Materials Included In Kit

Additional Materials Required

Prelab Preparation

Bromine water may be prepared before class and is safer and easier to work with than pure bromine. Prepare the solution and work with bromine in an operating fume hood. To prepare a saturated solution, combine 50 mL of 0.5 M sodium bromide solution with 50 mL of 0.5 M hydrochloric acid in a beaker and add 20 mL of 5% sodium hypochloride (bleach) solution.

Safety Precautions

Carry out all procedures in an operating fume hood. Cyclohexane, cyclohexene and toluene are flammable liquids and vapors. Keep away from heat, sparks and open flames. Hydrocarbons may cause drowsiness or dizziness if inhaled. Avoid breathing vapors or mist. Styrene and toluene are suspected reproductive hazards and may damage an unborn child. Work with these compounds in an operating fume hood only and do not use them if you are pregnant. Bromine water is a dilute solution of bromine; it is toxic by inhalation and will irritate skin and eyes. Avoid breathing the vapor and work with bromine water in an operating fume hood only. Calcium carbide is corrosive to skin and eyes. It reacts exothermically with water to produce flammable acetylene gas, which is toxic by inhalation. Keep the calcium carbide container closed at all times when not in use and do not use or place water near the container. Please review current Safety Data Sheets for comprehensive safety, handling and disposal information. Remind students to wash hands thoroughly with soap and water before leaving the laboratory.

Disposal

Please consult your current Flinn Scientific Catalog and Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Liquid hydrocarbon mixtures and wastes should be transferred in the hood to a properly labeled aqueous organic waste bottle for eventual licensed hazardous waste disposal. Record the amount and identity of the chemicals added to the waste container as well as the date. The chemicals may be identified as “mixed hydrocarbon waste.”

Lab Hints

• Do not store styrene after the inhibitor has been removed. Styrene will undergo a spontaneous, exothermic polymerization reaction due to inadvertent contamination with peroxide sources from air oxidation.
• The bromine and potassium permanganate tests may be extended to identify unknown or consumer hydrocarbon products. Suitable unknowns include mineral oil, turpentine and lamp or fuel oil.
• For a controlled study of the light-catalyzed bromination of toluene, carry out parallel reactions of toluene and bromine in two test tubes. Cover the first test tube with aluminum foil to block stray light and shine a black light on the second test tube. Compare the extent of reaction (color change) after five minutes.
• Toluene can react with bromine via two pathways, the light-catalyzed reaction discussed perviously (aliphatic substitution) and iron-catalyzed aromatic substitution. To demonstrate aromatic electrophilic substitution, mix toluene and bromine water in a test tube that is protected from light and add a micro spatula amount of iron filings. Iron will catalyze bromination of an aromatic C—H bond in the absence of light.
• Complete or “clean” combustion of acetylene may be demonstrated by collecting acetylene gas by water displacement in an inverted test tube. Fill a shallow pan such as a Pyrex crystallizing dish with water. Fill a large test tube about one-third full with water and invert it in the dish. Add one piece of calcium carbide to the water in the dish and move the test tube over the calcium carbide pellet to collect the acetylene gas being generated. When the test tube appears “empty,” remove it from the dish and invert it over a burning splint. Observe the intensity of the “pop” as well as the cleanliness of the tube following combustion.
• The preparation and combustion of acetylene was used in old-fashioned miner’s lamps to produce a bright light. Calcium carbide lamps are available for purchase on Internet sites, such as eBay. For a great chemistry video presentation of a miner’s lamp and the preparation and properties of acetylene, please visit the Flinn Scientific Teaching Chemistry Video Series at www.flinnsci.com to view the “Carbide Cannon” activity, presented by Bob Becker.

Answers to Prelab Questions

1. Covalent bonds are classified as polar or nonpolar based on the difference in electronegativity between two atoms. Why are C—H bonds (and hydrocarbons) considered nonpolar? How does this affect the physical properties of hydrocarbons, including their solubility and boiling points?

