Chapter 22 Alkanes and alkenes 22.1 Petroleum as a source of alkanes 22.2 Alkanes 22.3 Cracking and its industrial importance 22.4 Alkenes Key terms Progress check Summary Concept map P. 1 / 54
22.1 Petroleum as a source of alkanes Saturated and unsaturated hydrocarbons Petroleum consists mainly of hydrocarbons, which are compounds containing hydrogen and carbon only. Saturated: hydrocarbons containing only single bonds Unsaturated: those containing one or more carbon-carbon multiple bonds (C=C, C C) P. 2 / 54
Example (saturated) (unsaturated) Example 22.1 22.1 Petroleum as a source of alkanes P. 3 / 54
22.2 Alkanes Introducing alkanes Most of the hydrocarbons in petroleum are alkanes. They are saturated hydrocarbons. General formula: C n H 2n+2 They belong to the same homologous series the alkane series. P. 4 / 54
First member: Methane (CH 4 ) Followed by ethane (C 2 H 6 ), propane (C 3 H 8 ), butane (C 4 H 10 ) and so on Each carbon atom forms four single covalent bonds 22.2 Alkanes P. 5 / 54
Name Molecular formula Structural formula Ball-and-stick model Methane CH 4 Ethane C 2 H 6 Propane C 3 H 8 Butane C 4 H 10 Table 22.1 Formulae and ball-and-stick models of some alkanes. 22.2 Alkanes P. 6 / 54
Physical properties of alkanes Some physical properties of a few straight-chain alkanes have been discussed in Chapter 21. Chemical properties of alkanes Similar chemical properties because of their similar structures (only C H and C C bonds in the molecules). Alkanes are unreactive. They do not react with acids, alkalis, oxidizing agents (e.g. potassium permanganate) or reducing agents (e.g. sodium, sulphur dioxide). 22.2 Alkanes P. 7 / 54
They have only a few reactions. Figure 22.1 Sodium reacts with air but not with alkanes. That is why sodium is stored under paraffin oil (a mixture of liquid alkanes). Note: Alkanes are also called paraffins. (Latin: parum = little; affinitas = affinity). 22.2 Alkanes P. 8 / 54
Combustion Alkanes burn in air to give out heat. Thus, they often used as fuels. The general equation for the complete combustion of alkanes (or other hydrocarbons): y y C x H y (g) + (x + )O 2 (g) xco 2 (g) + H 2 O(l) 4 2 Example CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) 22.2 Alkanes P. 9 / 54
Under ordinary conditions, combustion of alkanes is often incomplete. Carbon dioxide, water, carbon monoxide and soot (unburnt carbon particles) would be produced. (a) (b) Figure 22.2 (a) LPG (consisting of lower alkanes) burns with a non-sooty blue flame. (b) A candle (consisting of higher alkanes) burns with a sooty yellow/orange flame. 22.2 Alkanes Class practice 22.1 P. 10 / 54
Reaction with halogens When hexane and bromine (dissolved in an organic solvent) are mixed in the dark, no reaction occurs But in sunlight, hexane reacts with bromine. The red-orange bromine solution is decolorized. 22.2 Alkanes P. 11 / 54
(a) sunlight the red-orange bromine solution is not decolorized in darkness hexane + bromine (dissolved in an organic solvent) (b) the red-orange bromine solution is decolorized Figure 22.3 Hexane does not react with bromine in the dark but it reacts in sunlight. Some sort of energy (such as light or heat) must be supplied for the reaction to take place. 22.2 Alkanes P. 12 / 54
Energy breaks the Br Br bonds in bromine molecules to form very reactive bromine atoms, starting the reaction. In direct sunlight, when more energy is available, the reaction takes place very rapidly. Methane also reacts with bromine in the presence of sunlight. 22.2 Alkanes P. 13 / 54
bromomethane CH 3 Br + Br 2 CH 2 Br 2 + HBr dibromomethane hydrogen bromide (as steamy fumes) CH 2 Br 2 + Br 2 CHBr 3 + HBr tribromomethane CHBr 3 + Br 2 CBr 4 + HBr tetrabromomethane 22.2 Alkanes P. 14 / 54
This is a chain reaction, so a mixture of products is usually obtained. A similar but faster reaction takes place when chlorine is used instead of bromine. The above reactions are examples of a type of reaction called substitution reaction. Learning tip The reactivity of the halogens towards alkanes follows the order F 2 > Cl 2 > Br 2 > I 2. 22.2 Alkanes P. 15 / 54
