17B: Distilling Aromatic Hydrocarbons

Transcription

1 Chapter 17 17B: Distilling Aromatic Hydrocarbons Key Question: How are aromatic compounds isolated? What do they have in common? Aromatic hydrocarbon molecules were originally discovered in spices and natural flavorings. In this investigation, students use the LabMaster with the condenser to distill small amounts of aromatic hydrocarbons contained in different spices. These different aromatic compounds are responsible for the odor or aroma of the different spices. The aromatic hydrocarbons are not soluble in water, and their boiling points are above 200 o C. This allows steam distillation to successfully separate a small percentage of these molecules. Reading Summary Questions and goals Students read section 17.1 (Carbon molecules) before performing this Investigation. Hydrocarbons contain only carbon and hydrogen atoms. An alkane is a hydrocarbon that contains only single bonds between carbon atoms. Alkanes are also called saturated hydrocarbons. Saturated refers to molecules containing only single bonds between carbon atoms. Hydrocarbons are nonpolar molecules, which means they do not contain polar bonds that are unbalanced. Hydrocarbons are structured as symmetric long chains. Some hydrocarbons, called aromatic hydrocarbons, contain the cyclic ring structure of benzene (C 6 H 6 ). Benzene is a six carbon ring structure with alternating double bonds between the carbons. The term aromatic refers to the fact that these compounds have distinct aromas, or smells. Spices and flavorings contain aromatic molecules. Hydrocarbons are held together by London dispersion forces. These forces can be relatively weak for small chain hydrocarbons, but become quite strong on longer ones. London dispersion forces increase with increasing molar mass of the hydrocarbon compound. Main Questions Learning Goals Key Vocabulary What molecules in spices are responsible for their aromas? How can aromatic hydrocarbon molecules be separated from spices? How does the process of distillation work? By the end of the Investigation, students will be able to: Explain how the distillation process works to separate molecules in substances. Understand that certain organic molecules contained in spices are responsible for their aromas. Describe some of the similarities between aromatic molecules contained in spices. hydrocarbon, alkane, unsaturated hydrocarbon, alkene, aromatic hydrocarbon, benzene, London dispersion force, nonpolar, polar, volatile 548 Chapter 17: Organic Chemistry

2 Materials and Setup Details 17 B Students work in groups of three to five at lab benches. Each group should have: LabMaster, heater and condenser Ice cubes Salt (NaCl) A few 25 mm test tubes Stirring rod Mortar and pestle Pipettes Boiling chips Variety of spices (powdered or fresh): vanilla beans, cloves, cinnamon, nutmeg, anise, allspice, caraway seed, cumin, orange peel, lemon peel, rose petals Preparation You will need to purchase the necessary spices, and an orange and a lemon if you wish to use the rind. Hints: If possible, use fresh spices for a better aroma. After crushing the spice, use some of the larger pieces, as they tend to stay at the bottom of the test tube more easily. Suggestions: Crush some regular ice cubes with a hammer to obtain smaller pieces of ice to use in the condenser. Solid ice lasts longer than actual crushed ice. Have a couple of small beakers of water heating on a hot plate. Students can easily add hot water from the beaker to their test tubes using an insulated glove. Teaching Time Preparation Assignments Misconceptions Outline of the Investigation One class period Purchase the spices that you would like to use. Fresh spices give a stronger aroma, but powdered spices also produce satisfactory results. Section 17.1 (Carbon molecules) in the Student Text before the Investigation. Students tend to think that foods contain only one type of molecule. Students are unaware that there are organic aromatic molecules in spices and flavorings that contribute specifically to their smell. 1 Setting up the experiment 2 Distilling your spice 3 Thinking about what you have learned Distilling Aromatic Hydrocarbons 549

3 . Investigation sections Part Ideas and Dialog Setting up the experiment Students prepare their spice and set up the condenser unit. Distilling your spice Here the aromatic compounds in each spice are distilled with the steam and small amounts are collected with the condensate. 1 Read through the steps in Part 1, and begin by preparing your spice. If fresh spices are available, they will yield a more fragrant distillate. Have the students crush it well with the mortar and pestle. Remind your students to stir the spice into the water thoroughly to be sure it becomes moistened. This is very important in the distillation process. Place 30 ml of hot water into the test tube. Be sure to add a boiling chip, so that the water does not boil over. After adding the spice, students need to fill the test tube with hot water, leaving about one inch of space at the top of the test tube. If the test tube is filled to the top, it will cause the solution to boil over into the condenser unit. Adding a boiling chip is important and helps to prevent vigorous boiling that could spill into the condenser unit. If hot water were to enter the condenser, it would stop condensing! Fill the top of the condenser with ice, pour salt over the top of the ice, and place the top of the condenser into the support clamp, holding the base of the condenser unit. Have students crush the ice cubes a little if they are large, so that the ice has good contact with the plastic. Crushed ice can be used, but it melts very rapidly and students need to be diligent about adding fresh ice often if it is used. 2 Heat the mixture, and record your observations it as it comes to a boil. Students should be able to see some vapor rising out of the test tube. After the mixture has boiled for a few minutes, they will be able to see condensate collecting in the condenser. The condensed steam will contain small amounts of immiscible aromatic organic molecules that are responsible for the smell and taste of the spice. The type of aromatic molecule depends upon the spice being distilled. The distillate will also contain small amounts of other volatile organic compounds that are carried out by the steam. Students will be able to identify the distinctive odor of their spice in the distillate. Even though the distillate is a clear, colorless liquid, the aroma of the spice will be easily identified. Continue to distill the mixture for 30 minutes. Watch your experiment carefully. Students will need to remove water from the top of the condenser and add ice regularly, so it is important that they watch their experiment carefully. It is a good idea for students to take turns, because the experiment must run for at least 30 minutes. Remind students to be careful when adding hot water to the test tube to maintain the water level. It may take a short time for the steam to build up once again once the water is added. 550 Chapter 17: Organic Chemistry

