In the Laboratory
First Day in Organic Lab Christine K. F. Hermann Department of Chemistry and Physics, Box 6949, Radford University, Radford, VA 24142 In our department, both chemistry majors and nonmajors take the same organic chemistry course. Recently, in my organic chemistry labs, I introduced a new experiment for the first day of lab. This particular experiment was designed to introduce the students to the techniques of reflux, distillation, gas chromatography, and the determination of boiling point and melting point during one lab period. One innovation in this experiment is that all of these techniques are introduced in one period. Another is that the students examine the reflux and distillation apparatus and then answer commonly asked questions on these techniques. This compares to the student merely setting up the glassware without understanding the process behind reflux and distillation. This experiment can be taught without the students knowing any organic chemistry. One goal of this type of experiment is to allow the students to discover, through the answering of questions on a worksheet, the concept of reflux and distillation and to observe the glassware correctly assembled. The glassware for both reflux and distillation was assembled and in operation upon the students’ arrival in lab. Different sources of heat were used in the reflux apparatus and the distillation apparatus. A further objective is to learn techniques for obtaining the physical constants. Practice is needed to master these techniques. Each student was given a liquid unknown and a solid unknown to be used for the determination of boiling point and melting point, and the identities of these unknowns were determined by comparing these values with a limited list of possibilities. The third objective of this experiment is familiarization with the gas chromatograph, since the students will be analyzing samples by this method later in the semester. After reading the discussion on gas chromatography, each student injected a sample under supervision, obtained a chromatogram, and calculated peak area and retention time. The following information and directions were given to the students. Reflux and Distillation In the laboratory, various glassware setups are utilized. When the glassware is assembled, a tiny bit of grease is used between the joints so that the glassware may be easily disassembled later. A reflux apparatus is used to heat solutions of organic compounds without incurring any loss of solvent, reagent, or product. Using distillation, liquid compounds may be purified by separating the desired liquid from the higher- or lower-boiling components. Both apparatuses contain condensers. Condensers consist of a glass tube enclosed in a larger glass tube. Water runs between the inner and outer tubes to provide cooling.
Reflux 1. Is there a stopper on top of the condenser? 2. Explain your answer to the previous question in terms of pressure and temperature, relating it to the ideal gas law.
852
3. What is happening to the liquid in the round- bottom flask? 4. Explain what happens when the vapors contact the condenser. 5. Where does the liquid (not the cooling water) go after it exits the condenser? 6. Gently touch the condenser. Is it hot or cold? 7. Which way does the water flow in the condenser— uphill or downhill? 8. Why?
Simple Distillation 1. Where does the liquid (not the cooling water) go after it exits the condenser? 2. How is this different from reflux? 3. Record the temperature on the thermometer. 4. What does this temperature indicate? 5. Is the simple distillation apparatus a closed system or an open system? 6. Explain your answer to the previous question in terms of pressure and temperature, relating it to the ideal gas law. 7. Which direction does the water travel—uphill or downhill? 8. Name the sources of heat that are used for the reflux and the distillation.
Physical Constants—Melting Point and Boiling Point Melting points and boiling points are key physical constants. The proximity of this temperature, experimentally obtained, to the literature value indicates the purity of the compound.
