Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds

Transcription

1 Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds Introduction There are four types of hydrocarbons: alkanes, alkenes, alkynes, and aromatic compounds, each type with different chemical properties. These properties can be used to classify unknown compounds. In this laboratory you will use some simple chemical tests to determine in which of these classes of hydrocarbon an unknown sample belongs. For simplicity's sake you will not test for the presence of alkynes in this laboratory, but appropriate test for these compounds do exist. You will run the tests for alkanes, alkenes, and aromatics on an unknown compound, along with some known representative compounds, to observe the effects of different reagents on these compounds. Alkanes are a rather unreactive family of compounds. They are capable of participating in only two common types of chemical reaction: combustible and free radical substitution reactions. Combustion (i. e. burning of fossil fuels) is obviously of great economic importance. Alkanes combust with a relatively clean flame. Alkenes, alkynes, and aromatic compounds combust with significant production of carbon monoxide and elemental carbon (soot). Free radical substitution consists of the replacement of hydrogen atoms with halogen atoms. These reactions occur only when the reactants are heated or exposed to ultraviolet light. This reaction can be run with fluorine, chlorine and bromine. The reaction with chlorine is by far the most common since fluorine is much too reactive and dangerous and the reaction with bromine requires much stronger heat or light. The radical substitutions with chlorine or bromine occur very, very slowly in a room with artificial illumination. These reactions can be summarized as follows: Combustion: Alkane + O 2 Substitution: Alkane + Cl 2 HighTemperature heat or light CO 2 + H 2 O + heat Alkyl chloride + HCl Alkenes are considered the most reactive of the hydrocarbon classes. They can undergo the same reactions as alkanes in the saturated parts of their structure, but in addition, undergo a wide variety of addition and oxidation reactions. The bond electrons readily react with reagents which are electron poor, such as electrophiles and oxidants. The reactions you will study in this laboratory are as follows: Addition of bromine: Alkene + Br 2 Alkyl dibromide Note: No heat or ultraviolet light is required for this reaction. The orange/brown Br 2 is dissolved in dichloromethane. The reaction proceeds with a loss of bromine color, which is a positive test for the presence of a double bond. This reaction occurs quickly. Caution: Even in the concentrations used, bromine is highly toxic and can cause burns. Saturated sodium Thiosulfate solution is provided to neutralize Br 2 that has been spilled on skin. Permanganate Oxidation: (Von Bayer test) 3 Alkene + 2 KMnO H 2 O 3 Glycol + 2 MnO KOH Note: In the course of this reaction the purple color of the aqueous potassium permanganate is replaced by solid brown manganese (IV) oxide (MnO 2 ). The color change is a Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds, Page 1

2 positive test for the presence of a double bond. Although this is a good test to differentiate alkanes and aromatics from alkenes, other materials besides alkenes react with aqueous KMnO 4. Addition of Hydrogen Sulfate: Alkene + H 2 SO 4 Alkyl hydrogen sulfate Note: Alkenes react with cold concentrated sulfuric acid and appear to go into solution. Alkanes and most aromatics do not react. Alkenes can also thicken or change color, which also indicates a reaction. Aromatic compounds, like alkanes, are relatively unreactive. They burn but, like alkenes, they have a very sooty flame, even sootier than alkenes. Aromatics also undergo electrophilic aromatic substitution reactions. Although aromatics are highly unsaturated, they do not participate in addition reactions like the alkenes, since the loss of double bonds would lead to a loss of resonance and aromatic character. These aromatic substitution reactions generally occur in the presence of Lewis acid catalysts. The aromatic reaction you will study in this laboratory is as follows Friedel-Crafts Alkylation: + R Cl AlCl 3 R + H Cl Note: The test you will perform in the lab, though more complex than that shown above, is basically the same reaction. The final product is a large molecule with a characteristic color. The production of a color, as listed in Table 1, is evidence of an aromatic compound CAUTION: Aluminum chloride, in its dry or anhydrous state, is extremely reactive and produces hydrochloric acid or hydrogen chloride gas when exposed to water or water vapor. This reaction should be performed in the hood. Table 1: Colors Produced by Aromatic Compounds in Friedel-Crafts Test Compound Structure Color produced in Friedel-Crafts Test Benzene Orange or red Naphthalene Blue or purple Anthracene Green Phenanthrene Purple Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds, Page 2

