Alkanes AS

Fractional Distillation: Industrially • Oil is pre-heated • then passed into column. • The fractions condense at different heights • The temperature of column decreases upwards • The separation depends on boiling point. • Boiling point depends on size of molecules. • The larger the molecule the larger the van der waals forces • Similar molecules (size, bp, mass) condense together • Small molecules condense at the top at lower temperatures • and big molecules condense at the bottom at higher temperatures. Key points to learn This is a physical process involving the splitting of weak van der waals forces between molecules Vacuum distillation unit • Heavy residues from the fractionating column are distilled again under a vacuum. • Lowering the pressure over a liquid will lower its boiling point. Vacuum distillation allows heavier fractions to be further separated without high temperatures which could break them down. Petroleum is a mixture consisting mainly of alkane hydrocarbons Petroleum fraction: mixture of hydrocarbons with a similar chain length and boiling point range N Goalby chemrevise.org 1 naptha (chemicals) fuel oil bitumen lubricating oils diesel oil kerosene (jet fuel) fuel gas (bottled) petrol/gasoline 20° C 40° C

Economic reasons for cracking • The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions. • To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked. • The products of cracking are more valuable than the starting materials (e.g. ethene used to make poly(ethene), branched alkanes for motor fuels, etc.) Cracking: conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds High Mr alkanes  smaller Mr alkanes+ alkenes + (hydrogen) This is a chemical process involving the splitting of strong covalent bonds so requires high temperatures. C. There are two main types of cracking: thermal and catalytic. They need different conditions and are used to produce different products produces mostly alkenes e.g. ethene used for making polymers and ethanol sometimes produces hydrogen used in the Haber Process and in margarine manufacture Thermal Cracking Conditions: High Pressure (7000 kPa) High Temperature (400°C to 900°C) Bonds can be broken anywhere in the molecule by C-C bond fission and C-H bond fission. C8H18  C6H14 + C2H4 C12H26  C10H22 + C2H4. d Conditions: Low pressure High Temperature (450°C) Zeolite Catalyst Catalytic Cracking Produces branched and cyclic alkanes and Aromatic hydrocarbons Used for making motor fuels Branched and cyclic hydrocarbons burn more cleanly and are used to give fuels a higher octane number. Cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used

COMBUSTION Alkanes readily burn in the presence of oxygen. This combustion of alkanes is highly exothermic, explaining their use as fuels. Complete Combustion C8H18(g) + 12.5 O2 (g)  8CO2 (g) + 9 H2O(l) Fuel : releases heat energy when burnt Incomplete combustion produces less energy per mole than complete combustion If there is a limited amount of oxygen then incomplete combustion occurs, producing CO (which is very toxic) and/or C (producing a sooty flame) CH4 (g) + 3 /2 O2 (g)  CO(g) + 2 H2O(l) CH4 (g) + O2 (g)  C(s) + 2 H2O(l) The products of complete combustion are CO2 and H2O. In excess oxygen alkanes will burn with complete combustion Incomplete Combustion Carbon (soot) can cause global dimming- reflection of the sun’s light. Pollution from Combustion SO2 can be removed from the waste gases from furnaces (e.g. coal fired power stations) by flue gas desulphurisation. The gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulphur dioxide in a neutralisation reaction Sulphur containing impurities are found in petroleum fractions which produce SO2 when they are burned. Coal is high in sulphur content, and large amounts of sulphur oxides are emitted from power stations. S+ O2  SO2 The calcium sulphite which is formed can be used to make calcium sulphate for plasterboard. SO2 + CaO  CaSO3 CH3SH+ 3O2  SO2 + CO2 + 2H2O SO2 will dissolve in atmospheric water and can produce acid rain. Pollutant Environmental consequence Nitrogen oxides (formed when N2 in the air reacts at the high temperatures and spark in the engine) NO is toxic and can form acidic gas NO2 NO2 is toxic and acidic and forms acid rain Carbon monoxide toxic Carbon dioxide Contributes towards global warming Unburnt hydrocarbons (not all fuel burns in the engine) Contributes towards formation of smog soot Global dimming and respiratory problems Catalytic converters These remove CO, NOx and unburned hydrocarbons (e.g. octane, C8H18) from the exhaust gases, turning them into ‘harmless’ CO2 , N2 and H2O. 2 CO + 2 NO  2 CO2 + N2 C8H18 + 25 NO  8 CO2 + 12½ N2 + 9 H2O Converters have a ceramic honeycomb coated with a thin layer of catalyst metals Platinum, Palladium, Rhodium – to give a large surface area. Global warming Carbon dioxide (CO2 ), methane (CH4 ) and water vapour (H2O) are all greenhouse gases. (They trap the Earth’s radiated infra red energy in the atmosphere). Water is the main greenhouse gas (but is natural), followed by carbon dioxide and methane. The Earth is thought to be getting warmer, and many scientists believe it is due to increasing amounts of greenhouse gases in the atmosphere. Carbon dioxide levels have risen significantly in recent years due to increasing burning of fossil fuels. Carbon dioxide is a particularly effective greenhouse gas and its increase is thought to be largely responsible for global warming. Nitrogen oxides form from the reaction between N2 and O2 inside the car engine. The high temperature and spark in the engine provides sufficient energy to break strong N2 bond N2 + O2  2NO N2 + 2O2  2NO2. Nitrogen Oxides NOx

