HYDROCARBONS: STRUCTURE & PROPERTIES

Transcription

1 YDROARBONS: STRUTURE & 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 ONERNS: Always wear safety goggles. ydrocarbons are flammable. Bromine is toxic and should be used in the hood. Observe waste disposal procedures. YDROARBONS: ydrocarbons 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 (saturated) Propane Alkene (unsaturated) Propene Alkyne (unsaturated) Propyne Aromatic (unsaturated) Benzene 1

2 Alcohols are hydrocarbons that contain an O bonded to a regular carbon. Phenols are aromatics that contain the alcohol group. Alcohol O Methanol (Methyl alcohol) Phenol O Phenol PYSIAL PROPERTIES: Volatility: Because of the small difference in electronegativities between carbon (=2.5) and hydrogen (=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. EMIAL PROPERTIES: ombustion: ydrocarbons 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. 4 2O 2 O O + + methane oxygen carbon dioxide 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. 2

3 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). Br Br + Br propene (colorless) bromine (brown) 1,2-dibromopropane (colorless) Bromine (Br 2 ) is a brown liquid so if a reaction occurs in which the Br 2 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. Oxidation: Some hydrocarbons are easily oxidized with potassium permanganate, KMnO 4. + KMnO 4 3 potassium permanganate (purple) propene (colorless) O O 3 1,2-propandiol (colorless) + MnO 2 manganese (IV) oxide (brown) Potassium Permanganate is purple so if a reaction occurs in which the KMnO 4 oxidizes the carbons we will see the purple KMnO 4 change to brown MnO 2. 3

4 PROEDURES: ATIONS: I. PYSIAL PROPERTIES: A. Volatility: 1. Obtain three small beakers of identical 1 size. Label 2 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. arefully weigh each beaker on the electronic balance 3 and record the masses on your report sheet. 4. Weigh each beaker again every 15 minutes 4 and record the masses. 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. ompare to determine relative volatility. 5 B. Solubility in Water: 1. Obtain 4 stoppered test tubes, each containing 2 mls of water Into tube #1 put 1 ml ethanol. ( 2 5 O) Into tube #2 put 1 ml of hexane. ( 6 14 ) Into tube #3 put 1 ml cyclohexene. ( 6 10 ) Into tube #4 put 1 ml toluene. 7 ( ) Stopper the tubes and shake each to mix. 3. heck each tube for layers. 8 Record your observations about the solubility of each hydrocarbon in water. Use S for soluble, PS for partially soluble, and I for insoluble. Save these tubes for use in Part I. NOTES: 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 ontinue working on other parts of the lab and come back every 15 minutes to weigh the beakers in this part. 5 A volatile liquid evaporates very easily. 6 Measure the 2 mls for the first tube with a graduated cylinder and then eyeball the levels of the rest to match. 7 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. 8 Sometimes the light optics due to curvature of the test tube looks like a layer when it is not. ompare 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. 4

5 . Density: 4. For each insoluble hydrocarbon in Part IB observe the positioning of the hydrocarbon layer relative to water. The more dense substance will be on the bottom. 9 The glassware must very dry. If any water is present your results may not be accurate. 5. Record the density of each hydrocarbon relative to water on the report sheet. Report M for more dense than water, L for less dense than water, and X if there is not enough evidence to determine. 6. Dispose of hydrocarbons in the designated waste containers. D. Solubility in Other ydrocarbons: 1. Obtain 3 dry 9 stoppered test tubes, each containing 1 ml of hexane ( 6 14 ) Into tube #1 put 1 ml ethanol. ( 2 5 O) Into tube #2 put 1 ml cyclohexene. ( 6 10 ) Into tube #3 put 1 ml toluene. ( ) Stopper the tubes and shake each to mix. 3. heck each tube for layers. Record your observations about the solubility of each hydrocarbon in hexane (a typical hydrocarbon solvent). 4. Dispose of hydrocarbons in the designated waste containers. II. EMIAL PROPERTIES: A. ombustion: 1. Line up 4 clean and dry 9 watch glasses about 6 inches apart in a fume hood. 10 Observe if the hydrocarbons ignited rapidly or if they were slow to ignite. Observe if there is a clean flame or a sooty flame. 2. Onto watch glass #1 put 10 drops ethanol. ( 2 5 O) Onto watch glass #2 put 10 drops of hexane. ( 6 14 ) Onto watch glass #3 put 10 drops cyclohexene. ( 6 10 ) Onto watch glass #4 put 10 drops toluene. ( ) 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 Record your observations. 5

6 B. Bromination: 1. Obtain 4 dry 9 stoppered test tubes. 2. Into tube #1 put 1 ml ethanol. ( 2 5 O) Into tube #2 put 1 ml hexane. ( 6 14 ) Into tube #3 put 1 ml cyclohexene. ( 6 10 ) Into tube #4 put 1 ml toluene. ( ) 3. Move your tubes to a fume hood 11 and into each tube drop 3 drops of Bromine (Br 2 ) solution 12. Swirl to mix if needed and record any results. 4. Dispose in the designated waste containers. Tubes containing bromine must go into bromine waste.. Oxidation: 1. Obtain 4 clean stoppered test tubes. 2. Into tube #1 put 1 ml ethanol. ( 2 5 O) Into tube #2 put 1 ml hexane. ( 6 14 ) Into tube #3 put 1 ml cyclohexene. ( 6 10 ) Into tube #4 put 1 ml toluene. ( ) 3. Into each tube drop 3 drops of Potassium Permanganate (KMnO 4 ) solution 13. Stopper and shake to mix well. Record any results. 4. Dispose in the designated waste containers. 11 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. 12 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. 13 KMnO 4 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 Br 2 that would allow easy mixing with our sample. Because KMnO 4 is dissolved in water here we have two layers that must be shaken to mix. 6

7 YDROARBONS REPORT: NAME DATE I. PYSIAL PROPERTIES: A. Volatility: #1 + Water Mass in grams #2 + Ethanol Mass in grams #3 + exane Mass in grams a. Mass at Start b. Mass lost in zero min O g O g O g c. Mass at 15 minutes d. Total g s lost from beginning (a-c) (a-c) (a-c) e. Mass at 30 minutes f. Total g s lost from beginning (a-e) (a-e) (a-e) g. Mass at 45 minutes h. Total g s lost from beginning (a-g) (a-g) (a-g) Total Mass Lost From the Beginning (in grams) Mass lost by Water (#1), Ethanol (#2), and exane (3#) 0 15 (a-c) 30 (a-e) 45 (a-g) Time (in minutes) Volatility Results Summary: Rank the substances in order of decreasing volatility. Most volatile > > Least volatile 7

8 B. Solubility in Water. Density ompared to Water D. Solubility in ydrocarbons Ethanol An Alcohol exane An Alkane yclohexene An Alkene Toluene An Aromatic Physical Properties Results Summary: 1. As a general rule, most hydrocarbons are than water. A. more volatile B. less volatile. there is no general rule 2. As a general rule, hydrocarbons are in water and in other hydrocarbons. A. soluble, soluble B. insoluble, soluble. insoluble, insoluble D. soluble, insoluble 3. As a general rule, most hydrocarbons are than water. A. more dense B. less dense. there is no general rule II. EMIAL REATIVITY: Ethanol An Alcohol A. ombustion exane An Alkane yclohexene An Alkene Toluene An Aromatic B. Bromination (reaction w/ Br 2 ). Oxidation (reaction w/ KMnO 4 ) 8