   Hydrocarbons are considered nonpolar because C and H have similar electronegativity values, 2.5 and 2.1, respectively. Hydrocarbons are immiscible with water and may be gases, liquids or solids at room temperature. Their boiling points are lower than those of polar molecules that have similar molecular weights. Example: The bp of methane (16 g/mole) is −164 °C, while the bp of ammonia (17 g/mole) is −33 °C.

2. What types of intermolecular forces are important in the properties of hydrocarbons? Explain the origin and strength of these forces.

   London dispersion forces are weak attractive forces between nonpolar molecules such as hydrocarbons. The average electron distribution in a nonpolar molecule is symmetrical. Electrons are in constant, random motion, however, and thus at any given instant a nonpolar molecule may have an uneven distribution of electrons and a temporary dipole moment. Because of mutual electron-electron repulsion, the negative end of a temporary dipole will polarize or distort the electron density in a neighboring molecule, resulting in an induced dipole moment. The strength of London dispersion forces increases in heavier atoms or molecules, which are more polarizable and more easily form temporary dipoles.

3. Candles contain paraffin wax, a hydrocarbon. When a test tube filled with cold water is placed over a burning candle, condensation appears on the outside of the test tube. If a large jar is inverted over the burning candle, and the gas in the jar is bubbled via suction through a solution containing calcium hydroxide, Ca(OH)₂, a white precipitate of CaCO₃ is observed. Explain these observations based on the products of combustion.

   The products of combustion are water and carbon dioxide. Water is visible as condensation on the walls of a test tube held above the burning candle. The formation of solid CaCO₃ is evidence for the presence of carbon dioxide via the following reaction.
   Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

4. What are the hazards of using bromine water? How can these hazards be avoided?

   Bromine water is toxic by inhalation and ingestion and is irritating to skin and eyes. Work with bromine in the hood to avoid breathing the vapor, and wear eye and skin protection to prevent exposure.

5. What special precautions are recommended when working with toluene and styrene? Explain.

   Toluene and styrene are suspected reproductive hazards and may harm or damage an unborn child. Always work with these substances in an operating fume hood to prevent exposure to the vapors. Women who are pregnant should discuss possible laboratory exposure to any chemical with their physicians.

6. Draw the structure of the monomer that is used to make polyvinyl chloride (PVC):
   {13797_PreLabAnswers_Figure_9}

   CH₂=CH—Cl

Answers to Questions

1. Compare and contrast the solubility of red food coloring and blue oil dye with hydrocarbons. Explain in terms of structure, polarity and intermolecular forces.

   Red food coloring is an aqueous liquid and is immiscible with hydrocarbons. The dye molecules did not dissolve in the hydrocarbons, which remained colorless. The blue oil dye was miscible with all the hydrocarbons and the dye itself distributed itself evenly in the combined liquid layer. Food dyes are highly polar, ionic compounds. Dissolving a polar solute in a nonpolar liquid requires breaking strong intermolecular forces between the dye and water molecules and replacing them with weaker intermolecular forces. This is thermodynamically not favored. Oil dyes are nonpolar, like the hydrocarbons and readily dissolve due to the entropy of mixing.

2. Which compound immediately decolorized bromine? Describe the observations and write a chemical equation for this reaction.

   Cyclohexene reacted immediately with bromine—the resulting product mixture consisted of two colorless liquid phases.

   {13797_Answers_Reaction_1}
3. Write a chemical equation for a possible light-catalyzed substitution reaction of bromine with cyclohexane. Did you observe any evidence for this reaction?

   The red color of bromine faded slightly after 5 minutes of irradiation with a black light. This was the only sign of the possible reaction shown below.

