THE CHEMISTRY OF ARENES
اسلاید 1: THE CHEMISTRYOF ARENESA guide for A level studentsKNOCKHARDY PUBLISHING
اسلاید 2: 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...www.argonet.co.uk/users/hoptonj/sci.htmNavigation is achieved by...either clicking on the grey arrows at the foot of each page orusing the left and right arrow keys on the keyboardARENESKNOCKHARDY PUBLISHING
اسلاید 3: CONTENTS Prior knowledge Structure of benzene Thermodynamic stability Delocalisation Electrophilic substitution Nitration Chlorination Friedel-Crafts reactions Further substitutionARENES
اسلاید 4: Before you start it would be helpful to… know the functional groups found in organic chemistry know the arrangement of bonds around carbon atoms recall and explain electrophilic addition reactions of alkenesARENES
اسلاید 5: STRUCTURE OF BENZENEPrimary analysis revealed benzene had...an empirical formula of CH anda molecular mass of 78 andaformula of C6H6
اسلاید 6: STRUCTURE OF BENZENEPrimary analysis revealed benzene had...an empirical formula of CH anda molecular mass of 78aformula of C6H6Kekulé suggested that benzene was...PLANARCYCLIC andHAD ALTERNATING DOUBLE AND SINGLE BONDS
اسلاید 7: STRUCTURE OF BENZENEHOWEVER...• it did not readily undergo electrophilic addition - no true C=C bond• only one 1,2 disubstituted product existed• all six C—C bond lengths were similar; C=C bonds are shorter than C-C• the ring was thermodynamically more stable than expected
اسلاید 8: STRUCTURE OF BENZENEHOWEVER...• it did not readily undergo electrophilic addition - no true C=C bond• only one 1,2 disubstituted product existed• all six C—C bond lengths were similar; C=C bonds are shorter than C-C• the ring was thermodynamically more stable than expectedTo explain the above, it was suggested that the structure oscillatedbetween the two Kekulé forms but was represented by neither ofthem. It was a RESONANCE HYBRID.
اسلاید 9: THERMODYNAMIC EVIDENCE FOR STABILITYWhen unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
اسلاید 10: THERMODYNAMIC EVIDENCE FOR STABILITY23- 120 kJ mol-1When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.C6H10(l) + H2(g) ——> C6H12(l)When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
اسلاید 11: THERMODYNAMIC EVIDENCE FOR STABILITY23- 120 kJ mol-1Theoretical- 360 kJ mol-1(3 x -120)When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.C6H10(l) + H2(g) ——> C6H12(l)Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexaneC6H6(l) + 3H2(g) ——> C6H12(l)When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
اسلاید 12: THERMODYNAMIC EVIDENCE FOR STABILITY23Experimental- 208 kJ mol-1- 120 kJ mol-1Theoretical- 360 kJ mol-1(3 x -120)When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.C6H10(l) + H2(g) ——> C6H12(l)Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexaneC6H6(l) + 3H2(g) ——> C6H12(l)Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scaleWhen unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
اسلاید 13: THERMODYNAMIC EVIDENCE FOR STABILITY23MORE STABLE THAN EXPECTEDby 152 kJ mol-1Experimental- 208 kJ mol-1- 120 kJ mol-1Theoretical- 360 kJ mol-1(3 x -120)When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.C6H10(l) + H2(g) ——> C6H12(l)Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexaneC6H6(l) + 3H2(g) ——> C6H12(l)Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scaleIt is 152kJ per mole more stable than expected.This value is known as the RESONANCE ENERGY.When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
اسلاید 14: THERMODYNAMIC EVIDENCE FOR STABILITY23MORE STABLE THAN EXPECTEDby 152 kJ mol-1Experimental- 208 kJ mol-1- 120 kJ mol-1Theoretical- 360 kJ mol-1(3 x -120)When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.C6H10(l) + H2(g) ——> C6H12(l)Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexaneC6H6(l) + 3H2(g) ——> C6H12(l)Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scaleIt is 152kJ per mole more stable than expected.This value is known as the RESONANCE ENERGY.When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
