EXPERIMENT 8 Reactions of Hydrocarbons

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1 EXPERIMENT 8 Reactions of Hydrocarbons Properties and Identification of Hydrocarbons Purpose: a) To identify saturated and unsaturated hydrocarbons using properties and reactions. b) Study substitution and addition reactions. Equipments test tube rack (1) test tube holder (2) 100 ml beaker medium test tube (6) Materials cyclohexene toluene n-hexane conc. H 2 SO 4 Br 2 /CCl 4 or Br 2 /CH 2 Cl % KMnO 4 10% Na 2 CO 3 unknowns hydrocarbons Discussion Hydrocarbons, compounds which contain only carbon and hydrogen, can be classified into several types, depending on their structure. Aliphatic hydrocarbons are divided into three classes: alkanes have only single bonds, and are said to be saturated; alkenes and alkynes have carbon-carbon double or triple bonds, and are said to be unsaturated. Aromatic hydrocarbons are cyclic compounds whose structure is related to that of benzene, with six -electrons in a six-member ring. Aliphatic Hydrocarbons such as Alkanes are relatively inert to chemical oxidizing agents such as neutral or alkaline permanganate, where alkenes are readily oxidized at room temperature. The change in color can be used as a test for a double bond, provided the molecule contains no easily oxidizable group. 1

2 Alkanes react slowly or not at all with bromine at room temperature in the dark, but in the presence of sunlight, substitution is fairly rapid: R-H + Br 2 /light R-Br + HBr (a substitution reaction) X X 2 /light + ( HBr ) The reaction is easily detected by loss of the bromine color and by evolution of hydrogen bromide. On the other hand, bromine adds rapidly at room temperature to alkenes in a reaction, which does not require light. Since the product is colorless, the bromine is rapidly decolorized when added to an alkene. X C = C X 2 / CH 2 Cl 2 C- C. Aromatic Hydrocarbons such as toluene will be used in each of the following experiments. Although formally unsaturated, C 6 H 6 CH 3 in the sense that it has multiple carbon-carbon double bonds, toluene does not give the usual reactions expected of an alkene. It is not easily oxidized, and preferably undergoes substitution rather than addition reactions. Permanganate converts cyclohexene into a diol. Since a syn-hydroxylation takes place, the reaction is thought to involve the formation of an intermediate cyclic manganate ester, which is readily hydrolyzed under the reaction conditions to yield the glycol. In the course of the reaction purple permanganate is reduced to brown manganese dioxide (MnO 2 ). Also this reaction is used as a qualitative test for the presence of an alkene (Baeyer's Test ). This decolorization of bromine water is often used as a test for a carbon-carbon double bond. The addition of bromine to cyclohexene gives a racemic trans product. Since the product is colorless, the bromine is rapidly decolorized when added to an alkene. X Br Br Br 2 CH 2 Cl 2 KMnO 4 H 2 O HO HO trans-dibromide cis- 1,2 -diol The following experiments illustrate some of the fundamental reactions of saturated, unsaturated, and aromatic hydrocarbons. The three classes sometimes react differently toward the same reagent, in which case it may be used to distinguish between them. 2

3 Experimental Procedure All tests should be carried out in dry test tubes, and observations should be recorded on the report sheet as each experiment is performed. Perform the tests (A), (B), (C), and (D) on the following materials 1. The saturated hydrocarbon (alkanes and cycloalkanes) cycloalkane, pentane, and heptane 2. The unsaturated hydrocarbon (alkenes and alkynes) cycloalkene and 2-pentene 3. The aromatic hydrocarbon - toluene and benzene (A) Bromine Test - In a small test tube, add 1 ml. of hydrocarbon to 3-4 ml. of 2% bromine in carbon Tetrachloride(Br 2 /CCl 4 ). Shake well and observe after two or three minutes. If the bromine is not decolorized prepare a second similar tube and place on tube in your laboratory locker and the other in bright sunlight. Allow both tubes to stand for ten to fifteen minutes and compare them. Observe the color of each tube, and whether or not hydrogen bromide was evolved and record the results. (B) Aqueous Potassium Permanganate (Baeyer's Test) In a small test tube, add 1 ml (10 drops). of hydrocarbon to a mixture of 3 ml. of dilute potassium permanganate solution (0.5 % KMnO 4 solution) and 3 ml. of dilute sodium carbonate solution (10% Na 2 CO 3 solution) and shake the tube for 1-2 minutes, and note the results. (Note -Dilute sulfuric acid may be substituted for sodium carbonate. This substitution gives a better test reagent for certain purpose since manganese dioxide is not precipitated in the acid solution) (C) Sulfuric Acid Test In a small test tube add 1 ml. of hydrocarbon, cautiously and with gentle shaking, to about 3 ml. of concentrated sulfuric acid. Shake the tubes well and note the results. Observe whether heat evolved and whether the hydrocarbon dissolves. Discard the contents by pouring them into a beaker containing at least 50 ml of water. Although alkanes are inert to cold, concentrated sulfuric acid, alkenes react by addition. The product, alkyl hydrogen sulfate, is soluble in concentrated sulfuric acid. (D) Solubility Add about 2 ml. of water in a small test tube and add 2 or 3 drops of the hydrocarbon to be tested. Shake the mixture to determine whether the hydrocarbon is soluble (a colorless second layer may be hard to see). Record your results and save the mixture for Test (F). (F) Relative Density Reexamine the mixtures prepared in part (E) and decide in each case whether the hydrocarbon is more dense (sinks) or less dense than water (floats). DISPOSAL: Please dispose all waste from these test tubes experiments in the appropriate waste container as directed by instructor. 3

4 EXPERIMENT 8 Properties of Hydrocarbons Data and Results (Prperties of Hydrocarbons) REPORT FORM Name Instructor Date Tests (A) and (B) are commonly used for unsaturation. Test (C) is also used to a limited extent for the same purpose. Record the results in a given tabular form such as that shown below. OBSERVATIONS ON HYDROCARBONS Reagent Used Alkane Toluene Alkene Unknown # Unknown # Br 2 /CH 2 Cl 2 (light) 0.5% KMnO 4 Conc. H 2 SO 4 H 2 O (solubility) (S,INS,SS) Relative Densities (L=low, H=high) 4

5 Pre-Laboratory Questions EXP 8 Due before lab begins. Answer in space provided. Name: 1. Draw skeletal (line bond) structures for the following compounds. cyclohexane cyclohexene toluene 2. What are the general formulas for alkanes and cycloalkanes? Alkanes cycloalkanes 3. Write equations for the reaction of 1-butene with the following reagents. a) Br 2 /CH 2 Cl 2 c) KMnO 4 / hot 4. How could you distinguish octane from 1-octene by a simple chemical test? 5. What would you expect the difference between reactivity of the following pairs? Please e explain your answers. a) hexane and Cyclohexane b) hexane and cyclohexene 5

6 Post-Laboratory Questions EXP 8 Due after completing the lab. Name: 1. Draw condensed structure formulas for the following compounds. cyclohexane cyclohexene toluene 2. What colors are bromine and potassium permanganate solutions? 3. What are the possible distinct alkenes structural formulas for a molecular formula of C 4 H 8? Please name each structural formulas. 4. Write equations for the reaction of cyclohexene with the following reagents. a) Br 2 /CH 2 Cl 2 b) KMnO 4 /cold 5. How could you distinguish 1-octanol from 1-octene by a simple chemical test? 6