Presentation on theme: "KNOCKHARDY PUBLISHING"— Presentation transcript:
1 KNOCKHARDY PUBLISHING AN INTRODUCTION TOTHE CHEMISTRYOF ALKANESKNOCKHARDY PUBLISHING
2 KNOCKHARDY PUBLISHING THE CHEMISTRY OF ALKANES INTRODUCTIONThis Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...Navigation is achieved by...either clicking on the grey arrows at the foot of each pageor using the left and right arrow keys on the keyboard
3 THE CHEMISTRY OF ALKANES CONTENTSStructure of alkanesPhysical properties of alkanesChemical properties of alkanesBreaking covalent bondsChlorination via free radical substitutionCrackingRevision check list
4 THE CHEMISTRY OF ALKANES Before you start it would be helpful to… Recall the definition of a covalent bondBe able to balance simple equationsBe able to write out structures for hydrocarbons
5 ALKANES General members of a homologous series general formula is CnH2n for non-cyclic alkanessaturated hydrocarbons - all carbon-carbon bonding is singlebonds are spaced tetrahedrally about carbon atoms.Isomerism the first example of structural isomerism occurs with C4H10BUTANE METHYLPROPANEStructural isomers have the SAME MOLECULAR FORMULA BUTDIFFERENT STRUCTURAL FORMULAThey possess different physical properties such as boiling point,melting point and density
6 HYBRIDISATION OF ORBITALS 11s22s2pThe electronic configuration of a carbon atom is 1s22s22p2
7 HYBRIDISATION OF ORBITALS 11s22s2pThe electronic configuration of a carbon atom is 1s22s22p2If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p311s22s2pThe process is favourable because the of arrangement of electrons; four unpaired and with less repulsion is more stable
8 HYBRIDISATION OF ORBITALS IN ALKANES The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent.Because one s and three p orbitals are used, it is called sp3 hybridisation2s22p22s12p34 x sp3
9 THE STRUCTURE OF ALKANES In ALKANES, the four sp3 orbitals of carbon repel each other into a TETRAHEDRAL arrangement with bond angles of 109.5º.Each sp3 orbital in carbon overlaps with the 1s orbital of a hydrogen atom to form a C-H bond.109.5º
10 PHYSICAL PROPERTIES OF ALKANES Boiling point increases as they get more carbon atoms in their formulamore atoms = greater intermolecular Van der Waals’ forcesgreater intermolecular force = more energy to separate the moleculesgreater energy required = higher boiling pointCH4 (-161°C) C2H6 (-88°C) C3H8 (-42°C) C4H10 (-0.5°C)difference gets less - mass increases by a smaller percentagestraight chains molecules have greater interaction than branched“The greater the branching, the lower the boiling point”Melting point general increase with molecular massthe trend is not as regular as that for boiling point.Solubility alkanes are non-polar so are immiscible with waterthey are soluble in most organic solvents.
11 CHEMICAL PROPERTIES OF ALKANES Introduction - fairly unreactive; (old family name, paraffin, meant little reactivity)- have relatively strong, almost NON-POLAR, SINGLE covalent bonds- they have no real sites that will encourage substances to attack themCombustion - make useful fuels - especially the lower members of the series- react with oxygen in an exothermic reactioncomplete CH4(g) O2(g) ——> CO2(g) H2O(l)combustionincomplete CH4(g) ½O2(g) ——> CO(g) H2O(l)the greater the number of carbon atoms, the more energy producedBUT the greater the amount of oxygen needed for complete combustion.Handy tip When balancing equations involving complete combustion, remember...every carbon in the original hydrocarbon gives one carbon dioxide andevery two hydrogen atoms gives a water molecule.Put the numbers into the equation, count up the O’s and H’s on the RHSof the equation then balance the oxygen molecules on the LHS.
12 Processes involving combustion give rise to a variety of pollutants... POLLUTIONProcesses involving combustion give rise to a variety of pollutants...power stations SO2 emissions produce acid raininternal combustion engines CO, NOx and unburnt hydrocarbonsRemovalSO2 react effluent gases with a suitable compound (e.g. CaO)CO and NOx pass exhaust gases through a catalytic converterCatalytic convertersIn the catalytic converter ... CO is converted to CO2NOx are converted to N2Unburnt hydrocarbons are converted to CO2 and H2Oe.g. 2NO CO ———> N CO2• catalysts are made of finely divided rare metals Rh, Pd, Pt• leaded petrol must not pass through the catalyst as the leaddeposits on the catalyst’s surface and “poisons” it, thus blockingsites for reactions to take place.
