Chemistry 150

 

Please have the following pages ready before class on Wednesday, January 23. Note that the different parts will be standard divisions in all lab writeups. For this particular writeup, please write an abstract and paper-clip it to the front of your individual writeup. The abstract and the carbon-copy pages of the write-up is due in class on Wednesday, January 30.

 

The on-line version of the procedure can be found at:

 

And this is what your lab notebook should look like:

 

                                                                        Your name, your partner’s name, date of experiment

 


Lab 2: The separation of organic compounds by polarity

   Part 1.  Purpose

 

Read the website:

 

http://chemweb.calpoly.edu/chem/124/124Experiments/OrgAnal/GCTheory.html

 

then read the rest of this write-up. You should have two sentences in your purpose: the first concerning what you are going to do with the gas chromatograph; and the second concerning what you are going to do with thin layer chromatography.

   Part 2. Materials and methods

 

Equipment: Molecular model kits; gas chromatograph (GC) and related supplies (glass syringe); thin layer chromatography (TLC) kit, including flexible TLC plates, capillary tubes, ruler, pencil, developing jar, ultraviolet (UV) lamp viewing box

 

Chemicals:

            GC: 5-component known solvent mixture (pentane, hexane, octane, 4-pentanone, toluene); 2-component unknown mixture

            TLC: 1% benzil in acetone; 1% benzoin in acetone; developing solvents, including toluene, 4-pentanone and hexane

 

Sketch the GC machine and label the parts you end up using.

 

Sketch the TLC kit and how you use the parts (label the parts!).

 

   Part 3. Procedure

 

You will work in pairs on this lab, and each pair will be assigned a specific time on the GC (there is only one machine for the whole class). Time management will be an issue, since you can’t do the TLC while you are running the GC.

 

Molecular model kit

Each of you will be asked to build a model of one of the molecules used in this lab. Make sure you get the instructor’s okay before displaying the model! You will then draw the structural formula (in the appropriate space; see below) of each molecule in your lab book.

 

Gas chromatograph

 

1. While you are waiting for your turn on the machine, sketch the structures of pentane, hexane, octane, 4-pentanone, and toluene, in your lab book under part 4 (original data).

 

2. Obtain a vial of the 5-component known mixture and a vial of an unknown 2-component mixture. The unknown mixture’s components are a subset of the known mixture.

 

3. Write down your unknown number (it will be on the side of the vial).

 

4. Make sure the gas chromatograph and computer are on, that gas is flowing through the column and that the computer software (Logger Pro) is running. Also make sure the glass syringe is clean by squirting a few microliters of acetone through it (note that the graduations on the syringe are in microliters).

 

5. Take up about 4 microliters of the known mixture into the syringe; try not to get much air in the syringe.

 

6. Locate injector port B and carefully (without bending the needle) insert the needle into the port. You should feel some resistance as the needle penetrates the rubber septum. As you inject the sample into the port, have your partner click the “collect” button on the computer display to begin data collection.

 

7. The run will end automatically after 3.5 minutes; you should have a display of five peaks corresponding, in order, to pentane first, then hexane, octane, 4-pentanone, and, finally, toluene. You should be able to get an exact retention time by using the table on the right side of the computer display.

 

8. Using the “integrator” function, determine the area under the curve (integral) for each peak. This measure will have strange units, but will be proportional to the volume of the particular component represented by the peak.

 

9. Once the retention time and integral information is recorded, rinse out the syringe with some acetone to clean it, then draw about 4 microliters of your unknown into the syringe. Again, minimize air uptake.

 

10. As before, inject the syringe contents into port B while clicking the “collect” button on the computer screen. When the run completes, simply note the presence or absence of a particular component of the known mixture in your unknown mixture by comparing retention times.

 

11. Rinse the syringe with acetone and reset the computer display for the next group.

 

Thin layer chromatography

 

1. Sketch the structures of benzil and benzoin in your lab book.

 

2. Obtain a TLC plate and the other equipment and chemicals needed for this section. Also obtain a piece of scrap TLC plate for “spotting practice”.

 

3. Draw some of the benzil or benzoin solution into a capillary tube (this will happen naturally so you do not need to apply suction). On the scrap TLC plate, practice making solution “spots” on the plate by gently touching the end of the capillary tube to the plate. Fluid will immediately transfer; the trick is to not make the spot bigger than a couple of millimeters. Let the spot dry (as the solvent evaporates), then respot the same point with the same size spot. Repeat this until there is no fluid left in the capillary, letting each spot dry fully between spottings. Do this until you have confidence that you can make a consistent spot.

 

4. On the real TLC plate, draw a line in pencil 1 centimeter away from the short edge of the plate. Use the lightest pressure necessary to make a line, since the TLC material will flake away if you press too hard. Along this line, at 5 to 7 millimeter intervals, make a small tick mark. This is where you will spot the plates with the solutions.

 

5. Using a separate capillary tube for each solution, spot each solution on a different tick mark. Remember, you should need to spot each solution several times on the same spot in order to use all the fluid in the capillary tube. Make sure you note where on the plate you spotted each solution. The side closest to the line will be called the bottom of the plate.