   {13797_Answers_Reaction_2}
4. The light-catalyzed reaction of bromine with toluene leads to substitution of the CH₃— group attached to the aromatic ring. (a) Write a chemical equation for this reaction. (b) What was the purpose of the litmus paper test in this reaction?
   1. {13797_Answers_Reaction_3}
   2. Litmus paper was used to test for the presence of HBr in the above reaction and differentiate a possible substitution reaction from an addition reaction product. The paper turned pink, which is consistent with the formation of acidic byproduct such as HBr via a substitution reaction. HBr is not produced when bromine adds to an unsaturated hydrocarbon.
5. Summarize the observed reactions of cyclohexane, cyclohexene, and toluene with bromine. Describe: (a) the relative rates of reaction for the three hydrocarbons; (b) the type of reaction that occurred; (c) whether a catalyst was required; and (d) the formation of byproducts.
   1. Cyclohexene reacted instantaneously with bromine Toluene reacted slowly in the absence of light and more rapidly when exposed to light. Cyclohexane reacted very, very slowly even when exposed to light.
   2. Cyclohexene reacts via an addition reaction, while toluene and cyclohexane undergo substitution reactions with bromine.
   3. The reactions of toluene and cyclohexane were catalyzed by ultraviolet light.
   4. An acidic byproduct (pink litmus paper) was observed in the reaction of toluene with bromine.
6. What were the observations for a positive test result in the reaction of cyclohexene with potassium permanganate? Write an equation for this reaction.

   The color of the reaction mixture changed from purple to dark brown, consistent with the disappearance of KMnO4 (purple solution) and the formation of MnO₂ (brown solid).

7. Adding iron to a mixture of toluene and bromine may catalyze a second type of substitution reaction, in which bromine replaces an aromatic C—H bond in the ring. (a) Draw structures for the three different substitution products that are possible in this aromatic substitution reaction. (b) Use resonance to explain why these are the only unique compounds that may be obtained.
   1. The three possible aromatic substitution products:
      {13797_Answers_Reaction_4}
   2. If the aromatic ring consisted of alternating single and double bonds, then the two structures shown below would be different (they would be isomers). However, due to resonance, these two products are in fact identical, and thus there are only three unique products.
      {13797_Answers_Reaction_5}
8. Describe observations for the chemical reaction that took place when styrene was heated.

   Styrene is a clear and colorless liquid. The liquid remained colorless during heating, but became “thicker” and more viscous. The product solidified to a hard, transparent film when poured and cooled.

9. Polymer solutions or polymer “melts” are generally viscous—thick and slow to pour. Viscosity is related to the difficulty molecules have in slipping past each other. Why do polymers have a higher viscosity than monomers or other small molecules?

   Viscosity is the resistance to flow of a liquid. On a molecular level, large polymer molecules or chains are easily entangled and jumbled, which prevents them from sliding past each other when poured. A plate of cooked spaghetti is a good analogy for visualizing the viscosity of polymer molecules.

10. Write a chemical equation for the reaction of calcium carbide with water and describe the experimental evidence for each product of this reaction.

    CaC₂(s) + 2H₂O(l) → H—C==C—H (g) + Ca(OH)₂ (s)
    Observations included bubbling due to the formation of acetylene gas and the formation of a white precipitate, Ca(OH)₂, which is only sparingly soluble in water.

11. Did complete or incomplete combustion occur when acetylene was burned? Write equations for the two possible combustion pathways. What factor determines which pathway will predominate?

    Complete combustion 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O
    Incomplete combustion 2C₂H₂ + O₂ → 4C + 2H₂O
    Incomplete combustion occurred, as evidenced by the formation of black soot, which looked like charcoal or elemental carbon. Complete combustion requires a high ratio of oxygen to acetylene. The amount of oxygen in air is the limiting factor in deciding whether complete or incomplete combustion will take place.

12. Fats and oils consist of long hydrocarbon chains attached to ester functional groups. (a) What do the terms saturated, unsaturated, and polyunsaturated imply about the structures of the hydrocarbon “tails” in fats and oils? (b) Predict the chemical reaction used to prepare “partially hydrogenated” vegetable oils.
    1. Saturated fats contain mainly C—C single bonds in the hydrocarbon tails. Unsaturated oils consist of hydrocarbon chains with at least one double bond, while polyunsaturated oils contain more than one C=C double bond in the hydrocarbon chains attached to the ester functional groups.
    2. Partially hydrogenated vegetable oils are obtained by reacting an unsaturated or polyunsaturated oil with hydrogen gas. Hydrogen adds across the double bonds in these molecules.
13. Explain the observed trend in the boiling points of straight-chain alkanes based on the strength of their intermolecular forces.
    {13797_Answers_Figure_10}
    The boiling points of alkanes increase in a predictable or regular manner as the size of an alkane molecule increases, based on the number of carbon atoms in the chain. This is due to an increase in the strength of London dispersion intermolecular forces between molecules as they get bigger. Higher temperatures and more energy are required to overcome these forces of attraction in the liquid phase and vaporize molecules.

