)

Quality of results

(20 points)

TOTAL

(171 points)

SCORE

(percent)


This is your report cover. Please fill it out and attach it to your prelab questions.

Eugenol Isolation Flow Chart

Place Ground Cloves in 100 mL Flask


with 40 mL of water
Steam Distill


Distillate

Pot Residue



Dispose of liquids down drain, cloves in trash

Extract with 8 mL CH2Cl2



Aqueous layer



Extract with 8 mL CH2Cl2



Extract with 8 mL CH2Cl2

Organic layer

Aqueous layer




Organic layer

Aqueous layer

Discard down drain





Flush down drain

Residue

Rotovap to dryness

subject to olfactory analysis



Filtrate

Dry over NaSO4 Gravity Filter

Combine organic layer
The Basics of Distillation

Distillation most commonly refers to the process of vaporizing a liquid in one vessel and then recondensing it in another vessel. It is usually used to separate one compound from another (others), but it can also be used to help to identify a compound because distillation is a reasonable way to determine the boiling point of a liquid.


For a distillation to occur at a reasonable rate, the compound in question must have a high vapor pressure. This is the same as saying it should have a low boiling point or that it should be volatile (N.B. volatile does not mean explosive). Vapor pressure refers to the rate at which molecules escape from a liquid (solid). If there is a high rate of escape or the vapor from the compound is exerting a lot of pressure against the atmosphere, then the compound has a high vapor pressure. Compounds that have a high vapor pressure at room temperature can often be detected by olfactory analysis. Diethyl ether has a high vapor pressure as does naphthalene (moth balls).
Boiling is not a necessary requirement for distillation, but it certainly accelerates the process. What is boiling? A liquid is boiling when the vapor pressure of the liquid equals that of the atmosphere giving the liquid the maximum rate of escape into the vapor phase. If a liquid has a low boiling point, its intermolecular forces are low and less energy is needed for molecules to escape into the vapor phase at their maximum rate. So if a liquid has a low boiling point, it has a higher vapor pressure at any temperature, but the maximum rate of escape can be achieved at a relatively low temperature. It is important to realize that like other phase changes, boiling is an equilibrium process. When a liquid is boiling the escaping tendency of the liquid into the gas phase equals that of the gas into the liquid phase.
As mentioned previously, boiling is not a necessary requirement for distillation, but it is usually done to speed up the collection of liquids in the receiver. The preceding discussion also opens up the idea that there is a relationship between the atmospheric pressure and the boiling point. That is, the more the atmosphere weighs down on the liquid, the greater the energy input must be to have those molecules escape at a high rate. On a high pressure day, our boiling points will go up and on a low pressure day they will go down. Now you may be a little more interested in what your weather person has to say each morning!
The relationship between atmospheric pressure and boiling point tells us that if we could reduce the atmospheric pressure in a controlled way, we could intentionally reduce the boiling point of a liquid. In principle, if the atmospheric pressure is lowered enough, liquids will boil at or below room temperature. In the laboratory, the atmospheric pressure is typically reduced by attaching a vacuum line to the distillation apparatus. In a couple of weeks you will use the rotary evaporators in the lab. These are really reduced pressure distillation apparatus. Reduced pressure distillation or vacuum distillation is useful when one wants to rapidly remove a volatile solvent from nonvolatile compounds, when minimal heating is important due to the thermal instability of a compound or when a compound has such a high boiling point that atmospheric distillation is not feasible.
First we will consider distillation as a purification method. There are fundamentally two different types of distillation that will be used routinely in this course, simple and fractional distillation. The simple distillation apparatus is shown below. Simple distillation is used in less critical separations of compounds. It can be used to separate a non-volatile solid from a liquid. For example, if one had a solution of sodium chloride in water, one could use this apparatus to distill the water from the sodium chloride. Of course, the sodium chloride would remain in the pot and the water would end up in the receiver. Simple distillation can sometimes be used to separate two volatile liquids provided that the two liquids have vastly different boiling points, say more than 50 degrees.
Simple distillation is a reasonable way to determine boiling point. One can load the pot with a pure unknown liquid and very carefully and slowly distill it. The still head temperature will quickly level off. The range of temperatures observed at the still head during the temperature plateau is the boiling range of the unknown. Since the boiling point is related to the structure of the compound, knowing the boiling point can be useful in identifying the compound.





