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LAB 15: HYDROCARBONS: STRUCTURE & PROPERTIES PURPOSE: To discover the physical and chemical properties of alkanes, alkenes, and aromatic hydrocarbons. To identify an unknown hydrocarbon by comparing it to known samples. SAFETY CONCERNS: Always wear safety goggles. Hydrocarbons are flammable. Bromine is toxic and should be used in the hood. Observe waste disposal procedures. HYDROCARBONS: Hydrocarbons are compounds made primarily of carbon and hydrogen. An alkane is a hydrocarbon containing only single bonds. An alkene contains one or more carbon-carbon double bonds. Alkynes have one or more carbon-carbon triple bonds. Aromatic hydrocarbons contain a circular pattern of double and single bonds where the double bonded electrons can travel completely around the ring. Saturated hydrocarbons contain no double bond or triple bonds. Alkanes are saturated. Unsaturated hydrocarbons contain double or triple bonds. Alkenes, Alkynes, and Aromatic hydrocarbons are all unsaturated. Examples: Alkane Alkene Alkyne Aromatic (saturated) (unsaturated) (unsaturated) (unsaturated) Propane Propene Propyne Benzene H H H H H H C C C H H H H C C H H H C H H H H H C C C H H H C C C C C C H H H Alcohols are hydrocarbons that contain an –OH bonded to a regular carbon. Phenols are aromatics that contain the alcohol group. Alcohol Phenol H H H H C O H H Methanol (Methyl alcohol) H C C C C H CH105 Lab 15: Hydrocarbons (F15) C C O H H Phenol 47 PHYSICAL PROPERTIES: Volatility: Because of the small difference in electronegativities between carbon (C=2.5) and hydrogen (H=2.1), hydrocarbons are characteristically nonpolar. Since hydrocarbons do not have partially negative or partially positive atoms there is minimal attraction between hydrocarbon molecules. Thus, hydrocarbons require less energy to evaporate or vaporize than do polar compounds. Small hydrocarbons require less energy to vaporize than large ones. They are highly volatile. Odor: The volatile nature of hydrocarbons causes them to vaporize and move through the air. These air-borne molecules reach our noses and we smell them. Many hydrocarbons have characteristic odors. Solubility: The nonpolar character of hydrocarbons allows them to have minimal attraction to other hydrocarbons but causes them to repel polar compounds such as water. Density: Nonpolar hydrocarbons have minimal attraction to each other so hold together loosely. They have low density. Polar compounds, on the other hand, attract each other and bind close. Water has a lot of hydrogen bonding and is particularly dense. CHEMICAL PROPERTIES: Combustion: Hydrocarbons easily combust in the presence of oxygen. They are commonly used for fuel. Wood, fuel oil, gasoline, diesel, and candle wax are all common flammable hydrocarbons fuels. CH4 methane + 2O2 CO2 oxygen carbon dioxide + 2H2O water Not all hydrocarbons will react with the same amount of oxygen so when burning in a normal atmosphere they may appear very different. Aromatic hydrocarbons, for example, burn very dirty in that they undergo incomplete combustion and produce a lot of soot. Small hydrocarbons and alcohols are more likely to completely combust and burn cleanly, with very little or no soot. Bromination: The double bond of an alkene reacts with halogens to form alkyl halide compounds. The pi bond (second bond) of the double bond is weak and breaks more easily than the sigma bond (first bond). H H C C H + Br2 CH3 propene (colorless) bromine (brown) Br Br H C C H CH3 H 1,2-dibromopropane (colorless) Bromine (Br2) is a brown liquid so if a reaction occurs in which the Br2 splits apart and the Br’s bond with carbons we will see the brown color disappear. Aromatic compounds do not react with halogens in the same way. The double bonds of an aromatic compound are stronger and so do not easily break. 48 CH105 Lab 15: Hydrocarbons (F15) Oxidation: Some hydrocarbons are easily oxidized with potassium permanganate, KMnO4. H H + C C H KMnO4 CH3 potassium permanganate (purple) propene (colorless) HO OH H C C H CH3 H + MnO2 manganese (IV) oxide (brown) 1,2-propandiol (colorless) Potassium Permanganate is purple so if a reaction occurs in which the KMnO4 oxidizes the carbons we will see the purple KMnO4 change to brown MnO2. CH105 Lab 15: Hydrocarbons (F15) 49 PROCEDURES: ACTIONS: NOTES: I. PHYSICAL PROPERTIES: A. Volatility: 1. Obtain three small beakers of identical1 size. Label2 them #1, #2, #3. 