Introduction

Hydrocarbons are organic compounds containing only carbon and hydrogen. This apparent simplicity in the structure of hydrocarbons is belied by the great diversity in the size or length of hydrocarbon molecules, the extent of branching in carbon−carbon chains, the variety of possible ring sizes and the presence of alkene, alkyne and aromatic functional groups.

Concepts

• Hydrocarbons
• Alkenes and alkynes
• Sigma and pi bonds
• Aromatic compounds
• Addition reactions
• Substitution reactions
• Oxidation
• Polymerization
• Combustion

Background

Hydrocarbons may be gases, liquids or solids depending on the size or molar mass of the molecules and the degree of branching in C—C chains and rings. Hydrocarbons with 1–4 carbon atoms are gases at room temperature, those with 5–12 carbon atoms are typically liquids, and hydrocarbons with > 16 carbon atoms are waxes or solids at room temperature. The boiling points of hydrocarbons generally increase in a smooth and predictable manner as the number of carbon atoms increases. This trend reflects a continuous increase in the strength of intermolecular forces, primarily London dispersion forces, as the size of a hydrocarbon molecule increases.

The ability of carbon atoms to form almost infinite C—C chains is illustrated by the formation of high-density polyethylene (HDPE), a plastic resin with an average molar mass of approximately one million (see Figure 1). The number of carbon atoms in a chain of this size is about 70,000!

Hydrocarbons are the primary constituents in natural gas, oil and coal, and many commercially important hydrocarbons are produced by petroleum refining. The top five hydrocarbons used in the manufacture of industrial and consumer products are ethylene, propylene, styrene, benzene and xylenes (see Figure 2). There are four major classes of functional group compounds for hydrocarbons. These are alkanes, alkenes, alkynes and aromatic compounds. The carbon skeleton in an alkane contains only C—C single bonds, which may be connected in chains or rings. Figure 3 shows the structures of three alkanes having the formula C₅H₁₂. Notice the carbon atoms may be joined in three different arrangements. These compounds are called isomers—they have the same molecular formula but different structural formulas. They also have different physical properties. Alkenes and alkynes contain one or more C=C double and C≡C triple bonds, respectively (see Figure 4). These compounds are referred to as unsaturated hydrocarbons. Introduction of a double or triple bond reduces the maximum number of hydrogens in the overall formula of an alkene or alkyne compared to an alkane with the same number of carbon atoms. Alkanes, including cyclic alkanes, do not react with common laboratory reagents. As a result, the C—C skeleton in an organic compound tends to remain intact in reactions involving other functional groups, which are often defined as the reactive groups in an organic molecule. Combustion is a notable exception to this general lack of reactivity. Although kinetically stable at room temperature, alkanes and other hydrocarbons form flammable vapors and liquids, or combustible solids, and will burn in the presence of oxygen and a source of ignition, such as heat, sparks or open flames. Combustion of natural gas, gasoline and fuel oils, and coal is highly exothermic and is the most important source of energy for manufacturing and transportation. Complete combustion of a hydrocarbon produces carbon dioxide and water, as shown in Equation 1 for the reaction of butane, C₄H₁₀. Replacing C—H bonds in alkanes with other atoms requires high temperatures, ultraviolet light or special metal catalysts. Consider chlorine and bromine, which are very reactive nonmetals (halogens). The halogens do not react with alkanes unless initiated or catalyzed by ultraviolet light (represented as hν). The resulting reactions are classified as substitution reactions in which a halogen atom (X) replaces a hydrogen atom in a C—H bond and HX is formed as a byproduct. See Equation 2; R may be any chain or ring of carbon atoms. Alkenes and alkynes are more reactive than alkanes because of the difference in bond strength between the sigma and pi bond components of a double or triple bond. The average carbon−carbon bond energy is 348 kJ/mole for a C—C single bond, 614 kJ/mole for a C=C double bond, and 839 kJ/mole for a C≡C triple bond. Both alkenes and alkynes are thus susceptible to reactions in which the pi bond is broken and new C—H or C—X bonds are formed.

