C15 Hydrocarbons (1).pdf

Chapter 15 Hydrocarbons Learning Outcomes Recall that carbon forms branched and unbranched chains and ring compounds Illustrate that carbon atoms can form single and double bonds Write formulae to represent simple organic compounds List the general characteristics of a homologous series...

Chapter 15 Hydrocarbons Learning Outcomes Recall that carbon forms branched and unbranched chains and ring compounds Illustrate that carbon atoms can form single and double bonds Write formulae to represent simple organic compounds List the general characteristics of a homologous series Deduce the homologous series given the fully displayed formulae of compounds Define structural isomerism Chapter 15 Hydrocarbons Learning Outcomes Write general and molecular formulae for the members of the alkane homologous series Write fully displayed structures and names of branched and unbranched alkanes and their isomers, given their molecular formulae Describe the reactions of alkanes Relate the characteristic reactions of alkanes to their structures Relate the properties of alkanes to their uses Chapter 15 Hydrocarbons Learning Outcomes Write general and molecular formulae for the members of the alkene homologous series Write fully displayed structures and names of branched and unbranched alkenes and their isomers, given their molecular formulae Describe the reactions of alkenes Relate the characteristic reactions of alkenes to their structures Relate the properties of alkenes to their uses  Chapter 15 Hydrocarbons Introduction to Hydrocarbons Hydrocarbons belong to a group of compounds called the organic compounds. This ability of carbon to form big molecules has enabled nature to form living matter, from simple cells to complex tissues. Gasoline or petrol is a mixture of hydrocarbons and is used primarily as fuel in internal combustion engines of cars.  Chapter 15 Hydrocarbons Bonding ability of carbon Carbon atoms are able to form a few types of covalent bonds with other carbon atoms or atoms of other elements: Single bonds: C-C, C-H, C-O, C-X (X = halogens) Double bonds: C=C, C=O Triple bonds: C≡ C, C≡N Organic compounds can be classified as unbranched, branched or ring compounds. There are five types of formulae which can be used to describe and identify organic compounds: 1. Molecular formula 2. Empirical formula 3. Full structural or displayed formula 4. Condensed structural formula 5. General formula  Chapter 15 Hydrocarbons Straight (unbranched) Chain Compounds Straight-chain molecules that are unbranched may contain single, double or triple bonds. E.g.: Single bond Double bonds Triple bonds Chapter 15 Hydrocarbons Branched- Chain Compounds Branched chain compounds consist of one or more carbons of a straight chain compound forming bonds with more than two carbons.  Chapter 15 Hydrocarbons Homologous Series A homologous series is a group or family of organic compounds that has the following characteristics: - They have the same general formula. - They show similar chemical properties because they have the same functional group. - Each member differs from the previous one by -CH2. - They show a gradual change in density, viscosity, flammability, melting point and boiling point. A functional group is a group of atoms that is responsible for the characteristic chemical properties of a homologous series. Alcohols have the –OH functional group, carboxylic acids have the –COOH functional group and alkenes contain a C=C bond. Chapter 15 Hydrocarbons Structural Isomerism Isomers are organic compounds with the same molecular formula but different structural formulae. Isomers that are in the same homologous series have similar chemical properties but differ slightly in melting points and boiling points. Isomers of butane (C4H10) Chapter 15 Hydrocarbons Alkane Homologous Series Alkanes have the general formula CnH2n+2, Their names end with “ane”. The carbon atoms are joined by single covalent bonds. Each member in the homologous series differs from the next one by -CH2. No. of Chemical name Molecular Relative Boiling carbon formula molecular point / ° atoms mass C 1 Methane CH4 16 –162 2 Ethane C2H6 30 –89 3 Propane C3H8 44 –42 4 Butane C4H10 58 –0.5 5 Pentane C5H12 72 36 Chapter 15 Hydrocarbons Structure of Alkanes Alkanes are made up of covalent molecules. Each