Qualitative Tests for Saturated and Unsaturated Hydrocarbons
Qualitative tests for Saturated and Unsaturated Hydrocarbons
Goals: To distinguish between saturated and unsaturated hydrocarbons using chemical tests: combustion test, bromine test and KMnO4 test.
Introduction: Compounds that contain only carbon and hydrogen are called hydrocarbons. Hydrocarbons are the simplest examples of organic, or carbon-based, compounds. The study of hydrocarbons and related molecules constitutes the branch of chemistry known as organic chemistry. Hydrocarbons can be obtained from natural sources or synthesized in a laboratory. Naturally obtained hydrocarbons include petroleum, natural gas, and coal. The atoms in these molecules are bound to each other by covalent bonds that determine many of their properties. In particular, hydrocarbons can be classified as saturated or unsaturated, depending on the kinds of bonds in them. Saturated Hydrocarbons are organic compounds that have only single bonds between two carbon atoms. These compounds are also known as alkanes. Some simple examples of open chain (acyclic) and closed chain (cyclic) saturated carbons are shown below.
Unsaturated hydrocarbons contain at least one double or a triple bond between two carbon atoms. These include alkenes ( ) and alkynes ( ). Aromatic hydrocarbons are another type of unsaturated hydrocarbon which contain a benzene ring. Some simple examples of unsaturated hydrocarbons are shown here.
Functional group: A functional group is defined as an atom or a group of atoms that are part of a larger molecule and partly determine its chemical and physical properties. The chemical properties of a functional group are relatively consistent from compound to compound, despite any other differences in the chemical structures. For example, the carbon-carbon double bond (C=C) in an alkene and the carbon-carbon triple bond (C≡C) in an alkyne are both considered functional groups, and both are active sites for chemical reactions involving these compounds.
Reactions for Hydrocarbons 1. Combustion Reaction: In a combustion reaction a hydrocarbon burns in the presence of excess oxygen to produce carbon dioxide (CO2) and water (H2O).
2. Bromine (Br2) test: A 1% solution of bromine (Br2) in carbon tetrachloride has a bright orange- red color. When drops of this Br2 solution are mixed into an alkene or alkyne (both clear, colorless liquids), its orange-red color disappears quickly as it reacts with the double or triple bonds present in the hydrocarbon. The electrons in the Br–Br bond and the extra electrons in the double or triple bond of the hydrocarbon rearrange to form new C–Br bonds. Since the bromine atoms are incorporated into the molecule, this is an example of an addition reaction.
On the other hand, saturated hydrocarbons (alkanes) do not have electron-rich double or triple bonds in them to react with the Br2 in this way. The bromine molecule can only react with an alkane through a much less favorable substitution reaction, in which a Br atom replaces an H atom bonded to a carbon. This substitution reaction occurs only in the presence of UV-light or heat, and even then proceeds very slowly. Thus, when drops of orange-red Br2 solution are added to a saturated alkane under typical room conditions, their color persists without fading.
While aromatic hydrocarbons such as benzene are unsaturated, the arrangement of electrons in the alternating double bonds is more stable than in an isolated double or triple bond. This means that aromatic double bonds do not undergo the decolorizing addition reaction with Br2, and the orange- red color of Br2 persists.
3. Baeyer’s reagent or Potassium Permanganate (KMnO4) test: When unsaturated hydrocarbons react with an aqueous solution of KMnO4, the purple color of KMnO4 disappears quickly and a dark brown precipitate forms. along with addition product diol of hydrocarbon.
Identification of Unknown compound: Use these tests for saturated and unsaturated hydrocarbons to determine the nature of an unknown compound provided to you by your instructor. Your unknown is one of the compounds used in this experiment.
Experimental Procedure A. Combustion Reaction Materials Required: Four evaporating dishes, cyclohexane, cyclohexene, toluene, and an unknown compound. Caution: The chemicals used here are highly flammable. Work in a fume hood. 1. Place 10-15 drops of cyclohexane in a clean evaporating dish. 2. Ignite the sample carefully using lighted splint or matchstick. Record your observations for this reaction, including the appearance of the flame and any smoke or residue that occurs. 3. Repeat steps 1-2 for cyclohexene, toluene and an unknown compound given to you by your instructor. Make sure to use a different evaporating dish for each sample. Note: Record the identity unknown compound (A or B) you are testing in your data sheet. 4. Compare your recorded observations for the unknown sample with your observations of the known sample tests to help identify your unknown.