 

6. Repeat steps 4 and 5 twice more, for a total of three identical TLC plates. You will be placing them in different developing solvents, so label the top of the plate with an identifying mark for the solvent (like “P” for 4-pentanone).

 

7. Shake each developing jar to thoroughly wet the filter paper inside; this is to prevent the solvent from rising up the TLC plate unevenly.

 

8. Carefully place each TLC plate in its appropriate developing jar, making sure the pencil line stays above the level of solvent in the jar. Let the top of the plate lean against the jar wall; make sure that the sides of the plate do not touch the sides of the jar!

 

9. Put the jar top on and leave the whole setup alone.

 

10. Capillary action should draw the solvent up each plate. When the solvent front (the wet part) reaches about two-thirds to three-fourths of the way to the top of the plate, carefully remove the plate from the jar and make a pencil mark to show how far up the plate the solvent got (you should do this quickly). Allow the plate to dry in the fume hood.

 

11. When all the plates are marked and dry, place a plate into the UV lamp viewing box. Make sure the lamp is on. Record the solvent front and spot distances (the spots will show up as dark smudges) as faithfully as you can on your TLC plate sketch. Do this for each plate, recording the results as you go.

 

12. The Rf factor is a way to describe the mobility of a particular compound on a TLC plate. To calculate the Rf for a given spot, simply divide the distance of the spot from the starting point by the distance of the solvent front from the starting point. Notice that you will always get a value between 0 and 1.

 

Waste disposal — Please cap the TLC developing jars and the known and unknown GC sample vials tightly when done. Use capillary tubes must be thrown into the glass disposal near the sink. Solid waste (such as TLC plates) may be discarded in the trash cans.

 

 

   Part 4.  Original data 

 

Gas chromatograph

 

While you are waiting for your turn on the machine, sketch the structures of the appropriate molecules in your lab book.

 

Write down the temperatures of the injector port, the detector and the column. If possible, note the flow rate and identity of the carrier gas.

 

Make a table of the name, boiling point, retention time (in seconds), integral (units) and presence/absence in the unknown mixture of each of the five components of the known mixture. This should end up being a 5 column by 6 row table (including headings). Give yourself plenty of room in this table!

 

Write down your unknown number (it will be on the side of the vial).

 

List the names of the two components in your unknown mixture.

 

Thin layer chromatography

 

Sketch the structures of the appropriate molecules in your lab book.

 

Make a full-scale sketch of each of the three TLC plates, noting the starting point and identity of each spot on each plate. Also, note which plate was developed in which solvent.

 

Make a table that shows the Rf values of each spot on all three plates. This sounds like a 9 column wide table. Give yourself enough room so that you can record a spot that ends up having a couple of Rf values.

 

 Part 5.  Calculated results 

 

None this time.

 

   Part 6.  Questions

 

1. In the original data table, there is a column for the different components’ boiling points. Look up the boiling points in a reference and write them in the table.

 

2. a. What is the structural difference between pentane, hexane and octane?

 

b. Generate a rule between a hydrocarbon’s chain length and its boiling point.

 

c. Look at your data and generate a rule between a hydrocarbon’s chain length and its retention time.

 

3. Note that toluene has fewer carbons than octane, yet the octane emerged from the GC first. Modify the question 2b rule to take into account this behavior. Hint: look at the boiling points.

 

4. a. What is the structural difference between 4-pentanone and pentane? In fact, label all of the molecules in the known mixture as “polar” or “nonpolar” overall, given the material you learned from Chapter 8.

 

b. The material in the GC column has the trade name Carbowax, and is a non-polar resin. Suggest a rule between a material’s retention time and its polarity compared to the polarity of the column material.

 

5. Using the integral information, give a whole number ratio of the five components in the known mixture to each other.

 

6. Why did you use a pencil on the TLC plate and not a pen?

 

7. What does the observation that the “spots” of organic material on the TLC plate appear dark under the UV lamp imply about the interaction of ultraviolet light and organic molecules?

 

8. a. Label benzil and benzoin as “polar” or “nonpolar” overall.

 

b. Which developing solvent gave the best separation between the benzil and benzoin? What was the polarity of this solvent? Generate a rule between a compound’s Rf value and its polarity compared to the polarity of the developing solvent.

 

   Part 7.  Conclusion

 

First sentence: State whether gas chromatography or thin layer chromatography or both is/are good methods for separating organic molecules

 

Next sentences: Give your unknown mixture number and its composition. Give the reason(s) (structural difference(s)) between your two component molecules that allowed the GC to separate them.

 

Next sentences: Which solvent was most effective at separating benzil from benzoin, and what was the reason this solvent was more effective than the others?

 

Last sentence: How confident are you in your results? In other words, was your technique and result good enough that you would feel this procedure could be given to the next group of students without modification? If you do not feel this way, suggest how the procedure could be modified to prevent some of the problems you may have encountered.

 

   Abstract

 

Name

School affiliation

 

Title of experiment

 

Summary of major results and methods of this experiment. Hint: you should be able to modify the middle sentences of the conclusion to work here.

 

Remember, less than 100 words.