Key point A substitution reaction is a chemical change in which an atom (or a group of atoms) of a molecule is replaced by another atom (or a group of atoms). Monosubstitution of methane with chlorine Substitution reactions of alkanes with halogens consist of three steps, including 1. initiation 2. propagation 3. termination 22.2 Alkanes P. 16 / 54
Example: monosubstitution of methane with chlorine Step 1 Initiation In this step, free radicals (or simply radicals), a very reactive species, are produced Learning tip In direct sunlight or when ignited, the mixture of methane and chlorine will explode. The reaction involved is: CH 4 (g) + 2Cl 2 (g) C(s) + 4HCl(g) 22.2 Alkanes P. 17 / 54
Key point A free radical (or simply radical) is an atom or a group of atoms with at least one unpaired electron. It is highly reactive and exists only momentarily. Cl Cl bond is broken by ultra-violet radiation (from diffuse sunlight) to give two chlorine radicals and start the chain reaction 22.2 Alkanes chlorine radical chlorine radical P. 18 / 54
Step 2 Propagation (a) Each chlorine radical combines with a hydrogen atom from a methane molecule to form a hydrogen chloride molecule and a methyl radical. methyl radical hydrogen chloride 22.2 Alkanes P. 19 / 54
(b) Some methyl radicals then combine with chlorine atoms from another chlorine molecule to form chloromethane molecules and other chlorine radicals. chloromethane chlorine radical 22.2 Alkanes P. 20 / 54
Step 3 Termination Some methyl radicals combine directly with chlorine radicals to form chloromethane molecules. C chloromethane Class practice 22.2 Activity 22.1 Experiment 22.1 Animation (Monosubstitution reaction of methane with chlorine) Experiment 22.1 22.2 Alkanes P. 21 / 54
22.3 Cracking and its industrial importance Supply and demand of petroleum fractions Due to the rapid development of the modern society, the supply of petrol, kerosene and gas oil cannot meet the demand. This problem can be solved by cracking. Heavy oils (in less demand) can be cracked to provide more petrol or kerosene (in greater demand). P. 22 / 54
Key point Cracking is the process of breaking down large molecules (usually long-chain molecules of carbon compounds) into smaller ones. 22.3 Cracking and its industrial importance P. 23 / 54
Cracking of petroleum fractions During the cracking process, the heavy fractions are heated in the absence of air. Cracking is a chemical reaction, involving the breaking and formation of covalent bonds. Usually a catalyst of aluminium oxide mixed with silicon dioxide is used. Thus, the process is called catalytic cracking. It is carried out in a catalytic cracker in an oil refinery. Think about Animation (Cracking of oil) 22.3 Cracking and its industrial importance P. 24 / 54
Heavy fractions such as fuel oil are usually cracked to produce extra petrol. Molecules in fuel oil are quite large (over 25 carbon atoms per molecule). For simplicity, consider a smaller molecule (decane C 10 H 22 ). 22.3 Cracking and its industrial importance Figure 22.4 A catalytic cracker in an oil refinery. P. 25 / 54
C 10 H 22 C 6 H 14 + C 4 H 8 cracking decane hexane but-1-ene hot catalyst 22.3 Cracking and its industrial importance P. 26 / 54
C 10 H 22 C 8 H 18 + C 2 H 4 cracking decane octane ethene Figure 22.5 A decane molecule (C 10 H 22 ) may be cracked at various points along the chain. Here are two of the many possible ways. 22.3 Cracking and its industrial importance P. 27 / 54
Products obtained from the cracking of fuel oil vary with the conditions used. In general, the products include alkanes (with the number of carbon atoms less than 10) and alkenes. 22.3 Cracking and its industrial importance P. 28 / 54
Types of cracking Two common types of cracking: Thermal cracking Catalytic cracking Thermal cracking: It does not use catalyst. It takes place at higher temperatures and pressures. 22.3 Cracking and its industrial importance P. 29 / 54