The fractional distillation apparatus is shown below. Obviously, the only difference in the set up is the insertion of a column between the still pot and the still head. It doesn't look like much, but that column can make the world of difference in a separation depending on its construction and size. Fractional distillation can be used for very critical separations. In principle, one could separate any two liquids provided there is a difference in boiling point. Again though, the ability to separate is totally dependent on the equipment and the way the distillation is carried out. To understand this better this better some basic comprehension of distillation theory is needed.




Fractional Distillation Apparatus



For a fractional distillation of two volatile liquids to be successful, it is imperative that the two liquids form a solution and that the solution behave in an ideal fashion, i.e., when the liquids vaporize, the vapor phase must be enriched in the more volatile component. It should make sense that if one component has a lower boiling point than the other it should have weaker forces and should have a higher vapor pressure. This being the case, the molecules should leave the solution at a higher overall rate thus producing an enrichment.


It is also important to realize that when heating (or even at room temperature) both types of molecules are vaporizing simultaneously, though hopefully one escapes in greater quantity than the other. This is one of the most common misconceptions about purification through distillation - that the more volatile component just jumps off the surface of the solution, leaving the less volatile component behind. This would be true of water in a solution of sodium, but not true at all of a solution of two reasonably volatile liquids.
The total vapor pressure of a binary solution of two volatile components A and B can be described as follows.

Pr XAP°A +XBP°B


The above equation is a combination of Raoult's law and Dalton's law of partial pressures. What it is basically saying is that the total pressure of the system is the result of the partial pressure of each of the components making up the solution. The partial pressures in turn depend on the amount of each component and the vapor pressure of the individual component. So if one had an equal quantity of diethyl ether (b.p. 34.6°C) and hexane (b.p. 69°C), diethyl ether molecules would be escaping at a higher rate. If one had unequal quantities of two compounds having the same vapor pressures, then the one in larger quantity would be escaping at a higher rate.
To understand enrichment or how purification occurs better let us first consider a simple distillation. In theory, a simple distillation apparatus has one theoretical plate. A theoretical plate is one enrichment. An enrichment is a single vaporization resulting in an enhancement of the more volatile component relative to the less volatile component. Suppose we have a 50:50 mixture of two compounds A and B, respectively. Lets say that when this solution is initially vaporized it is enriched to 60:40, A:B. If the simple distillation apparatus truly has only one enrichment then the material collected in the receiver of the distillation apparatus would be 60:40. If further enrichment is desired, one could take the 60:40 material and add it to a clean still pot and re-distill it. Upon re-vaporization it might go to 70:30, A:B and so on until the desired purity is achieved. Obviously this is not a very efficient way to go about a purification. Please note that I am making the enrichment increments up in a convenient manner for the sake of simplicity. As you will see later, enrichments do not occur in such even increments.
Switching to a fractional distillation set up will greatly improve matters. In the fractional distillation apparatus a column has been added. The column can vary quite a bit with regard to its structure. Generally, columns are better if they are longer and packed with a material that provides a large surface area. What does the column do? Well consider the hypothetical 50:50, A:B mixture again. This mixture is heated to boiling and it vaporizes. Lets assume it is enriched to 60:40, A:B as with the simple distillation. At this point, it is important to think about the boiling point of this solution. If you could take the thermometer from the still head and lower it into the distillation pot, you would find that the boiling point of the 50:50 mixture lies between the boiling point of the two individual compounds. What is the boiling point of the vaporized, enriched 60:40 material like? It should be closer to the boiling point of pure A which means the boiling point should be lower than that of the 50:50 mixture. Being lower boiling means that the enriched vapor phase condenses at a lower temperature. It rises up and hits a cooler part of the apparatus and condenses. It then starts to flow back into the pot. But being lower boiling means that it doesn't have to flow as low to revaporize. It is important at this point to realize that the column has a temperature gradient. The set up is hottest at the bottom and is cooler as one approaches the still head. So the 60:40 flows not quite as low and revaporizes again to say 70:30. The 70:30 has an even lower boiling point so it travels higher in the column before recondensing. It also does not have to drop as low to revaporize. Its boiling point is lower. The condensation/revaporization cycles continue over and over again until hopefully nearly pure material is obtained at the top of the column. This would be the material that is first condensed and collected in the receiver.
From the above discussion it is evident that a surface on which the liquid can condense is important. If the column were very short and/or not packed with a material to provide this surface, fewer condensation/ vaporization cycles would occur and the material initially condensed into the receiver would not be as pure. As mentioned previously another term for one of these enrichment cycles is a theoretical plate. The more efficient a column is the shorter the height equivalent of a theoretical plate (HETP). A very efficient column has many theoretical plates. You will discover through our investigations that our lab columns do not have many theoretical plates.
Once again it must be emphasized that the increments of enrichment given above were quite ridiculous and were created to make the explanation of enrichment a bit simpler to follow. The following vapor phase composition diagram describes the quantitative aspects of enrichment better. In reading this diagram realize that the X axis corresponds to the percent composition of the solution. The Y axis corresponds to temperature in degrees Celsius. There are actually two curves on the diagram. The upper, convex curve corresponds to the vapor phase composition curve. The lower, concave cure corresponds the liquid phase composition. The point "A" given at the far left of the curve corresponds to the boiling point of pure A. The point `B' given at the far right is the boiling point of pure B. Now how does the curve work? Suppose You have a 50:50 solution of A:B. The 50:50, A:B is vaporized. If it is an ideal solution, it will become enriched in the vapor phase with respect to A. To find out the vapor phase composition extend a line from 50:50 A:B on the X axis to the liquid phase composition curve. Then draw a perpendicular until you intersect the vapor phase composition curve. Then draw a perpendicular down to the percentages on the X axis. This ratio, 82:18, A:B is the vapor phase composition. The process we just went through is the equivalent of one enrichment, plate or simple distillation.