carbon atom is joined to four other atoms by single covalent bonds. The structure of a methane molecule is drawn as Name of alkane Molecular formula Structural formula Methane CH4 Ethane C2H6 Propane C3H8 Butane C4H10 Pentane C5H12 Chapter 15 Hydrocarbons Chemical Properties of Alkanes The alkanes are generally unreactive due to their saturated carbon-carbon bonds. However, they can undergo two types of chemical reactions. Combustion of Alkanes Alkanes undergo combustion in air or oxygen to form carbon dioxide and water vapour. The following two equations illustrate this. CH4(g) + 2O2(g) 🡪 CO2(g) + 2H2O(g) 2C2H6(g) + 7O2(g) 🡪 4CO2(g) + 6H2O(g) Chapter 15 Hydrocarbons Chemical Properties of Alkanes Substitution Reactions with Halogens Alkanes react with halogens, such as fluorine, chlorine and bromine under UV light or heat to form a mixture of halogenoalkanes. E.g. methane reacts with chlorine as follows: CH4 + Cl2 🡪 CH3Cl + HCl CH3Cl + Cl2 🡪 CH2Cl2 + HCl CH2Cl2 + Cl2 🡪 CHCl3 + HCl CHCl3 + Cl2 🡪 CCl4 + HCl This type of reaction in which the hydrogen atom of the hydrocarbon is replaced by the halogen atom is called a substitution reaction. Chapter 15 Hydrocarbons Uses of Alkanes and Their Derivatives Positive Combustion of alkanes are highly exothermic, so they are widely used as fuels. Natural gas (mainly methane) – used in power stations, factories and more recently as LNG (Liquefied Natural Gas) for running motor vehicles. Liquefied petroleum gas (LPG) – used for cooking and running motorcars. Substituted alkanes such as chlorofluorocarbons or CFCs are non-flammable and not very toxic – used as refrigerants, propellants for aerosols, making polystyrene or polyurethane foam, solvents for dry cleaning and for general degreasing purposes. Negative Unfortunately, CFCs are largely responsible for destroying the ozone layer. The carbon-chlorine bonds break to generate chlorine free radicals, which destroy the ozone layer. CFCs are now being replaced by more environmentally friendly compounds.  Chapter 15 Hydrocarbons Quick Check 1 1. Consider the two isomers of pentane (an alkane after butane in the homologous series) shown below. Are the following molecules also isomers of pentane? Explain why. 2. What is the chemical formula of an alkane with (a) eight carbon atoms; (b) 20 carbon atoms? Solution  Chapter 15 Hydrocarbons Solutions to Quick Check 1 1. Yes, because they have the same molecular formula (C5H12) but different structural formula. 2. (a) C8H18 (b) C20H42 Return  Chapter 15 Hydrocarbons Alkene Homologous Series The alkenes are another family of hydrocarbons, ie. they contain hydrogen and carbon only. All alkenes share the following characteristics: They have the general formula CnH2n, where n is an integer. Their names end with ‘ene’. There is a carbon-carbon double bond present in the molecule. They are said to be unsaturated as other atoms can be added to the molecule when the carbon-carbon double bond opens up.  Chapter 15 Hydrocarbons Alkene Homologous Series Chemical Molecular Relative molecular Boiling point / ° name formula mass C Ethene C 2H 4 28 –104 Propene C 3H 6 42 –48 Butene C 4H 8 56 –6 Pentene C5H10 70 30.1 There is no organic compound with the formula CH2. Just like the alkanes, as the number of carbon atoms increases, the molecules become bigger and heavier. As the number of carbon atoms increases, the density, melting point and boiling point increases. The first three alkenes are gases, the next few are liquids and the rest are solids. Chapter 15 Hydrocarbons Structure of Alkenes Alkenes have a carbon-carbon double covalent bond. Name of Molecular Structural Formula alkene Formula Ethene C2H4 Propene C3H6 But-1-ene C4H8 (1-butene) Pent-1-ene C5H10 (1-pentene)  Chapter 15 Hydrocarbons Isomers of Alkenes For propene, if we place the C=C double bond on the third carbon atom, we will get a structure which is exactly the same as the original when it is rotated through 180°. Hence, there is no isomer for propene. It is possible for the C=C double bond to be in another position in the carbon chain forming new isomers. For example, butene has three isomers as shown below: (i) (ii) (iii)  Chapter 15 Hydrocarbons Chemical Properties of Alkenes The alkenes are much more reactive compared to alkanes. Alkenes have an unsaturated carbon-carbon double bond which can be easily broken and other atoms can be added to form a molecule with single bonds. Alkenes take part in many important chemical reactions and provide a basis for the manufacturing of many important materials and chemicals in the industry.  