Melting Point Obtain a melting point capillary tube that has been sealed on one end. If both ends are open, use the Bunsen burner to seal one end. Place a few crystals of the solid into the melting point tube. Tap the crystals, in powder form, down to the closed end of the capillary tube. Only about 1 mm of sample should be present in the melting point tube. The sample is now ready for the melting point to be measured. The instructor will demonstrate the proper technique for obtaining the melting point. A range of temperatures is obtained. The first temperature is recorded when the solid appears to become wet or shrinks slightly. The second temperature is recorded when the entire mass of crystals has been converted to a liquid. Identify the unknown from these compounds: benzophenone (mp 48 °C), benzocaine (mp 92 °C), benzoic acid (mp 122 °C), and salicylic acid (mp 158 °C). Unknown number ____________________________ Melting point of the unknown _________________ Identity of the unknown ______________________ Boiling Point Boiling point can be measured by several methods. The three basic methods are an ultramicro boiling point
Journal of Chemical Education • Vol. 73 No. 9 September 1996
In the Laboratory
(1), a micro boiling point (2), and macro boiling point (2). The instructor will demonstrate one of these techniques. Identify the unknown from these possibilities: ethanol (bp 78 °C), 2-pentanone (bp 101 °C), cyclohexanone (bp 156 °C), and acetophenone (bp 202 °C). Unknown number ____________________________ Boiling point of the liquid _____________________ Identity of the liquid _________________________
B
A
Gas Chromatography The gas chromatograph is fitted with a column packed with Carbowax on a silica support. The temperature of the column is set at 115 °C and of the injection port and detector ovens at 150 °C. A mixture of ethanol, 1-propanol, 1-butanol, and 1-pentanol will be analyzed. Chromatography is a method that separates a mixture into its components by distribution between two phases, one mobile and one stationary. A mobile phase is usually a liquid or gas; it carries the mixture through the column. A stationary phase is a solid or liquid; it slows passage of the individual compounds through the column, usually by polar attraction. In most cases, the nonpolar compounds come off the column first, followed by the polar compounds. In gas chromatography, the mobile phase is an inert gas and the stationary phase is usually a solid or silicon oil. The retention times of the peaks correspond to the time it takes the individual compounds to pass through the column. The percentage composition of each peak corresponds to the percentage of each component in the original mixture. The amount of sample used for the gas chromatograph varies from a few tenths of a microliter to 20 µL. Too much sample will cause the column to become overloaded, resulting in the merging of peaks with one another. The sample is injected into the gas chromatograph quickly. If the injection is too slow, the chromatogram ends up looking like a mountain range instead of individual sharp peaks. As soon as the sample is injected, the needle is removed from the injection port. NOTE: The plunger of the syringe must have some pressure on it at all times. If the user puts no pressure on the plunger, it may fly out from the barrel of the syringe and become airborne as soon as the sample is vaporized.
Analyzing the Sample Obtain the gas chromatogram under the supervision of the instructor. Calculate the retention times, peak areas, and percentage compositions for the four peaks by referring to the example calculations demonstrated in Figures 1 and 2 . Peak A B C D
Retention time (min) _____________ _____________ _____________ _____________
Peak area _________ _________ _________ _________
Composition (%) _________ _________ _________ _________
0
time
distance for peak A
distance for peak B t
for peak A =
distance for peak A ____________________ chart speed
t
for peak B =
distance for peak B ____________________ chart speed
R
R
Figure 1. Calculation of Retention Time.
H
H
B
W B A
W A
Area of peak A = height A X width A (measured at half height) Area of peak B = height B X width B (measured at half height) Area of peak A Percent composition of peak A = _____________________ x 100 Areas of peak A + peak B Area of peak B Percent composition of peak B = _____________________ x 100 Areas of peak A + peak B
Figure 2. Calculation of Peak Area and Molar Percent Composition.
Conclusions This procedure assumes that the student will appear in the laboratory unprepared, since there is no lecture before my first labs and the students have not been informed of the nature of the lab experiment. The handout is the only item that the student receives relating to the first experiment. Verbal explanations are added as needed.
The students were graded according to the correct answers given to questions. I assisted them in all of the techniques and explained the techniques in greater detail. The questions were geared toward some of the most common questions that I have received as each technique is introduced. The explanations above provide only a basic understanding of the concepts.
Vol. 73 No. 9 September 1996 • Journal of Chemical Education
853
In the Laboratory
I find that the timing of this lab allows students to become familiar with some of the techniques they will use throughout the semester. This lab gives the students a better understanding of each technique when it is encountered later in the semester. Some common mistakes, such as stoppering a reflux condenser or attaching water lines backwards, have been minimized since this lab has been introduced.
854
Literature Cited 1. Mayo, D. W.; Pike, R. M.; Trumper, P. K. Microscale Organic Laboratory, 3rd ed.; Wiley: New York, 1994; pp 44–45. 2. Shriner, R. L.; Fuson, R. C.; Curtin, D. Y.; Morrill, T. C.; Hermann, C. K. F. The Systematic Identification of Organic Compounds, 7th ed.; Wiley: New York, 1997.
Journal of Chemical Education • Vol. 73 No. 9 September 1996