Now to get the idea of what happens in a fractional distillation, this process would be repeated over and over, one step for each plate in the column. So the enriched, material would recondense and then revaporize. Upon revaporization it would enrich to as indicated on the graph and so on. You will note that enrichments get progressively smaller as one approaches pure "A". This is significant for you in lab this week because if you start with an unknown that is richer in cyclohexane your overall enrichment will be less that someone who starts with a sample that is richer in toluene even though there may be no variation in technique or equipment from you to your neighbor.


It is important to recognize that other factors influence the purity of a sample. Generally speaking, the faster a distillation is done, the cruder the material will be at the top of the column. For a distillation to go faster, one normally increases the rate of heating. With greater heat input, the column ends up being hotter and has less of a temperature gradient. With a smaller temperature gradient, fewer enrichments occur. To understand this better think about a theoretical distillation occurring over a column having the exact same temperature as the still pot. The material in the pot would vaporize and enrich, but the column would be so hot that the material would not recondense until it hit the condenser. Without multiple recondensations in the column, little enrichment will occur. Even though a slower distillation is better, we usually have to compromise a bit so that we can finish these labs in a reasonable amount of time. So though your purity will not be its absolute best, stick to the prescribed distillation rates so that you will not have to stay overnight to finish the lab. We usually aim for a rate of one drop per second going into the receiver. If your rate is less than that at any time during the procedure increase the voltage. If it is more, decrease the voltage.

There are a few more terms you should absorb at this point. The term holdup refers to the amount of liquid that must be left in the distillation apparatus to avoid distilling to complete dryness. We usually stop a distillation when the liquid level is just above the boiling stones. Stopping at this point avoids the possible explosion of peroxides that may have formed during vigorous heating. When this is done liquid is left in the pot, but also on every surface in the still. Holdup is a source of material loss in a distillation. The term throughput refers to the rate at which one distills. The higher the throughput, the lower the purity.


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