4 Investigation page Sample answers Teaching tips Set up a LabMaster as a demonstration unit complete with the condenser unit, test tube, condensate collector, and ice. Using your apparatus, explain the overall setup to your students. It helps the students to have this to refer to as they prepare their own setup. Ask students if they or their family grows spices. Sometimes students will have access to some fresh herbs, and may enjoy distilling and sharing something of their own. Rose petals and citrus peel can also be distilled. If using the citrus peel, have students shave off just the surface of the orange/lemon rind. Grating some of the peel gives good results, and is better than a mortar and pestle in this case. It does take quite a while to obtain a very small amount of condensate. The distillation needs to run for at least 30 minutes, which does not include setup. You may find that you need an extended period to complete this investigation, in order not to be rushed. If you only have 50 minutes or so, prepare your students setups the day before. If other classes are not using the LabMasters, students can set everything up ahead of time with the exception of ice, hot water and spices. It is important that the water-spice mixture does not boil over into the condenser unit. Caution students to take turns carefully watching their experiment. It is tempting for them to become distracted in conversation while waiting for the distillate to form. 17 B: Distilling Aromatic Hydrocarbons 551

5 Investigation sections Part Ideas and Dialog Thinking about what you have learned Here students analyze the distillation process and study some of the aromatic molecules. 3 Why do you think we set the LabMaster to 125 o C and not 150 o C? Why did we add a boiling chip? Students may need help with this question. They may guess that a higher temperature may cause over heating. Depending upon whether or not they have used a boiling chip before, they may know its purpose is to prevent superheating. Boiling chips provide cavities for solvent vapor bubbles to form. Using a boiling chip allows tiny vapor bubbles to form, and helps to keep the solution evenly heated. Without a boiling chip the water can become superheated and release large bubbles, causing spattering. Can you speculate what too high a temperature could do to the aromatic compound in your spice? Too high a temperature may cause the aromatic molecule to break down. While most of the aromatic compounds in the listed spices have boiling points within the range of 230 o C to 290 o C, they are all very volatile at low temperatures. Heating them in water at a lower temperature allows the organic molecules to be swept out with the water vapor. Why do we add salt to the ice to melt it? Why not just use crushed ice? Students may remember colligative properties, and say that the salt ions lower the freezing temperature of the ice/water mixture. Using crushed ice does not change the actual freezing temperature of 0 o C for ice. Adding salt allows the ice/water mixture to stay liquid at a lower temperature, and therefore make far greater contact with the surface of the condenser. This lets more vapor get condensed, because the average temperature of the condenser s surface is much colder. What do you notice about the aromatic molecules? Are there similar features of these molecules? Based on the molecular structures shown in Part 3, students will likely say that they all contain a cyclic carbon structure. All of them contain the benzene ring, except for carvone in caraway seeds. Most of the molecular structures contain several double bonds outside the benzene ring. Students could say that aromatic hydrocarbons contain at least one cyclic ring structure. More information can be found in the text on page Chapter 17: Organic Chemistry

6 Investigation page Sample answers Example Answers 3a. We set the LabMaster to 125 o C so that the water did not overheat and boil into the condenser. Adding the boiling chip keeps the water from forming large bubbles, which would cause splattering of the water-spice mixture. 3b. Too high of a temperature could cause the molecules of the aromatic hydrocarbons to decompose and break apart. 3c. We add salt to the ice to lower the freezing point below the normal 0 o C freezing temperature. This allows the condenser to become colder and helps to facilitate the efficiency of the condensation process. Crushed ice does not cause the freezing temperature to be depressed below 0 o C. 3d. Each of the molecules shown in Part 3, contain an aromatic hydrocarbon in their molecular structure. All of the spice molecules contain a benzene ring, except for carvone. Overall, the aromatic molecules contain several double bonds in their structures. 17 B: Distilling Aromatic Hydrocarbons 553