2. Into beaker #1 put 5 mL of water. Into beaker #2 put 5 mL of ethanol. Into beaker #3 put 5 mL of hexane. 3. Carefully weigh each beaker on the electronic balance3 and record the masses on your report sheet. 4. Weigh each beaker again every 15 minutes4 and record the masses. 1 The beakers must have identical surface areas so that each of the liquids has the same amount exposed for evaporation. 2 Some beakers have a white patch where labels can be written with pencil. Otherwise use a grease pencil to clearly distinguish your beakers. 3 Use the same balance for each measurement to avoid errors. 4 Continue working on other parts of the lab and come back every 15 minutes to weigh the beakers in this part. 5 5. On the report sheet make a graph of the total mass lost by each substance every 15 minutes. Draw a straight line through the graph points to show the linear relationship for the evaporation of each liquid. Compare to determine relative volatility.5 A volatile liquid evaporates very easily. 6 Never smell a chemical directly but instead gently “waft” the vapors toward your nose. 7 B. Odor: 6. Waft the vapors6 of each of the 3 beakers used in Part I and record your observations on the report sheet. 7 We are using hexane to represent the odor of all hydrocarbons although not all hydrocarbons smell exactly the same. Compare the general properties of the odor of water, hydrocarbon, and alcohol. 8 C. Solubility in Water: 1. Obtain 4 stoppered test tubes, each containing 2 mLs of water.8 Measure the 2 mLs for the first tube with a graduated cylinder and then eyeball the levels of the rest to match. 9 2. Into tube #1 put 5 drops ethanol. (C2H5OH) Into tube #2 put 5 drops of hexane. (C6H14) Into tube #3 put 5 drops cyclohexene. (C6H10) Into tube #4 put 5 drops toluene. 9 (C6H5CH3) Stopper the tubes and shake each to mix. 3. Check each tube for layers. Add more drops of a particular reagent if you can’t determine the solubility with the amount given. 10 4. Record your observations about the solubility of each hydrocarbon in water. Use ‘S’ for soluble, and ‘I’ for insoluble. Save these tubes for use in Part ID. 50 CH105 Lab 15: Hydrocarbons (F15) We are using toluene, (methyl benzene) as a representative of the family of aromatics. It is less toxic than benzene because it is not as flat (planar) so therefore does not intercollate as readily into our spiral DNA. 10 Sometimes the light optics due to curvature of the test tube looks like a layer when it is not. Compare to plain water in a tube. If you are still not certain that an insoluble layer is forming then add a couple more drops of reagent and see if your layer gets bigger. D. Density: 11 5. For each insoluble hydrocarbon in Part IC observe the positioning of the hydrocarbon layer relative to water. The more dense substance will be on the bottom. The glassware must very dry. If any water is present your results may not be accurate. 6. Record the density of each hydrocarbon relative to water on the report sheet. Report M for more dense than water, and L for less dense than water. 7. Dispose of hydrocarbons in the designated waste containers. E. Solubility in Other Hydrocarbons: 1. Obtain 3 dry11 stoppered test tubes, each containing 5 drops of hexane (C6H14).8 2. Into tube #1 put 5 drops ethanol. (C2H5OH) Into tube #2 put 5 drops cyclohexene. (C6H10) Into tube #3 put 5 drops toluene. (C6H5CH3) Stopper the tubes and shake each to mix. 3. Check each tube for layers. Add more drops of a particular reagent if you can’t determine the solubility with the amount given. Record your observations about the solubility of each hydrocarbon in hexane (a typical hydrocarbon solvent). 4. Dispose of hydrocarbons in the designated waste containers. 