اسلاید 15: HYBRIDISATION OF ORBITALS - REVISIONThe electronic configuration of a carbon atom is 1s22s22p211s22s2p
اسلاید 16: HYBRIDISATION OF ORBITALS - REVISIONThe electronic configuration of a carbon atom is 1s22s22p211s22s2pIf 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 of the arrangement of electrons; four unpaired and with less repulsion is more stable
اسلاید 17: HYBRIDISATION OF ORBITALS - REVISIONThe four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent.2s22p22s12p3 4 x sp3HYBRIDISEsp3 HYBRIDISATION
اسلاید 18: HYBRIDISATION OF ORBITALS - REVISIONAlternatively, only three orbitals (an s and two p’s) combine or HYBRIDISE to give three new orbitals. All three orbitals are equivalent. The remaining 2p orbital is unchanged.2s22p22s12p3 3 x sp22pHYBRIDISEsp2 HYBRIDISATION
اسلاید 19: In ALKANES, the four sp3 orbitals repel each other into a tetrahedral arrangement.In ALKENES, the three sp2 orbitals repel each other into a planar arrangement and the 2p orbital lies at right angles to themSTRUCTURE OF ALKENES - REVISION
اسلاید 20: Covalent bonds are formed by overlap of orbitals.An sp2 orbital from each carbon overlaps to form a single C-C bond.The resulting bond is called a SIGMA (δ) bond.STRUCTURE OF ALKENES - REVISION
اسلاید 21: The two 2p orbitals also overlap. This forms a second bond; it is known as a PI (π) bond. For maximum overlap and hence the strongest bond, the 2p orbitals are in line.This gives rise to the planar arrangement around C=C bonds.STRUCTURE OF ALKENES - REVISION
اسلاید 22: two sp2 orbitals overlap to form a sigma bond between the two carbon atomsORBITAL OVERLAP IN ETHENE - REVIEWtwo 2p orbitals overlap to form a pi bond between the two carbon atomss orbitals in hydrogen overlap with the sp2 orbitals in carbon to form C-H bondsthe resulting shape is planar with bond angles of 120º
اسلاید 23: STRUCTURE OF BENZENE - DELOCALISATIONThe theory suggested that instead of three localised (in one position) double bonds, the six p (p) electrons making up those bonds were delocalised (not in any oneparticular position) around the ring by overlapping the p orbitals. There would be nodouble bonds and all bond lengths would be equal. It also gave a planar structure.6 single bonds
اسلاید 24: STRUCTURE OF BENZENE - DELOCALISATION6 single bondsone way to overlapadjacent p orbitalsThe theory suggested that instead of three localised (in one position) double bonds, the six p (p) electrons making up those bonds were delocalised (not in any oneparticular position) around the ring by overlapping the p orbitals. There would be nodouble bonds and all bond lengths would be equal. It also gave a planar structure.
اسلاید 25: STRUCTURE OF BENZENE - DELOCALISATION6 single bondsone way to overlapadjacent p orbitalsanotherpossibilityThe theory suggested that instead of three localised (in one position) double bonds, the six p (p) electrons making up those bonds were delocalised (not in any oneparticular position) around the ring by overlapping the p orbitals. There would be nodouble bonds and all bond lengths would be equal. It also gave a planar structure.
اسلاید 26: STRUCTURE OF BENZENE - DELOCALISATION6 single bondsone way to overlapadjacent p orbitalsdelocalised piorbital systemanotherpossibilityThe theory suggested that instead of three localised (in one position) double bonds, the six p (p) electrons making up those bonds were delocalised (not in any oneparticular position) around the ring by overlapping the p orbitals. There would be nodouble bonds and all bond lengths would be equal. It also gave a planar structure.