Alkenes and alkynes undergo characteristic addition reactions with a variety of reagents, including the halogens (e.g., X₂ = Cl₂, Br₂), hydrogen (with a metal catalyst), and water (in the presence of an acid catalyst). Figure 5 shows several examples of addition reactions, in which a compound of the type X—Y “adds across” the carbon atoms in the double bond, breaking the pi bond and forming new single bonds. Unsaturated hydrocarbons also react readily with potassium permanganate and other strong oxidizing agents. Reaction of an alkene with the permanganate ion, MnO₄^–, produces a diol, with two OH groups attached to the original C=C atoms in the alkene (Equation 3). Bromine and potassium permanganate are used in qualitative tests to detect the presence of alkene and alkyne functional groups in a compound. Positive test results are interpreted based on the disappearance of color or color changes for bromine (red) and potassium permanganate (purple), respectively.

Ethyne, more commonly known as acetylene, C₂H₂, is the simplest alkyne. It is produced commercially by the reaction of calcium carbide, CaC₂, with water (Equation 4). The primary use of acetylene is in welding torches. Complete combustion produces an extremely hot flame, > 3300°C, when acetylene burns in pure oxygen. In air, acetylene burns with a dark sooty flame. The production of carbon is characteristic of the incomplete combustion of alkynes and aromatic compounds with a high degree of unsaturation. Alkenes are the primary feedstock for many common polymers and plastics, including polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Polymers are long, chain-like molecules composed of multiple repeating units of smaller molecules, called monomers, which are joined together by a chemical reaction. Addition polymers are formed when alkenes containing one or more C=C double bonds add to each other. Polymerization reactions typically require a catalyst to initiate the reaction, but once a reaction starts it will continue as a chain reaction until thousands of monomer units have been combined.

Polystyrene, a rigid, clear plastic, is made by heating styrene, CH₂=CH—C₆H₅, with a free-radical catalyst such as dibenzoyl peroxide. The catalyst breaks apart in the presence of heat or light to produce benzoyl radicals. See Equations 5 and 6. Aromatic compounds are a special class of hydrocarbons derived from benzene, C₆H₆, or polycyclic analogs such as naphthalene and anthracene (see Figure 6). The presence of six pi electrons in a cyclic ring system confers unique stability (resonance stabilization) on these compounds. The structure of benzene appears to show alternating single and double bonds. According to x-ray and other evidence, however, all of the C—C bonds in benzene are identical. This apparent anomaly may be explained in terms of valence bond theory (two equivalent resonance forms) or molecular orbital theory, in which six pi electrons are delocalized and equally shared by all the carbon atoms in the ring (see Figure 7). Due to the unique stability of the aromatic ring system, benzene and other aromatic compounds typically undergo substitution reactions in which the aromatic ring is retained. For example, benzene reacts with bromine in the presence of an iron catalyst to produce bromobenzene and hydrogen bromide (Equation 7).

Experiment Overview

The purpose of this activity is to investigate the properties of a variety of hydrocarbons, including cyclohexane, cyclohexene, toluene, styrene and acetylene.

Materials

Prelab Questions

1. Covalent bonds are classified as polar or nonpolar based on the difference in electronegativity between two atoms. Why are C—H bonds (and hydrocarbons) considered nonpolar? How does this affect the physical properties of hydrocarbons, including their solubility and boiling points?
2. What types of intermolecular forces are important in the properties of hydrocarbons? Explain the origin and strength of these forces.
3. Candles contain paraffin wax, a hydrocarbon. When a test tube filled with cold water is placed over a burning candle, condensation appears on the outside of the test tube. If a large jar is inverted over the burning candle, and the gas in the jar is bubbled via suction through a solution containing calcium hydroxide, Ca(OH)₂, a white precipitate of CaCO₃ is observed. Explain these observations based on the products of combustion.
4. What are the hazards of using bromine water? How can these hazards be avoided?
5. What special precautions are recommended when working with toluene and styrene? Explain.
6. Draw the structure of the monomer that is used to make polyvinyl chloride (PVC), (CH₂—CH)_n.