13 BREAKING COVALENT BONDS There are 3 ways to split the shared electron pair in an unsymmetrical covalent bond.UNEQUAL SPLITTINGproduces IONSknown as HETEROLYSIS orHETEROLYTIC FISSIONEQUAL SPLITTINGproduces RADICALSknown as HOMOLYSIS orHOMOLYTIC FISSION• If several bonds are present the weakest bond is usually broken first• Energy to break bonds can come from a variety of energy sources - heat / light• In the reaction between methane and chlorine either can be used, however...• In the laboratory a source of UV light (or sunlight) is favoured.
14 FREE RADICALS TYPICAL PROPERTIES • reactive species (atoms or groups) which possess an unpaired electron• their reactivity is due to them wanting to pair up the single electron• formed by homolytic fission (homolysis) of covalent bonds• formed during the reaction between chlorine and methane• formed during thermal cracking• involved in the reactions taking place in the ozone layer
15 CHLORINATION OF METHANE Reagents chlorine and methaneConditions UV light or sunlight - heat is an alternative energy sourceEquation(s) CH4(g) Cl2(g) ——> HCl(g) CH3Cl(g) chloromethaneCH3Cl(g) + Cl2(g) ——> HCl(g) CH2Cl2(l) dichloromethane CH2Cl2(l) + Cl2(g) ——> HCl(g) CHCl3(l) trichloromethane CHCl3(l) Cl2(g) ——> HCl(g) CCl4(l) tetrachloromethaneMixtures free radicals are very reactive - they are trying to pair their electronwith sufficient chlorine, every hydrogen will eventually be replaced.
16 CHLORINATION OF METHANE Reagents chlorine and methaneConditions UV light or sunlight - heat is an alternative energy sourceEquation(s) CH4(g) Cl2(g) ——> HCl(g) CH3Cl(g) chloromethaneCH3Cl(g) + Cl2(g) ——> HCl(g) CH2Cl2(l) dichloromethane CH2Cl2(l) + Cl2(g) ——> HCl(g) CHCl3(l) trichloromethane CHCl3(l) Cl2(g) ——> HCl(g) CCl4(l) tetrachloromethaneMixtures free radicals are very reactive - they are trying to pair their electronwith sufficient chlorine, every hydrogen will eventually be replaced.Mechanism Mechanisms portray what chemists think is going on in the reaction,whereas an equation tells you the ratio of products and reactants.Chlorination of methane proceeds via FREE RADICAL SUBSTITUTION because the methane is attacked by free radicals resulting inhydrogen atoms being substituted by chlorine atoms.The process is a chain reaction.In the propagation step, one radical is produced for each one used
17 CHLORINATION OF METHANE The single dots represent UNPAIRED ELECTRONS Initiation Cl2 ——> 2Cl• RADICALS CREATEDThe single dots represent UNPAIRED ELECTRONSDuring initiation, the WEAKEST BOND IS BROKEN as it requires less energy.There are three possible bonds in a mixture of alkanes and chlorine.Average bond enthalpy kJ mol-1The Cl-Cl bond is broken in preference to the others as it is the weakest and requires requires less energy to separate the atoms.
18 CHLORINATION OF METHANE Propagation Cl• + CH4 ——> CH3• HCl RADICALS USED andCl2 + CH3• ——> CH3Cl Cl• then RE-GENERATEDFree radicals are very reactive because they want to pair up their single electron.They do this by abstracting a hydrogen atom from methane; a methyl radical is formedThe methyl radical is also very reactive and attacks a chlorine moleculeA chlorine radical is produced and the whole process can start over again
19 CHLORINATION OF METHANE Termination l• + Cl• ——> Cl2 RADICALS REMOVEDCl• + CH3• ——> CH3ClCH3• + CH3• ——> C2H6Removing the reactive free radicals brings an end to the reaction.This is not very likely at the start of the reaction because of their low concentration.