B. Bromine Test Materials Required: Four test tubes, test tube rack, 1% bromine solution (in carbon tetrachloride), cyclohexane, cyclohexene, toluene, unknown compound. Chemical Safety: Handle the bromine solution very carefully. Contact with liquid bromine can cause severe burns. If bromine is spilled on skin, wash the area with lots of water. Do not inhale or smell the bromine vapors; it has strong pungent odor.
Caution: Work in a fume hood.
1. Take 4 test tubes and place them in the test tube rack. 2. Add about 2 mL of cyclohexane, cyclohexene, toluene and your unknown compound to the four test tubes separately, and label them with the name of the appropriate compound. 3. Add about 10-15 drops of bromine solution to each of these tubes. Shake gently to mix the solutions. 4. Observe whether the orange-red color of bromine disappears or not. If the orange-red color disappears, a reaction has occurred, confirming the presence of an unsaturated double or triple bond in the compound.
C. Baeyer’s reagent or Potassium Permanganate (KMnO4) Test Materials Required: Four test tubes, test tube rack, potassium permanganate solution, cyclohexane, cyclohexene, toluene, unknown compound. 1. Take 4 test tubes and place them in the test tube rack. 2. Add about 2 mL of cyclohexane, cyclohexene, toluene and your unknown compound to the four test tubes separately, and label them with the name of the appropriate compound.
3. Add about 10-15 drops of 1% KMnO4 solution to each of these tubes. Shake gently to mix the solutions. 4. Observe whether the purple color changes to a brown colored solid within 1 minute. The appearance of the brown colored solid in the test tube indicates the reaction has occurred, and confirms the presence of an unsaturated bond in the compound.
D. Identification of Unknown From your test results from part A, B and C, determine if the unknown is a saturated (alkane) or an unsaturated (alkene) organic compound.
Name: ______Course/section: ______
Instructor Name: ______Date: ______
Pre-Lab Questions 1. How many bonds does a carbon atom form in a hydrocarbon?
2. What is a functional group?
3. What functional groups are present in alkenes, alkynes and toluene?
4. What is a combustion reaction? Write a balanced chemical equation for combustion of benzene (C6H6).
5. What would you expect to observe when Br2 reacts with 2-butyne?
6. What would you expect to observe when an aqueous solution of KMnO4 reacts with 2-butene?
7. Draw the structures for the following hydrocarbons.
Hydrocarbon Structure Hydrocarbon Structure
Cyclohexane Benzene (C6H12) (C6H6)
Cyclohexene Toluene (C6H10) (C7H8)
Name: ______Course/section: ______
Instructor Name: ______Date: ______
Data Sheet
A. Combustion Reaction Hydrocarbon Observation when ignited in Balanced Chemical Equation for the presence of oxygen Combustion Cyclohexane (C6H12)
Cyclohexene (C6H10)
Toluene (C7H8)
Unknown ______
B. Bromine Test Structural formula of Hydrocarbon Observations product (if reaction occurs)
Cyclohexane (C6H12)
Cyclohexene (C6H10)
Toluene (C7H8)
Unknown ______
C. Baeyer’s reagent or Potassium Permanganate Test (KMnO4) Structural formula of Hydrocarbon Observations product (if reaction occurs)
Cyclohexane (C6H12)
Cyclohexene (C6H10)
Toluene (C7H8)
Unknown ______
D. Identification of Unknown compound
Observations Unknown Saturated or compound Combustion test Bromine test KMnO4 test Unsaturated?
Post Lab questions 1. Complete the following reactions and name the products.
2. Toluene contains a benzene ring with three double bonds in it, but still does not show any signs of undergoing a reaction with bromine solution. Why?
3. Why doesn’t cyclohexane decolorize the orange-red color of the bromine or the purple color of KMnO4 solutions?