12 You will be given a sample of an unknown hydrocarbon that you will endeavor to characterize by class. You unknown will be either an alkane, an alkene, or an aromatic. II. CHEMICAL PROPERTIES: A. Combustion (Instructor Demo): 1. Line up 5 clean and dry11 watch glasses about 6 inches apart in a fume hood. 2. Onto watch glass #1 put 10 drops ethanol. (C2H5OH) Onto watch glass #2 put 10 drops of hexane. (C6H14) Onto watch glass #3 put 10 drops cyclohexene. (C6H10) Onto watch glass #4 put 10 drops toluene. (C6H5CH3) Onto watch glass #5 put 10 drops unknown hydrocarbon.12 13 Observe if the hydrocarbons ignited rapidly or if they were slow to ignite. Observe if there is a “clean” flame or a “sooty” flame. 3. Quickly go down the row and ignite each liquid with a burning wood splint and compare the colors and types of flames produced by each hydrocarbon as they burn together. 13 4. Record your observations. CH105 Lab 15: Hydrocarbons (F15) 51 B. Bromination: 1. Obtain a dry11 white spot plate. 2. Into well #1 put 3 drops ethanol. (C2H5OH) Into well #2 put 3 drops hexane. (C6H14) Into well #3 put 3 drops cyclohexene. (C6H10) Into well #4 put 3 drops toluene. (C6H5CH3) Into well #5 put 3 drops unknown hydrocarbon. 3. Move your plate to a fume hood14 and into each well drop 1 drop of Bromine (Br2) solution15. Stir to mix if needed and record any results. 4. Dispose in the designated waste containers. Bromine must go into “bromine waste”. C. Oxidation: 1. Obtain a clean white spot plate. 2. Into well #1 put 3 drops ethanol. (C2H5OH) Into well #2 put 3 drops hexane. (C6H14) Into well #3 put 3 drops cyclohexene. (C6H10) Into well #4 put 3 drops toluene. (C6H5CH3) Into well #5 put 3 drops unknown hydrocarbon. 3. Into each well drop 1 drop of Potassium Permanganate (KMnO4) solution16. Stir to mix if needed. Record any results. 4. Dispose in the designated waste containers. D. Unknown Hydrocarbon: 5. You have observed the behavior of an unknown compound in combustion, bromination, and oxidation. On the report sheet characterize your unknown as an alkane, alkene, or an aromatic and then explain why you believe it to be so. 52 CH105 Lab 15: Hydrocarbons (F15) 14 Bromine is highly toxic. Keep the bottle stoppered and do this reaction only in the fume hood. You may choose to wear protective gloves. If you spill bromine inform your instructor immediately for help with proper clean up. 15 Bromine is soluble both in water and in hydrocarbon solvents. If we use a solution of Bromine dissolved in water here we would have two layers since hydrocarbons are insoluble in water. By using bromine dissolved in a hydrocarbon we can mix the bromine with our hydrocarbons more easily. 16 KMnO4 is soluble in water but is not soluble in hydrocarbon solvents. We do not have the option of using a hydrocarbon solvent as we did with Br2 that would allow easy mixing with our sample. Because KMnO4 is dissolved in water here we have two layers that must be stirred or shaken to mix. LAB 15: HYDROCARBONS NAME_____________ DATE______________ PRE LAB EXERCISES: 1. Draw the structural formula for each of these hydrocarbons: Ethanol Hexane Cyclohexene Toluene 2. Match the following terms with the structures they represent: ____ Alcohol A. C. H H C C H ____ Alkene E. H H CH2 CH3 CH3 H H H H H H H C C C C H H H H H H ____ Alkane B. D. H H H OH H C C C C H H H H H ____ Alkyne H H C C C H H ____ Aromatic 3.___ Black soot is given off when certain hydrocarbons are burned because __ A. B. C. D. the fuel is undergoing complete combustion in the presence of oxygen. carbon dioxide and water mix to give a black powder. there is not enough oxygen present in the environment to combust that much fuel. more than one of these. 4.___ Several drops of a bromine solution were added to a liquid. The red-brown color of the bromine immediately disappeared. The compound may have been A. a saturated hydrocarbon B. a cycloalkane D. an alkane E. an alkene 5. C. an aromatic hydrocarbon Complete and balance the following equations: A. H H C C H C H H H + Br2 B. C5H12 + O2 CH105 Lab 15: Hydrocarbons (F15) 53 54 CH105 Lab 15: Hydrocarbons (F15) LAB 15: HYDROCARBONS NAME___________________ PARTNER_________DATE___ REPORT: I. PHYSICAL PROPERTIES: A. Volatility: a. Mass at Start b. Mass lost in zero min c. Mass at 15 minutes d. Total g’s lost from beginning e. Mass at 30 minutes f. Total g’s lost from beginning g. Mass at 45 minutes h. Total g’s lost from beginning i. Mass at 60 minutes j. Total g’s lost from beginning #1 + Water #2 + Ethanol #3 + Hexane Mass in grams Mass