New Town Secondary School Alkanes and Alkenes 2023

Page 1 New Town Secondary School Secondary 4E Pure Chemistry Name: ( ) Class: Date: Organic Chemistry – Alkanes and Alkenes Syllabus At the end of this topic, students should be able to: 11.2b Describe the alkanes as a homologous series of saturated hydrocarbons with the general formula CnH2n+2. 11.2c Draw the structures of branched and unbranched alkanes, C1 to C4, and name the unbranched alkanes methane to butane. 11.2d Define isomerism and identify isomers. 11.2e Describe the properties of alkanes (exemplified by methane) as being generally unreactive except in terms of combustion and substitution by chlorine. 11.3a Describe the alkenes as a homologous series of unsaturated hydrocarbons with the general formula CnH2n. 11.3b Draw the structures of branched and unbranched alkenes, C2 to C4, and name the unbranched alkenes ethene to butene. 11.3c Describe the manufacture of alkenes and hydrogen by cracking hydrocarbons and recognise that cracking is essential to match the demand for fractions containing smaller molecules from the refinery process. 11.3d Describe the difference between saturated and unsaturated hydrocarbons from their molecular structures and by using aqueous bromine. 11.3e Describe the properties of alkenes (exemplified by ethene) in terms of combustion, polymerisation and the addition reactions with bromine, steam and hydrogen. 11.3f State the meaning of polyunsaturated when applied to food products. 11.3g Describe the manufacture of margarine by the addition of hydrogen to unsaturated vegetable oils to form a solid product.

Page 2 Alkanes Alkanes are saturated hydrocarbons with the general formula of CnH2n+2. This means that each alkane molecule contains only carbon and hydrogen atoms, and each carbon aom is covalently bonded to four other atoms. As with all homologous series, the physical properties of individual members changes with increasing mass/length of carbon chain. Name Formula Melting Point (°C) Boiling Point (°C) State at r.t.p. Density (g/cm3) Flammable Methane CH4 – 182 – 162 Gas 0.424 Most Ethane C2H6 – 183 – 89 Gas 0.546 Propane C3H8 – 188 – 42 Gas 0.584 Butane C4H10 – 138 0 Gas 0.601 Pentane C5H12 – 130 36 Liquid 0.626 Hexane C6H14 – 95 69 Liquid 0.659 Heptane C7H16 – 91 98 Liquid 0.684 Octane C8H18 – 57 126 Liquid 0.703 Nonane C9H20 – 54 151 Liquid 0.718 Decane C10H22 – 30 174 Liquid 0.730 Least Isomerism In organic chemistry, molecules with the same molecular formula but different structural formula are known as structural isomers. Types of structural isomers • Organic molecules with the same molecular formula but differ in their structure with respect to the position of the functional group. (Positional isomers) • Organic molecules with the same molecular formula but differ in their structure with respect to the length of the longest carbon chain. (Chain Isomers) Positional Isomers Propanol (C3H7OH) Propan-1-ol Propan-2-ol Propan-1-ol Number of positional isomers for propanol: HCCCO H H H H H H H H C C C H H H H O H H H H C C C O H H H H H H H

Page 3 Chain Isomers C4H10 Butane 2 - Methylpropane Unbranched C4H10 with all carbon atoms joined up in a row Branched alkane since it has a branch of a side chain –CH3 Boiling point = – 0.5 °C Boiling point = – 11.7 °C Naming isomers Name of organic molecule: 2,3–dimethylbutane 2,3 – di methyl butane Position of substituent / side chain Number of (same) substituent / side chain Name of substituent / side chain Name of longest straight chain Side Chain (alkyl group) / Substituent methyl – CH3 chloro – Cl ethyl – CH2CH3 bromo – Br propyl – CH2CH2CH3 iodo – I butyl – CH2CH2CH2CH3 H C C C C H H H H H H H H H H C C C H H H H H H CH H H H C C H C H H CH H H H H H H C C C H H H H H H CH H H H C C C C H H H C H CH H H H H H H H H