Chapter 15 Hydrocarbons Chemical Properties of Alkenes Combustion of Alkenes Alkenes undergo combustion in air or oxygen to form carbon dioxide and water vapor. Alkenes do not burn as well as the alkanes and generally produce a more sooty flame. Due to an insufficient supply of air, the combustion is incomplete and carbon monoxide and soot are produced. The complete combustion reactions of alkenes: C2H4(g) + 3O2(g) 🡪 2CO2(g) + 2H2O(g) 2C3H6(g) + 9O2(g) 🡪 6CO2(g) + 6H2O(g)  Chapter 15 Hydrocarbons Chemical Properties of Alkenes Addition Reactions with Halogens Alkenes react readily with halogens to form haloalkanes. The reaction takes place quickly, without the need of UV light or catalyst. E.g. Ethene reacts with chlorine to form dichloroethane. C 2H 4 + Cl2 🡪 C2H4Cl2 ethene chlorine 1,2-dichloroethane This type of reaction in which the chlorine atoms are added across the C=C double bond is called an addition reaction. 1,2-dichloroethane  Chapter 15 Hydrocarbons Chemical Properties of Alkenes Test for unsaturation Alkenes react with aqueous bromine (bromine water) and cause the bromine solution to turn from brown to colourless. This reaction can be used as a test for unsaturation of any organic compound. The equation for the reaction is: C2H4 + Br2 🡪 C2H4Br2 ethene bromine 1,2-dibromoethane  Chapter 15 Hydrocarbons Chemical Properties of Alkenes Addition reaction with steam (hydration) Alkenes react with water (in the form of steam) and in the presence of a catalyst to form alcohols. E.g.ethene reacts with steam at 300 °C and with phosphoric(V) acid as catalyst, to form ethanol. C2H4(g) + H2O(g) 🡪 C2H5OH(l) ethene water vapour ethanol It is a very important reaction used for the manufacture of ethanol in the industry. The ethanol produced is usually mixed with a little methanol to make it undrinkable and the mixture is used as ‘industrial alcohol’.  Chapter 15 Hydrocarbons Chemical Properties of Alkenes Addition reaction with hydrogen (hydrogenation) Alkenes react with hydrogen in the presence of the catalyst nickel at a moderately high temperature to form alkanes. E.g. C2H4(g) + H2(g) 🡪 C2H6(g) ethene hydrogen ethane The addition of hydrogen to carbon-carbon double bonds is used in the industry to change vegetable oils into fats, which can be used for making margarine. When hydrogen is added to the unsaturated vegetable oil, the melting point of the oil increases, and the oil becomes solid (fats). Chapter 15 Hydrocarbons Uses of Alkenes Alkenes are very versatile in their uses and are the starting materials for various chemicals. E.g. ethene and propene are the starting materials for polymers like poly(ethene), poly(propene), polyvinylchloride (PVC) and Teflon etc. Alkenes are also used for the manufacture of industrial chemicals such as alcohols and aldehydes. The lower alkenes can be used as fuel and can be obtained by the cracking of kerosene or petrol.  Chapter 15 Hydrocarbons Quick Check 2 The diagram below shows the structural formula of an organic compound. (a) Name the organic compound. (b) Name the homologous series to which this compound belongs. Solution Chapter 15 Hydrocarbons Solutions to Quick Check 2 (a) Propene (b) Alkene Return  Chapter 15 Hydrocarbons Polyunsaturated Food Unsaturated hydrocarbons contain carbon-carbon double bonds. Polyunsaturated oils are oils that contain two or more C=C bonds in the molecules. Due to the geometry of these C=C double bonds, the molecules are harder to pack closely together and thus are liquids at room temperature. Saturated oils which are mostly straight-chain molecules are more easily packed and hence, they exist as solids at room temperature.  Chapter 15 Hydrocarbons Polyunsaturated Food Saturated fats like animal fats and oils, butter and cream are unhealthy in the diet as they raise the blood cholesterol of a person. Polyunsaturated oils and fats are healthier as they do not clot the blood vessels and nor raise the blood cholesterol level. E.g. polyunsaturated oils are found in sunflower oil, palm oil, fish oil, soya oil, linseed oil and nuts.