20 CHLORINATION OF METHANE OVERVIEWInitiation Cl2 ——> 2Cl• radicals createdPropagation Cl• CH ——> CH3• HCl radicals used andCl CH3• ——> CH3Cl Cl• then re-generatedTermination Cl• + Cl• ——> Cl2 radicals removedCl• + CH3• ——> CH3ClCH3• + CH3• ——> C2H6SummaryDue to lack of reactivity, alkanes need a very reactive species to persuade them to reactFree radicals need to be formed by homolytic fission of covalent bondsThis is done by shining UV light on the mixture (heat could be used)Chlorine radicals are produced because the Cl-Cl bond is the weakestYou only need one chlorine radical to start things offWith excess chlorine you get further substitution and a mixture of chlorinated products
21 CHLORINATION OF METHANE RADICALSPRODUCEDInitiationPropagationTerminationRADICALS USEDAND REGENERATEDRADICALSREMOVED
22 CHLORINATION OF METHANE Furtherpropagation If excess chlorine is present, further substitution takes placeThe equations show the propagation steps for the formation of...dichloromethane Cl• + CH3Cl ——> CH2Cl• HClCl2 + CH2Cl• ——> CH2Cl Cl•trichloromethane Cl• + CH2Cl2 ——> CHCl2• + HClCl CHCl2• ——> CHCl Cl•tetrachloromethane Cl• + CHCl3 ——> CCl3• + HClCl CCl3• ——> CCl Cl•Mixtures Because of the many possible reactions there will be a mixture of products.Individual haloalkanes can be separated by fractional distillation.
23 CRACKING THERMAL Converts heavy fractions into higher value products Involves the breaking of C-C bonds in alkanesConverts heavy fractions into higher value productsTHERMAL proceeds via a free radical mechanismCATALYTIC proceeds via a carbocation (carbonium ion) mechanismTHERMALHIGH PRESSURE kPaHIGH TEMPERATURE °C to 900°CFREE RADICAL MECHANISMHOMOLYTIC FISSIONPRODUCES MOSTLY ALKENES e.g. ETHENE for making polymers and ethanolPRODUCES HYDROGEN ... used in the Haber Process and in margarine manufactureBonds can be broken anywhere in the molecule by C-C bond fission or C-H bond fission
24 CRACKING CATALYTIC Converts heavy fractions into higher value products Involves the breaking of C-C bonds in alkanesConverts heavy fractions into higher value productsTHERMAL proceeds via a free radical mechanismCATALYTIC proceeds via a carbocation (carbonium ion) mechanismCATALYTICSLIGHT PRESSUREHIGH TEMPERATURE °CZEOLITE CATALYSTCARBOCATION (IONIC) MECHANISMHETEROLYTIC FISSIONPRODUCES BRANCHED AND CYCLIC ALKANES, AROMATIC HYDROCARBONSUSED FOR MOTOR FUELSZEOLITES are crystalline aluminosilicates; clay like substances
25 What should you be able to do? REVISION CHECKWhat should you be able to do?Recall and explain the physical properties of alkanesRecall the use of alkanes as fuelsRecall and explain the different ways to break a covalent bondWrite balanced equations representing combustion and chlorinationUnderstand the conditions and mechanism of free radical substitutionRecall the conditions and products from thermal and catalytic crackingCAN YOU DO ALL OF THESE? YES NO
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27 Try some past paper questions WELL DONE!Try some past paper questions
29 PETROCHEMICALSIn the past, most important organic chemicals were derived from coal.Nowadays, natural gas and crude petroleum provide an alternative source.the composition of crude petroleum varies according to its sourceit is a dark coloured, viscous liquidconsists mostly of alkanes with up to 40 carbon atoms, plus water, sulphur and sandcan be split up into fractions by fractional distillationdistillation separates the compounds according to their boiling pointat each level a mixture of compounds in a similar boiling range is taken offrough fractions can then be distilled further to obtain narrower boiling rangessome fractions are more important - usually the lower boiling point oneshigh boiling fractions may be broken down into useful lower boiling ones - CRACKINGCONTENTS
30 PETROCHEMICALS Boiling C’s per Name of Use(s) range / °C molecule fraction< LPG (Liquefied Calor GasPetroleum Gas) Camping GasGASOLINE PetrolNAPHTHA PetrochemicalsKEROSINE Aviation FuelGAS OIL Central Heating Fuel> LUBRICATING OIL Lubrication Oil> FUEL OIL Power Station FuelShip Fuel> WAX, GREASE CandlesGrease for bearings> > 50 BITUMEN Road surfacesRoofingCONTENTS