Page 4 Reactions of Alkane Because alkanes are saturated, where the carbon–carbon single (C–C) bonds and the carbon–hydrogen (C–H) bonds are strong, alkanes are generally unreactive. However, alkanes can undergo three types of reactions – combustion, substitution and cracking. Combustion Combustion is a chemical reaction whereby a reactant is burnt in the presence of air. Oxygen in the air reacts with the reactant to produce carbon dioxide and water in the form of steam. Combustion of alkanes are highly exothermic. For alkanes, the combustion reaction is represented by the general equations below: word equation: alkane + oxygen gas ® carbon dioxide + steam chemical equation: CxHy + (x + y4 ) O2 ® x CO2 + y2 H2O In the case of incomplete combustion, when the reactant is burnt in insufficient oxygen, soot and carbon monoxide may also be produced. Substitution In the presence of light as the catalyst, alkanes can react with chlorine gas. Each hydrogen atom in the alkane molecule will be substituted one at a time by one chlorine atom. If left for a long time, all the hydrogen atoms will eventually be substituted. Using methane as an example, the following substitution occurs in the presence of light: Step 1: + uv light + chloromethane Step 2: + uv light + dichloromethane Step 3: + uv light + trichloromethane Step 4: + uv light + tetrachloromethane HC H H H Cl Cl HC H H Cl H Cl HC H H Cl Cl Cl HC H Cl Cl H Cl HC H Cl Cl Cl Cl Cl C H Cl Cl H Cl Cl C H Cl Cl Cl Cl Cl C Cl Cl Cl H Cl

Page 5 Cracking Smaller alkanes are useful because they can burn easily. When the size and mass of the molecules increases, they become more difficult to burn, and are therefore not as efficient as fuels. Cracking allows for the breaking down of large alkane molecules to produce smaller useful molecules like alkenes and smaller alkanes or hydrogen gas. It is carried out by heating the molecule to a high temperature (600 °C) and passing it over a hot catalyst (Al2O3 or SiO2). There are two different reaction pathways resulting from cracking. This lead to the formation of different products. Pathway I (form alkane) Pathway II (form hydrogen gas) Large Alkane ® Alkenes + Smaller Alkanes Large Alkane ® Alkenes + Hydrogen Gas C10H22 ® C5H10 + C5H12 C10H22 ® 2 C5H10 + H2 Cracking is important because it can cater to the demand for smaller molecule fractions (for petrol) from the refinery process. It is also important because it produces hydrogen and alkenes. Hydrogen is required in the Haber Process while alkenes are starting materials to make ethanol and plastics.

Page 6 Alkenes Alkenes are UNsaturated hydrocarbons with the general formula of CnH2n. This means that each alkene molecule contains at least one carbon-carbon double covalent bond. If the alkene molecule contains more than one carbon-carbon double bond, we say that the molecule is polyunsaturated. As with all homologous series, the physical properties of individual members changes with increasing mass/length of carbon chain. Reactions of Alkene Because alkenes are unsaturated, where each carbon atom is covalently bonded to only three other atoms, this allows for other atoms to be added to it and results in alkenes being more chemically reactive than alkanes. Alkenes undergoes combustion and addition reactions. Combustion Similar to alkanes, alkenes react with excess oxygen gas in air to produce carbon dioxide and steam as follows: word equation: alkene + oxygen gas ® carbon dioxide + steam chemical equation: CxHy + (x + y4 ) O2 ® x CO2 + y2 H2O However, alkenes burn with a sootier flame than alkanes with a similar number of carbon atoms as alkenes have a relatively higher percentage of carbon than corresponding alkenes. Less steam is produced in alkenes than alkanes of a similar number of carbon atoms. Try writing down both balanced equation for combustion of ethane and ethene respectively

Page 7 Addition The carbon-carbon double bond in alkenes are reactive and thus will readily undergo addition reactio