Synthesis of chloroalkanes Reaction of alkanes with bromine / chlorine in UV light In the presence of UV light alkanes react with chlorine to form a mixture of products with the halogens substituting hydrogen atoms. In general, alkanes do not react with many reagents. This is because the C-C bond and the C-H bond are relatively strong Overall Reaction CH4 + Cl2  CH3Cl + HCl methane chloromethane This is the overall reaction, but a more complex mixture of products is actually formed To understand this reaction fully we must look in detail at how it proceeds step by step. This is called its mechanism The MECHANISM for this reaction is called a FREE RADICAL SUBSTITUTION It proceeds via a series of steps: STEP ONE: Initiation STEP TWO: Propagation STEP THREE: Termination STEP ONE Initiation Cl2  2Cl. Essential condition: UV light The UV light supplies the energy to break the Cl-Cl bond. It is broken in preference to the others as it is the weakest. The bond has broken in a process called homolytic fission. each atom gets one electron from the covalent bond When a bond breaks by homolytic fission it forms Free Radicals. Free Radicals do not have a charge and are represented by a DEFINITION A Free Radical is a reactive species which possess an unpaired electron CH4 + Cl.  HCl + .CH3 STEP TWO Propagation .CH3 + Cl2  CH3Cl + Cl. The chlorine free radicals are very reactive and remove an H from the methane leaving a methyl free radical The methyl free radical reacts with a Cl2 molecule to produce the main product and another Cl free radical All propagation steps have a free radical in the reactants and in the products. As the Cl free radical is regenerated, it can react with several more alkane molecules in a CHAIN REACTION STEP THREE Termination .CH3 + Cl .  CH3Cl .CH3 + .CH3  CH3CH3 Collision of two free radicals does not generate further free radicals: the chain is TERMINATED. Minor step leading to impurities of ethane in product. Write this step using structural formulae and don’t use molecular formulae N Goalby chemrevise.org 4 Applying the mechanism to other alkanes The same mechanism is used: Learn the patterns in the mechanism STEP ONE Initiation Br2  2Br . Essential condition: UV light Example: Write mechanism of Br2 and Propane Br2 splits in the same way as Cl2 CH3CH2CH3 + Br.  HBr + CH3CH2CH2 . STEP TWO Propagation CH3CH2CH2 . + Br2  CH3CH2CH2Br + Br. Remove one H from the alkane to produce a radical To the radical produced in the previous step add a Br STEP THREE Termination CH3CH2CH2 . + Br.  CH3CH2CH2Br CH3CH2CH2 . + CH3CH2CH2 .  CH3CH2CH2CH2CH2CH3 5 N Goalby chemrevise.org Further substitution Excess Cl2 present will promote further substitution and could produce CH2Cl2 , CHCl3 and CCl4 These reactions could occur CH3Cl + Cl2  CH2Cl2 + HCl CH2Cl2 + Cl2  CHCl3 + HCl CHCl3 + Cl2  CCl4 + HCl CH3Cl + Cl.  HCl + .CH2Cl . CH2Cl + Cl2  CH2Cl2 + Cl . Example propagation steps that would lead to further substitution You should be able to write overall reaction equations for various reactions Example 1. Write the overall reaction equation for the formation of CCl4 from CH4 + Cl2 CH4 + 4 Cl2  CCl4 + 4 HCl Example 2. Write the overall reaction equation for the formation of CFCl3 from CH3F + Cl2 CH3F + 3 Cl2  CFCl3 + 3 HCl Note HCl is always the side product – never H2