اسلاید 27: STRUCTURE OF BENZENE - DELOCALISATION6 single bondsone way to overlapadjacent p orbitalsdelocalised piorbital systemanotherpossibilityThis final structure was particularly stable andresisted attempts to break it down through normalelectrophilic addition. However, substitution of anyhydrogen atoms would not affect the delocalisation.The theory suggested that instead of three localised (in one position) double bonds, the six p (p) electrons making up those bonds were delocalised (not in any oneparticular position) around the ring by overlapping the p orbitals. There would be nodouble bonds and all bond lengths would be equal. It also gave a planar structure.
اسلاید 28: STRUCTURE OF BENZENE
اسلاید 29: STRUCTURE OF BENZENEANIMATIONThe animation doesn’t work on early versions of Powerpoint
اسلاید 30: WHY ELECTROPHILIC ATTACK?TheoryThe high electron density of the ring makes it open to attack by electrophilesHOWEVER...Because the mechanism involves an initial disruption to the ringelectrophiles will have to be more powerful than those which reactwith alkenes. A fully delocalised ring is stable so will resist attack.
اسلاید 31: WHY SUBSTITUTION?TheoryAddition to the ring would upset the delocalised electron system Substitution of hydrogen atoms on the ring does not affect the delocalisation Overall there is ELECTROPHILIC SUBSTITUTION ELECTRONS ARE NOT DELOCALISEDAROUND THE WHOLE RING - LESS STABLE STABLE DELOCALISED SYSTEM
اسلاید 32: ELECTROPHILIC SUBSTITUTIONTheory The high electron density of the ring makes it open to attack by electrophiles Addition to the ring would upset the delocalised electron system Substitution of hydrogen atoms on the ring does not affect the delocalisation Because the mechanism involves an initial disruption to the ring, electrophiles must be more powerful than those which react with alkenes Overall there is ELECTROPHILIC SUBSTITUTION
اسلاید 33: ELECTROPHILIC SUBSTITUTIONTheory The high electron density of the ring makes it open to attack by electrophiles Addition to the ring would upset the delocalised electron system Substitution of hydrogen atoms on the ring does not affect the delocalisation Because the mechanism involves an initial disruption to the ring, electrophiles must be more powerful than those which react with alkenes Overall there is ELECTROPHILIC SUBSTITUTION Mechanism• a pair of electrons leaves the delocalised system to form a bond to the electrophile• this disrupts the stable delocalised system and forms an unstable intermediate• to restore stability, the pair of electrons in the C-H bond moves back into the ring• overall there is substitution of hydrogen ... ELECTROPHILIC SUBSTITUTION
اسلاید 34: ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATIONReagentsconc. nitric acid and conc. sulphuric acid (catalyst)Conditionsreflux at 55°CEquation C6H6 + HNO3 ———> C6H5NO2 + H2O nitrobenzene
اسلاید 35: ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATIONReagentsconc. nitric acid and conc. sulphuric acid (catalyst)Conditionsreflux at 55°CEquation C6H6 + HNO3 ———> C6H5NO2 + H2O nitrobenzeneMechanism
اسلاید 36: ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATIONReagentsconc. nitric acid and conc. sulphuric acid (catalyst)Conditionsreflux at 55°CEquation C6H6 + HNO3 ———> C6H5NO2 + H2O nitrobenzeneMechanismElectrophile NO2+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction... 2H2SO4 + HNO3 2HSO4¯ + H3O+ + NO2+ acid base
اسلاید 37: ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATIONReagentsconc. nitric acid and conc. sulphuric acid (catalyst)Conditionsreflux at 55°CEquation C6H6 + HNO3 ———> C6H5NO2 + H2O nitrobenzeneMechanismElectrophile NO2+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction... 2H2SO4 + HNO3 2HSO4¯ + H3O+ + NO2+ acid baseUse The nitration of benzene is the first step in an historically important chain of reactions. These lead to the formation of dyes, and explosives.