Safety Precautions

Carry out all procedures in an operating fume hood. Cyclohexane, cyclohexene and toluene are flammable liquids and vapors. Keep away from heat, sparks and open flames. Hydrocarbons may cause drowsiness or dizziness if inhaled. Avoid breathing vapors or mist. Styrene and toluene are suspected reproductive hazards and may damage an unborn child. Work with these compounds in an operating fume hood only and do not use them if you are pregnant. Bromine water is a dilute solution of bromine; it is toxic by inhalation and will irritate skin and eyes. Avoid breathing the vapor and work with bromine water in an operating fume hood only. Calcium carbide is corrosive to skin and eyes. It reacts exothermically with water to produce flammable acetylene gas, which is toxic by inhalation. Keep the calcium carbide container closed at all times when not in use and do not use or place water near the container. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Please follow all normal laboratory safety guidelines and wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Properties of Hydrocarbons

1. Obtain 1 mL each of the following liquids in clean, dry test tubes (1−6).
   {13797_Procedure_Table_1}
2. Add 2–3 drops red food coloring to each test tube 1–3 and observe the color and appearance of each mixture.
3. Add 2–3 drops of blue oil dye to each test tube 4–6. Observe the color and appearance of each mixture.
4. Pour the contents of the test tubes into an appropriately labeled waste beaker (flammable liquid). Rinse the test tubes well with acetone, and wipe clean with a paper towel as needed to remove traces of dyes.
5. Refill test tubes 1–6 as shown in step 1.
6. Observe the color of bromine water and add 10 drops of bromine water to each test tube 1–3. Stopper each tube with a cork and observe any immediate changes in the color and appearance of each mixture.
7. Observe any mixtures that did not immediately react for an additional 2–3 minutes for signs of chemical change.
8. Place any test tubes that did not react under a black light for 2–3 minutes. Observe any changes in color and appearance.
9. Uncork each test tube 1–3 and place a piece of moistened blue litmus paper in the mouth of the test tube. (Hold the litmus paper with forceps or tweezers to avoid skin contact.) Observe any color changes.
10. Add 10 drops of 1% potassium permanganate solution to each test tube 4–6. Observe any changes in the color and appearance of each mixture.
11. Pour the contents of the test tubes into an appropriately labeled waste beaker. Rinse the test tubes with acetone and wipe clean with a paper towel as needed.

Polymerization of Styrene

1. Set up a boiling water bath: Fill a 250-mL beaker about half-full with water and add a boiling stone. Heat on a hot plate at a medium-high setting.
2. Styrene contains an inhibitor to prevent unwanted polymerization. Remove the inhibitor by passing the styrene through a short column of alumina (aluminum oxide). See Figure 8: Place a small plug of cotton into a glass Pasteur pipet, and push the cotton down into the neck of the pipet using copper wire. Fill the pipet with about 1 cm of alumina.
   {13797_Procedure_Figure_8_Removing the inhibitor from styrene}
3. Obtain 1 mL of styrene in a clean disposable pipet and slowly drip the liquid through the alumina column in the Pasteur pipet. Collect about 20 drops of purified styrene in a clean and dry Pyrex test tube.
4. Using a micro spatula, add 2 grains of dibenzoyl peroxide to the styrene and place the test tube in the boiling water bath for about 10 minutes.
5. After 10 minutes, remove the test tube from the boiling water bath using a test tube clamp and carefully pour the contents of the test tube onto a large Pyrex watch glass. Allow the product to harden and cool and record observations.

Preparation of Acetylene

1. Fill a medium test tube about one-third full with water and add one small pebble or nugget of calcium carbide to the water.
2. Observe any signs of a chemical reaction for one minute.
3. Light a wooden splint and place it over the mouth of the test tube.
4. Observe evidence for combustion and possible products.
5. Extinguish the lighted splint in a beaker of water, if needed, and allow the test tube to cool for a few minutes before pouring the reaction mixture into the beaker as well. Make sure that all of the solid calcium carbide has reacted before neutralizing and disposing of the reaction mixture.

Student Worksheet PDF

13797_Student1.pdf