Catalyst added? Thermal cracking no Catalytic cracking yes (usually aluminium oxide mixed with silicon dioxide) Temperature higher (about 700 C) lower (about 500 C) Pressure Quality control of products higher (about 30 atm) fair lower (about 1 atm) good Table 22.2 Differences between thermal cracking and catalytic cracking. 22.3 Cracking and its industrial importance P. 30 / 54
Importance of cracking Cracking is very important in the petroleum industry for two reasons: To produce extra petrol To produce alkenes Figure 22.6 Alkenes are commonly used to make the raw materials for the production of plastics. 22.3 Cracking and its industrial importance P. 31 / 54
Cracking in the laboratory Liquid paraffin is a mixture of alkanes. Figure 22.7 Liquid paraffin is a mixture of alkanes. 22.3 Cracking and its industrial importance P. 32 / 54
hard-glass test tube broken pieces of unglazed porcelain glass wool soaked with liquid paraffin strong heat gaseous products obtained from cracking water Figure 22.8 Cracking liquid paraffin in the laboratory. 22.3 Cracking and its industrial importance P. 33 / 54
SBA note Medicinal paraffin or kerosene may be used to replace liquid paraffin for cracking. Broken pieces of porous pot, pumice stone or aluminium oxide may also be used as a catalyst. Example 22.2 Skill corner 22.1 Experiment 22.2 Class practice 22.3 Experiment 22.2 22.3 Cracking and its industrial importance P. 34 / 54
22.4 Alkenes Introducing alkenes Alkenes are unsaturated hydrocarbons with the general formula C n H 2n (n = 2, 3, 4...). Alkenes belong to the same homologous series the alkene series. They are usually obtained from the cracking of petroleum fractions. P. 35 / 54
Name Molecular formula Structural formula Ball-and-stick model Ethene C 2 H 4 Propene C 3 H 6 But-1-ene C 4 H 8 But-2-ene C 4 H 8 Table 22.3 Formulae and ball-and-stick models of some alkenes. 22.4 Alkenes P. 36 / 54
Physical properties of alkenes Alkenes show a gradual change in physical properties as the number of carbon atoms in the molecules increases Name Condensed formula Melting point ( C) Boiling point ( C) Density at 20 C (g cm 3 ) Ethene CH 2 =CH 2 169 104 Propene CH 3 CH=CH 2 185 47.7 But-1-ene CH 3 CH 2 CH=CH 2 185 6.3 Pent-1-ene CH 3 (CH 2 ) 2 CH=CH 2 165 30.0 0.641 Hex-1-ene CH 3 (CH 2 ) 3 CH=CH 2 140 63.5 0.674 Table 22.4 Physical properties of some straight-chain alkenes. 22.4 Alkenes P. 37 / 54
Chemical properties of alkenes All alkenes have the same functional group, so they have similar chemical properties. Because of the presence of the carbon-carbon double bond, alkenes are unsaturated. They are much more reactive than alkanes. 22.4 Alkenes P. 38 / 54
Combustion Alkenes burn in excess oxygen to form carbon dioxide and water. For example, 2CH 3 CH=CH 2 (g) + 9O 2 (g) 6CO 2 (g) + 6H 2 O(l) propene Normally, the oxygen in the air is insufficient for complete combustion. Therefore, unburnt carbon particles are produced when alkenes burn. Alkenes burn with a luminous, smoky flame. 22.4 Alkenes Think about P. 39 / 54
Reactions with halogens When an alkene reacts with bromine (dissolved in an organic solvent) under room conditions, the red-orange bromine solution is decolorized rapidly. A bromine atom is added to each of the doubly bonded carbon atoms. colourless propene (unsaturated) red-orange colourless 1,2-dibromopropane (saturated) 22.4 Alkenes P. 40 / 54
Hex-1-ene also reacts with bromine (dissolved in an organic solvent) Br 2 (dissolved in an organic solvent) hex-1-ene bromine solution decolorized Figure 22.9 Hex-1-ene (an alkene) decolorizes bromine (dissolved in an organic solvent) rapidly. 22.4 Alkenes P. 41 / 54
Key point An addition reaction is a reaction in which two or more molecules react to give a single molecule. Alkenes undergo addition reactions with bromine and chlorine in non-aqueous solvents (e.g. organic solvent) or in water. Example 22.3 22.4 Alkenes P. 42 / 54
Reaction with acidified potassium permanganate solution Alkenes rapidly decolorize acidified potassium permanganate solution. propene from acidified potassium permanganate solution (purple) propane-1,2-diol (a colourless liquid) The purple permanganate ion MnO 4 (aq) is reduced to very pale pink (almost colourless) manganese(ii) ion Mn 2+ (aq). 22.4 Alkenes P. 43 / 54
