Experiment Free Radical Chlorination of Heptane In today s experiment, you will be chlorinating n-heptane with chlorine atoms (radicals) to form monochlorination products of that alkane. You will analyze the product mixture by gas chromatography to compare the amount of substitution of primary hydrogens versus secondary hydrogens. For the most part, alkanes are among the least reactive of organic compounds. Of course they burn, and this combustion of alkanes and other hydrocarbons provides the energy that powers nearly all commercial transportation. The other major reaction of alkanes is free radical halogenation. In this reaction, a halogen atom is formed by one means or another. Historically, they were first formed by the homolytic cleavage of the halogen-halogen bond by ultraviolet light. uv hν X X 2 X Bromine atoms can be generated this way in liquid solutions or by N-bromosuccinimide, NBS, which you will learn more about later in the semester. Working with a toxic gas like chlorine, however, is difficult without using special equipment. However, if the chlorine is generated in a liquid solution and immediately irradiated with ultraviolet light, chlorine atoms are formed in solution without the use of any special equipment. In this experiment, this is done by reacting sodium hypochlorite (NaClO), from liquid bleach, with hydrochloric acid in the presence of light as shown in the following sequence of equations. NaClO + 2HCl Cl 2(g) light Cl 2(g) + NaCl + H 2 O 2Cl Once formed, the chlorine radical can react with the heptane as shown below: R H + Cl R + HCl In this first propagation step, chlorine radical abstracts a hydrogen to form an alkyl radical and HCl gas. The alkyl radical then reacts with another molecule of Cl 2 to form the halogenated hydrocarbon product. This also regenerates Cl radical to continue propagation of the chain reaction. R Cl Cl RCl + Cl
Unlike bromine, which is very selective, chlorine atoms are more reactive and give multiple monochlorination products. Gas chromatography of the product mixture will be used to analyze the monochlorination products that are formed. Pre-lab Preparation Before coming to lab, you must do the following in your lab notebook: 1. Write structural formulas for all of the monochlorination products you would expect from this reaction. 2. Write in tabular form the relevant physical constants for heptane and the monochlorination products you have listed above 3. Answer the following questions about the procedures given below. a. What has been done in this experiment to minimize the reaction of the monochlorination products with additional chlorine radicals to give di- and trichloro products? (Consider possible ways to prevent multiple chlorinations.) b. What is the purpose of the sodium bicarbonate washes in the purification of the monochlorinated heptanes. c. The gas chromatogram of your product mixture should show separation of the primary from the secondary products. Referring to your physical property data, predict which compounds you would expect to elute from the column first the primary product or the mixture of secondary products? Explain. Experimental Procedure! Safety Considerations! Hydrochloric acid can cause burns. Use caution when handling HCl and clean any spills immediately.! Bleach can irritate your skin and damage clothes. Clean any drips or spills immediately.! Hydrochloric acid evolves as a gas forming in the heptane during the reaction. Do not use any unvented caps in this laboratory procedure.
