(O Level Chem) Alkanes and Alkenes Teaching & Learning Notes

1. Carbon is a Group IV and Period 2 element and so it has four valence electrons and two shells. Using its four valence electrons, a carbon atom is able to form a maximum of four covalent bonds. After forming four bonds, its shell is completely full and so the carbon atom can never form five or more bonds. Carbon is well known for its ability, called catenation, to form covalent bonds with other carbon atoms to form large structures such as diamond and graphite and long chains of organic compounds. Living things are made of carbohydrates, proteins and fats that are long-chain carbon compounds.

2. Fossil fuels include natural gas, crude oil and coal. They are called fossil fuels because they were made naturally from decomposing prehistoric plant and animal remains. Since living things are made of compounds of carbon, fossil fuels are made of compounds of carbon too. Coal is a black colored rock mainly made of carbon. Natural gas is a mixture of hydrocarbons consisting mainly of methane. Crude oil is a thick, often black, liquid consisting of various hydrocarbons of different relative molecular masses.

3. Hydrocarbons are covalent compounds. Between atoms in the hydrocarbon molecules, there are strong covalent bonds. Between molecules, however, there are weak van der Waals forces. The larger the hydrocarbon molecule, the greater the van der Waals forces which require more energy to overcome. That is why hydrocarbons with greater molecular masses have higher melting and boiling points than hydrocarbons with lower molecular masses. Now, arrange the compounds, butane C₄H₁₀, heptane C₇H₁₆, and methane CH₄, in order of increasing melting and boiling points.

4. Crude oil needs to be separated into fractions that are useful to us. Since it is a mixture of hydrocarbons with different boiling points, crude oil can be separated using fractional distillation. At the oil refinery, crude oil is heated to about 350 ºC to evaporate most of its compounds. The gases are pumped into the bottom of a fractionating tower about 10 m high. Hydrocarbons with higher boiling points will condense first and are collected at the lower parts of the tower. Hydrocarbons with lower boiling points will rise to the cooler top of the tower before they condense. The fractions obtained include petrol or gasoline (C₅-C₁₀, bp~120 ºC) used as a fuel in cars, naphtha (C₅-C₉, bp~70 ºC) used as feedstock for the chemical industry, kerosene or paraffin (C₁₀-C₁₆, bp~170ºC) for heating and cooking and for aircraft engines, diesel (C₁₄-C₂₀, bp~270 ºC) for diesel engines, lubricating oil (C₂₀-C₅₀) used as lubricants, polishes and waxes. The residue (>C₇₀), bitumen, would be used for making road surface.

5. A homologous series is a group of compounds with a general formula and similar chemical properties. The compounds in a homologous series show a gradual change in physical properties. For example their melting points and viscosity increase and flammability decreases as their molecular mass and size of molecules increase. The homologous series that we will study in O-level are alkanes, alkenes, alcohols and carboxylic acids. Alkanes and alkenes are hydrocarbons because they contain hydrogen and carbon only. Alkanes are also called saturated hydrocarbons because they only contain C-C single bonds, while alkenes are called unsaturated hydrocarbons because they contain C=C double bonds.

6. Alkanes is a homologous series of saturated hydrocarbons with a general formula of C_nH_2n+2. The first member of alkanes is methane CH₄ which has the lowest melting and boiling points. Methane is easily burned and so we say it is very flammable. The next three members are ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀ which are also flammable gases. Pentane C₅H₁₂ and hexane C₆H₁₄ are flammable liquids. The greater the molecular mass of the liquid alkanes, the more viscous and the less flammable they are. The following diagram shows the full structural formulas of straight-chained alkanes up to butane. In drawing full structural formulas, all the bonds must be shown. Now, it is your turn to draw the straight-chained pentane and hexane. Check that all the structures of alkanes agree with the general general formula C_nH_2n+2.

7. Straight-chained alkanes are unbranched alkanes. From C₄ onwards, branched alkanes exist. This is an example of isomerism. Isomers are molecules with the same molecular formula but different structural formulas. Butane C₄H₁₀, for example, can only form two isomers, n-butane which is the unbranched butane, and isobutane which is the branched butane. Pentane C₅H₁₂can form three isomers, namely n-pentane, iso-pentane and neo-pentane. The more carbon atoms there are in the alkane, the more isomers it can form. Try drawing the isomers of hexane C₆H₁₄. How many isomers can hexane form?

8. Alkanes are generally less reactive than other classes of organic compounds because it requires a lot of energy to break their strong C-H and C-C covalent bonds. Alkanes can undergo two kinds of reactions, namely, combustion and free radical substitution. In plentiful supply of oxygen, methane burns to form carbon dioxide and water. If the supply of oxygen is limited, then incomplete combustion occurs and the poisonous carbon monoxide is produced. Use the following values to calculate the enthalpy of combustion of 1 mole of methane. The incomplete combustion of methane is less exothermic than its complete combustion. 