اسلاید 38: ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATIONReagentschlorine and a halogen carrier (catalyst)Conditionsreflux in the presence of a halogen carrier (Fe, FeCl3, AlCl3)chlorine is non polar so is not a good electrophilethe halogen carrier is required to polarise the halogenEquation C6H6 + Cl2 ———> C6H5Cl + HClMechanismElectrophile Cl+ it is generated as follows... Cl2 + FeCl3 FeCl4¯ + Cl+ a Lewis Acid
اسلاید 39: FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATIONOverviewAlkylation involves substituting an alkyl (methyl, ethyl) groupReagents a halogenoalkane (RX) and anhydrous aluminium chloride AlCl3Conditionsroom temperature; dry inert solvent (ether)Electrophilea carbocation ion R+ (e.g. CH3+)EquationC6H6 + C2H5Cl ———> C6H5C2H5 + HCl
اسلاید 40: FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATIONOverviewAlkylation involves substituting an alkyl (methyl, ethyl) groupReagents a halogenoalkane (RX) and anhydrous aluminium chloride AlCl3Conditionsroom temperature; dry inert solvent (ether)Electrophilea carbocation ion R+ (e.g. CH3+)EquationC6H6 + C2H5Cl ———> C6H5C2H5 + HClMechanismGeneralA catalyst is used to increase the positive nature of the electrophile and make it better at attacking benzene rings.AlCl3 acts as a Lewis Acid and helps break the C—Cl bond.
اسلاید 41: FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATIONCatalystanhydrous aluminium chloride acts as the catalystthe Al in AlCl3 has only 6 electrons in its outer shell; a LEWIS ACIDit increases the polarisation of the C-Cl bond in the haloalkanethis makes the charge on C more positive and the following occurs RCl + AlCl3 AlCl4¯ + R+
اسلاید 42: FRIEDEL-CRAFTS REACTIONS - INDUSTRIAL ALKYLATIONIndustrialAlkenes are used instead of haloalkanes but an acid must be presentPhenylethane, C6H5C2H5 is made by this methodReagentsethene, anhydrous AlCl3 , conc. HClElectrophileC2H5+ (an ethyl carbonium ion)EquationC6H6 + C2H4 ———> C6H5C2H5 (ethyl benzene)Mechanismthe HCl reacts with the alkene to generate a carbonium ionelectrophilic substitution then takes place as the C2H5+ attacks the ringUseethyl benzene is dehydrogenated to produce phenylethene (styrene);this is used to make poly(phenylethene) - also known as polystyrene
اسلاید 43: FRIEDEL-CRAFTS REACTIONS OF BENZENE - ACYLATIONOverviewAcylation involves substituting an acyl (methanoyl, ethanoyl) groupReagents an acyl chloride (RCOX) and anhydrous aluminium chloride AlCl3Conditionsreflux 50°C; dry inert solvent (ether)ElectrophileRC+= O ( e.g. CH3C+O )EquationC6H6 + CH3COCl ———> C6H5COCH3 + HClMechanismProductA carbonyl compound (aldehyde or ketone)
اسلاید 44: FURTHER SUBSTITUTION OF ARENESTheoryIt is possible to substitute more than one functional group.But, the functional group already on the ring affects...• how easy it can be done • where the next substituent goesGroup ELECTRON DONATING ELECTRON WITHDRAWINGExample(s) OH, CH3 NO2Electron density of ring Increases DecreasesEase of substitution Easier HarderPosition of substitution 2,4,and 6 3 and 5
اسلاید 45: FURTHER SUBSTITUTION OF ARENESExamplesSubstitution of nitrobenzene is...• more difficult than with benzene• produces a 1,3 disubstituted productSubstitution of methylbenzene is…• easier than with benzene• produces a mixture of 1,2 and 1,4 isomeric productsSome groups (OH) make substitution so mucheasier that multiple substitution takes place
اسلاید 46: STRUCTURAL ISOMERISM1,3-DICHLOROBENZENEmeta dichlorobenzeneRELATIVE POSITIONS ON A BENZENE RING1,2-DICHLOROBENZENEortho dichlorobenzene1,4-DICHLOROBENZENEpara dichlorobenzeneCompounds have similar chemical properties but different physical properties
اسلاید 47: THE CHEMISTRYOF ARENESTHE END© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
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