1. First, assemble an apparatus in which the chlorine gas will be generated. See the photograph below. Obtain a polyethylene bottle, a dropper cap and a piece of tubing with a syringe needle attached to one end and a vented cork attached to the other end. Connect the tubing to the dropper cap of the polyethylene bottle using the adapter on the end of the syringe. This assembled apparatus is your chlorine generator; when you add the bleach and HCl to this apparatus, chlorine will be generated and will flow out of the tubing. You will also need a 50 ml beaker to hold the polyethylene bottle to prevent it from tipping over. You must carry out all of the procedures in steps 2, 3, and 4 be1ow in the fume hood. 2. Obtain a small test-tube, add 3 ml of heptane, and then place it in a test-tube rack in the hood. Fit the vented cork which is attached to the polyethylene tubing into the top of the test-tube so that the end of the polyethylene tubing is positioned at the bottom of the test tube, fully immersed in the heptane. Rest this test-tube set-up back in the rack. vented stopper for test-tube with heptane chlorine generator with bleach and HCl
3. Obtain a lamp for irradiation of the sample and position it near the test tube and turn it on. Now you are ready to add the bleach and HCl to the dropper bottle to begin generating chlorine. First add 5 ml of bleach to the polyethylene bottle followed by two 1.5 ml portions of 6.0 M HCl. (Have 3 ml of HCl ready in a graduated cylinder and add about half to the dropper bottle.) Immediately after adding the HCl, replace the dropper cap to force the chlorine gas into the tubing. You should see bubbles of Cl 2 flowing out of the end of the tube which is immersed in the heptane. Swirl the test-tube as the sample is irradiated by the light from the lamp. Once the bubbles slow, add the remaining HCl and cap quickly to generate more chlorine gas. Note: If you do not see bubbling in the heptane for several minutes, you will not obtain sufficient product. If necessary, empty the dropper bottle and repeat the bleach and HCl additions until you are certain that you achieved several minutes of bubbling while irradiating with the lamp. 4. While the reaction proceeds, continue to swirl the tube in front of the light and occasionally swirl the bleach solution to continue generating chlorine. Note any color changes that you see come and go from the heptane solution as the reaction progresses. After allowing the reaction to proceed for approximately four or five minutes, you should see that the bubbling has slowed. When the bubbling stops, it is very important to immediately remove the tubing from the hexane to prevent the product from being pulled back into the bleach solution by the vacuum created as the chlorine generator cools. You should, however, continue to irradiate the test tube for another minute or so after removing the tubing. Your reaction should be complete at this point. Empty the contents of the bleach/hcl apparatus into the waste container. 5. You will need to do a couple of washings to remove residual HCl (a by-product generated in the heptane by the chlorination reaction). First, pour the heptane/chloroheptanes mixture into a large vial and then add 1 ml of 5% sodium bicarbonate solution. Cap the vial and shake to neutralize any acid. Remove the cap frequently to vent. Allow the layers to separate. (Note that you are basically using the vial as a separatory funnel for this small scale reaction). Decide which layer is the aqueous layer and remove the aqueous layer by drawing it into a disposable pipette and transferring it to a small beaker. (To do this, you will need to squeeze the air out of the bulb, insert the pipet all the way to the bottom of the bottom layer, and slowly release the pressure on the bulb to draw the bottom layer into the pipet.) Save all layers in case of error. Now wash the organic layer with 1 ml of deionized water and again remove the water layer. Pour the finished organic sample into another clean, dry vial (be certain to leave any traces of water behind when you transfer) and add enough calcium chloride pellets to dry the sample. 6. Run a gas chromatogram of the sample to analyze the products that were formed. You should observe that separation is achieved on the GC between the primary and secondary products, but within the various secondary products, no separation is observed. Therefore, because their structures and boiling points are so similar, all of the secondary products will elute together as one peak. You will also see a very large peak for unreacted heptane, which will elute with a shorter retention time than the chlorinated products. Look for a reference GC chromatogram of heptane posted in the lab near the GC.
7. Dispose of waste into the waste container and clean all vials, test-tubes and glassware. Post-Lab and Report Requirements The lab report for this experiment will consist of the following: 1. Write a detailed mechanism showing how the chlorine atom (radical) reacts with heptane to form one of the products you have just written. Your mechanism must show all chain propagating steps and chain terminating steps. You do not have to show how chlorine gas was formed by reaction of the bleach with HCl. 2. Based on the number of secondary versus primary hydrogens in a molecule of heptane, calculate the expected ratio of substitution at the secondary position compared to primary positions, assuming all hydrogens to be equally reactive. 3. Assuming that your two chloroheptane GC peaks are triangles, calculate the areas of the two peaks. (Do this my measuring the width of the peak at half-height and multiplying this by the height of the peak.) How does the ratio of the 2 :1 peak areas compare to the 2 H:1 H ratio you calculated in question 2 above? If these ratios differ, give a detailed explanation of why they differ. 4. If you could measure a yield for this experiment, you would find that you did not get a 100% yield of your products. Give chemical reasons why you didn t.