in grams Mass in grams Og Og Og (a-c) (a-c) (a-c) (a-e) (a-e) (a-e) (a-g) (a-g) (a-g) (a-i) (a-i) (a-i) Mass lost by Water (#1), Ethanol (#2), and Hexane (3#) Total Mass Lost From the Beginning (in grams) 0 15 (a-c) 30 (a-e) 45 (a-g) 60 (a-i) Time (in minutes) Volatility Results Summary: Rank the substances in order of decreasing volatility. Most volatile > > A. On the given structures of ethanol label any partially positive (+) or partially negative (-) atoms. B. Draw a dotted line (…..) to show Hydrogen bonding H H H C C O H H H Least volatile H H H O C C H H H forming between the 2 ethanol molecules given. Explanation/Analysis: Water Why did each of the three liquids behave as they did?.Be Specific . Ethanol CH105 Lab 15: Hydrocarbons (F15) Hexane 55 Water Ethanol Hexane An Alcohol A Hydrocarbon B. Odor Ethanol Hexane Cyclohexene Toluene An Alcohol An Alkane An Alkene An Aromatic C. Solubility in Water D. Density Compared to Water E. Solubility in Hydrocarbons Physical Properties Results Summary: _____1. As a general rule, most hydrocarbons are _____ than water. A. more volatile B. less volatile C. there is no general rule _____2. As a general rule, hydrocarbons are _____ in water and _____in other hydrocarbons. A. soluble, soluble C. insoluble, soluble B. insoluble, insoluble D. soluble, insoluble _____3. As a general rule, most hydrocarbons are _____ than water. A. more dense B. less dense C. there is no general rule II. CHEMICAL REACTIVITY: Ethanol Hexane Cyclohexene Toluene An Alcohol An Alkane An Alkene An Aromatic A. Combustion B. Bromination (rxn w/ Br2) C. Oxidation (rxn w/ KMnO4) D. Unknown # _____ Belongs to what family of compounds? (circle one) Alkane Alkene Explanation: 56 CH105 Lab 15: Hydrocarbons (F15) Aromatic Unknown # _________ Chemical Reactivity Results Summary: _____4.The identity of the black soot that is given off when certain hydrocarbons are burned is ___ A. impurities that were present in the original sample. B. a mixture of carbon monoxide and carbon dioxide.. C. elemental carbon. D. none of these. _____5.Which of the following generally gives the most soot when burned? A. Alcohols B. Alkanes C. Alkenes D. Aromatics _____6.List all of the following that generally undergo an immediate reaction with Br2? A. Alcohols B. Alkanes C. Alkenes D. Aromatics _____7.List all of the following that generally undergo an immediate reaction with KMnO4? A. Alcohols B. Alkanes C. Alkenes D. Aromatics Write balanced chemical equations to represent the following reactions: 9. Complete Combustion of Hexane: (Use structures not formulas for hexane) 10. Reaction of Cyclohexene with Bromine: (Use structures not formulas for cyclohexene & product) Related Exercises: 5.___ Two structures are identical when A. they have the same molecular formula B. they have the same combustion products C. they can be superimposed on each other. 6.___ Do the following structural formulas represent the same or different molecules? H Br H H H H A. same H C C H H C C Br H H H H B. different H C C H H Br C. not enough information CH105 Lab 15: Hydrocarbons (F15) 57 58 CH105 Lab 15: Hydrocarbons (F15) LAB 15: HYDROCARBONS: Worksheet Name ____________ 1. Draw structural formulas and give names for the 2 isomers of C3H7Br. Name Name 2. Draw structural formulas and give names for the 4 isomers of C3H6Br2. Name Name Name Name 3. Draw structural formulas and give names for the 2 isomers of C4H10. Name Name 4. Draw structural formulas and give names for the 3 isomers of C5H12. Name Name Name 5. Draw structural formulas and give names for the 3 isomers of C2H2Br2 Name Name CH105 Lab 15: Hydrocarbons (F15) Name 59 6. Draw structural formulas and give names for the 5 cyclopentane isomers of C5H8Br2. Name Name Name Name Name 7. Draw structural formulas and give names for the 5 isomers of C6H14. Name Name Name Name Name 14.___ For geometric (cis-trans) isomerism to occur, must a double bond be present in the structure? A. Yes B. No C. Not enough information 15.___ Do the carbon atoms in an ethene have freedom of rotation? A. Yes B. No C. Depends on the temperature. 16.___ Do the carbon atoms in a cyclopentane have freedom of rotation? A. Yes B. No C. Depends on the temperature. 60 CH105 Lab 15: Hydrocarbons (F15)