3 Procedure For the following tests, use samples of, (or petroleum ether),,, (or turpentine) and an unknown sample. As you perform the tests, record the results on the data sheet. Ignition (Combustion) Test: For liquids, place 3-4 drops of material on a watch glass, ignite the liquid and observe whether a sooty flame results. Many samples will be difficult to ignite with a match, and you will find that a Bunsen burner will reliably ignite all samples that you are testing. For solids place a small amount of sample on a spatula and place it in the flame of the Bunsen burner. Again observe and record results. CAUTION: Keep all other flammable substances well away from the Bunsen burner Perform this test in the hood. Bromine Test (to be performed in the HOOD): Place 2 drops of the liquid (or about 15 mg of solid) in a 10 x 75 mm test tube and add to this 0.5 ml of dichloromethane, CH 2 Cl 2. Next, add 5 to 10 drops, with shaking, of a 2 % solution of Br 2 dissolved in dichloromethane. The disappearance of the orange bromine color as the reagent is added is a positive test for the presence of the double bond of an alkene. Potassium Permanganate (Von Bayer) Test: On a spot plate place 0.5 ml of acetone and 2 drops (or about 15 mg of solid). Next, add drop wise with stirring two to three drops of an aqueous 1 % KMnO 4 solution. The test is positive if the purple KMnO 4 color disappears and is replaced by a brown precipitate of manganese(iv) oxide. Do not count changes that occur after several minutes. Run a blank test with acetone to insure that it is not interfering with the test. Sulfuric Acid Solubility Test: Place one ml of cold concentrated sulfuric acid in a dry 10 x 75 mm test tube and add one drop of the liquid sample from a Pasteur pipet (or a few crystals of solid) directly into the sulfuric acid. Swirl to ensure mixing and observe. Concentration gradients (wavy lines in the mixture) and disappearance of liquid or solid indicate solubility. A color change or change in viscosity of the sulfuric acid is also evidence of reaction. CAUTION: Concentrated sulfuric acid is extremely corrosive and reacts violently with water. Friedel-Crafts Test (to be performed in the HOOD.): Place 100 mg of AlCl 3 in a test tube. Heat the tube gently with a Bunsen burner to sublime some of the AlCl 3 on to the side of the tube. Allow the tube to cool to room temperature. Mix one drop (40-50 mg of a solid) of the sample to be tested with 3-4 drops of dichloromethane. Next, carefully add this mixture to the tube, allowing it to pass over the sublimed AlCl 3. Watch for the formation of color where the reagents come into contact (see the colors listed in Table 1 for aromatic classification). Production of a color is a positive test for the presence of aromatic compounds. Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds, Page 3

4 Report Name Chemical Tests for Alkanes, Alkenes and Aromatic Compounds Part A Combustion Test Part B Sulfuric Acid Test Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds, Page 4

5 Part C Potassium Permanganate Test Part D Bromine Test Part E Friedel-Crafts Test Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds, Page 5

6 What can you determine about your unknown? Your unknown is probably not one of the samples tested, but based on the five tests performed, classify it as an alkane, alkene, or aromatic compound and give reasons for your choice. Problems: 1. Complete and balance the following: (If no reaction occurs, just indicate by N. R.) a. CH 2 =CH 2 + KMnO 4 + H 2 O b. C 6 H 6 + O 2 c. + Br 2 d. + KMnO 4 + H 2 O e. + H 2 SO 4 2. An unknown compound was known to be benzene (C 6 H 6 ) or or. It burned with a very sooty flame and when bromine in dichloromethane was added to the sample, the brown color remained. What was the unknown? Explain why you made the choice you did. Chemical Tests for Alkanes, Alkenes, and Aromatic Compounds, Page 6