Bond	Bond energy (kJ/mol)
O=O	498
C-O	358
O-H	463
C=O	732
C-H	413

9. If methane and chlorine gases are mixed together in the dark at room temperature, there is no reaction. In the presence of ultraviolet, however, a chain reaction occurs. The ultraviolet has provided the activation energy for the reaction to occur by breaking the Cl-Cl bond to form the reactive chlorine atoms. This is the initiation stage. The Cl atom is called a free radical because it has an unpaired valence electron. A chlorine radical is written as Cl•. With the Cl• radical, the propagation to change methane CH₄ into methyl chloride CH₃Cl occurs. The Cl• is reformed to attack more CH₄ molecules. In effect, the Cl• radical acts as a catalyst to change CH₄ to CH₃Cl. The reaction stops or terminates when radicals combine together to form molecules. Write down the overall equation for the chain reaction by combining equations 2 and 3 together.

10. Alkenes are unsaturated hydrocarbons with the general formula C_nH_2n. The functional group of alkenes is C=C. Alkenes are called unsaturated because the carbon atoms are not bonded to the maximum number of 4 atoms. The first member of the alkenes has two, not one, carbon atoms and it is called ethene C₂H₂. Note the difference that the names of alkanes end with -ane while the names of alkenes end with -ene. Draw the structural formulas of unbranched but-1-ene, pent-1-ene and hex-1-ene. The number “1” in the names indicate that the C=C is at the first carbon atom. The melting and boiling points and viscosity increase down the series of alkenes, just like the trend in alkanes.

11. Like alkanes, alkenes undergo combustion to form carbon dioxide and water. Practice writing out the chemical equation for the complete combustion of ethene. Alkenes typically undergo a type of reactions called addition since two more atoms can be attached to the carbon atoms of each C=C bond. Although the C=C bond (bond energy = 610 kJ/mol) is stronger than the C-C bond (bond energy = 350 kJ/mol), alkenes are more reactive than alkanes. This is because one of the two bonds in C=C is easily broken.

12. When ethene and hydrogen are passed over nickel catalyst at 150 ºC, the hydrogen is added to the C=C and the saturated ethane is formed. This addition reaction is also called hydrogenation. Solid margarine is manufactured from liquid vegetable oils by hydrogenation too. Vegetable oils are polyunsaturated meaning that the chain of molecules contains several C=C bonds. 

13. When a mixture of gaseous ethene and steam is passed over a catalyst of phosphoric(V) acid at 300 ºC and 60-70 atm, ethanol C₂H₅OH is formed. This is the industrial manufacture of ethanol from ethene. In the chapter on alcohols, you will learn about fermentation which is another method of making ethanol. Note that ethanol can also be changed back to ethene by dehydration using concentrated sulfuric acid at 170 ºC.

14. How do we test for the presence of alkenes? In other words, how do you test for unsaturation in an organic compound? We use bromine. When ethene, for example, is blown into bromine that is  dissolved in an organic solvent such as tetrachloromethane under standard laboratory conditions with no ultraviolet, the reddish-brown liquid is decolorized. 1,2-dibromoethane, which is the addition product, is formed. Note that if aqueous bromine is used instead, the product is 2-bromo-ethanol.

15. How is ethene manufactured? Crude oil, unfortunately, contains too many large hydrocarbons which are useless because they are viscous and are not flammable. There is not enough small hydrocarbons to meet demand. Cracking is needed to break large hydrocarbon molecules to produce small hydrocarbons such as ethene. For example, C₁₅H₃₂ is vaporized and passed over a very fine zeolite catalyst powder at 500 ºC to form ethene, propene and octane C₈H₁₈. Try to write down the equation for this reaction which is an example of thermal decomposition. Cracking can also be done in the laboratory. 

16. The ethene and propene produced from the cracking of C₈H₁₈ are useful for making plastics. At high pressure of 10-80 atm and 70-300 ºC in the presence of an aluminum-based catalyst, the gaseous ethene is converted to white solid pellets of poly(ethene) which is our familiar plastic. The ethene molecules have joined together by addition reaction. Each poly(ethene) molecule can be made by 1000 to 1,000,000 ethene molecules and so there is a sharp increase in molecular mass from reactant to product. Poly(ethene) is an example of polymers which are very long molecules made up of repeating units called monomers. The formation of polymers by the addition of unsaturated monomers is called addition polymerization. Try to show the equation for the addition polymerization of propene. Use “n” to denote a very large number.

Draw a flow chart to show all the reactions of ethane and ethene that you have learned.