Learning tip Acidified potassium permanganate solution is a powerful oxidizing agent and it can easily cause further oxidation of the product diol. Cold and dilute alkaline potassium permanganate solution can be used to produce diol. Alkene is oxidized to a diol. This is an addition reaction, with two OH groups being added across the double bond. It is also a redox reaction. 22.4 Alkenes P. 44 / 54
Hex-1-ene also reacts with acidified potassium permanganate solution acidified KMnO 4 solution hex-1-ene acidified KMnO 4 solution decolorized Figure 22.10 Hex-1-ene (an alkene) decolorizes acidified potassium permanganate solution rapidly. Example 22.4 Experiment 22.3 Class practice 22.4 Experiment 22.3 22.4 Alkenes P. 45 / 54
Key terms 1. addition reaction 加成反應 2. alkane 烷烴 3. alkene 烯烴 4. catalytic cracking 催化裂解作用 5. cracking 裂解作用 6. free radical/radical 自由基 / 基 7. initiation 引發 8. propagation 傳播 9. saturated 飽和的 10. substitution reaction 取代反應 P. 46 / 54
11. termination 終止 12. thermal cracking 熱裂解作用 13. unsaturated 不飽和的 Key terms P. 47 / 54
Progress check 1. How can we distinguish between saturated and unsaturated hydrocarbons from their structural formulae? 2. What would happen during the combustion of alkanes? 3. What would happen during the substitution reaction of alkanes with halogens? 4. What are the steps involved in the monosubstitution of methane with chlorine? 5. How can we obtain smaller molecules including alkanes and alkenes from petroleum fractions? P. 48 / 54
6. Why is cracking an important industrial process? 7. How can we carry out cracking of a petroleum fraction in the laboratory? 8. What would happen when alkenes react with bromine? 9. What would happen when alkenes react with acidified potassium permanganate solution? 10.How can we carry out chemical tests for unsaturated hydrocarbons? Progress check P. 49 / 54
Summary 22.1 Petroleum as a source of alkanes 1. Hydrocarbons can be classified into saturated hydrocarbons and unsaturated hydrocarbons. 22.2 Alkanes 2. Most of the hydrocarbons in petroleum are alkanes, with the general formula C n H 2n+2. 3. Alkanes are unreactive. However, they can react with halogens (in sunlight) and burn in air. P. 50 / 54
4. A substitution reaction is a chemical change in which an atom (or a group of atoms) of a molecule is replaced by another atom (or a group of atoms). Example: CH 4 + Br 2 CH 3 Br + HBr 5. In general, substitution reactions of alkanes consist of three steps, including initiation, propagation and termination. Free radicals, a very reactive species, are involved in the reaction process. 6. All alkanes have similar chemical properties, but alkanes with larger molecules react more slowly. Summary P. 51 / 54
22.3 Cracking and its industrial importance 7. Cracking is the process of breaking down large molecules (usually long-chain molecules of carbon compounds) into smaller ones. 8. Cracking heavy oils produces lighter petroleum fractions. That is, cracking long-chain alkanes produces short-chain alkanes. Alkenes are also produced. 9. Cracking is important in the petrochemical industry for two reasons: It produces extra petrol (as fuel for motor vehicles) Summary P. 52 / 54
It produces alkenes (as raw materials for the production of other synthetic products) 22.4 Alkenes 10. Alkenes are a homologous series of unsaturated hydrocarbons with the general formula C n H 2n. Every alkene molecule contains a carbon-carbon double bond. 11. Alkenes are more reactive than alkanes. They undergo addition reactions. 12. An addition reaction is a reaction in which two or more molecules react to give a single molecule. Summary P. 53 / 54
Concept map Petroleum fractions heating in the absence of air cracking saturated unsaturated hydrocarbons hydrocarbons ALKANES ALKENES Other substances chemical properties chemical properties substitution reaction three steps involved 1. Initiation 2. Propagation 3. Termination Combustion Carbon dioxide produces Water Addition reaction P. 54 / 54