THE CHEMISTRY OF ARENES A guide for A level students KNOCKHARDY PUBLISHING 2008 SPECIFICATIONS

INTRODUCTION This 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 page orusing the left and right arrow keys on the keyboard ARENES KNOCKHARDY PUBLISHING

CONTENTS Prior knowledge Structure of benzene Thermodynamic stability Delocalisation Electrophilic substitution Nitration Chlorination Friedel-Crafts reactions Further substitution ARENES

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 alkenes ARENES

STRUCTURE OF BENZENE Primary analysis revealed benzene had... an empirical formula of CH and a molecular mass of 78 and a molecular formula of C 6 H 6

STRUCTURE OF BENZENE Primary analysis revealed benzene had... an empirical formula of CH and a molecular mass of 78 a molecular formula of C 6 H 6 Kekulé suggested that benzene was... PLANAR CYCLIC and HAD ALTERNATING DOUBLE AND SINGLE BONDS

STRUCTURE OF BENZENE HOWEVER... 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

STRUCTURE OF BENZENE HOWEVER... 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 To explain the above, it was suggested that the structure oscillated between the two Kekulé forms but was represented by neither of them. It was a RESONANCE HYBRID.

THERMODYNAMIC EVIDENCE FOR STABILITY 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.

THERMODYNAMIC EVIDENCE FOR STABILITY kJ mol -1 When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C 6 H 10 (l) + H 2 (g) ——> C 6 H 12 (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.

THERMODYNAMIC EVIDENCE FOR STABILITY kJ mol -1 Theoretical kJ mol -1 (3 x -120) When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C 6 H 10 (l) + H 2 (g) ——> C 6 H 12 (l) Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane C 6 H 6 (l) + 3H 2 (g) ——> C 6 H 12 (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.

THERMODYNAMIC EVIDENCE FOR STABILITY 23 Experimental kJ mol kJ mol -1 Theoretical kJ mol -1 (3 x -120) When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C 6 H 10 (l) + H 2 (g) ——> C 6 H 12 (l) Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane C 6 H 6 (l) + 3H 2 (g) ——> C 6 H 12 (l) Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale 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.

THERMODYNAMIC EVIDENCE FOR STABILITY 23 MORE STABLE THAN EXPECTED by 152 kJ mol -1 Experimental kJ mol kJ mol -1 Theoretical kJ mol -1 (3 x -120) When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C 6 H 10 (l) + H 2 (g) ——> C 6 H 12 (l) Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane C 6 H 6 (l) + 3H 2 (g) ——> C 6 H 12 (l) Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale It 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.

THERMODYNAMIC EVIDENCE FOR STABILITY 23 MORE STABLE THAN EXPECTED by 152 kJ mol -1 Experimental kJ mol kJ mol -1 Theoretical kJ mol -1 (3 x -120) When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C 6 H 10 (l) + H 2 (g) ——> C 6 H 12 (l) Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane C 6 H 6 (l) + 3H 2 (g) ——> C 6 H 12 (l) Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale It 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.

HYBRIDISATION OF ORBITALS - REVISION The electronic configuration of a carbon atom is 1s 2 2s 2 2p 2 1 1s 2 2s 2p

HYBRIDISATION OF ORBITALS - REVISION The electronic configuration of a carbon atom is 1s 2 2s 2 2p 2 1 1s 2 2s 2p If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s 2 2s 1 2p 3 1 1s 2 2s 2p The process is favourable because of the arrangement of electrons; four unpaired and with less repulsion is more stable

HYBRIDISATION OF ORBITALS - REVISION The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent. 2s 2 2p 2 2s 1 2p 3 4 x sp 3 HYBRIDISE sp 3 HYBRIDISATION

HYBRIDISATION OF ORBITALS - REVISION Alternatively, 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. 2s 2 2p 2 2s 1 2p 3 3 x sp 2 2p HYBRIDISE sp 2 HYBRIDISATION

In ALKANES, the four sp 3 orbitals repel each other into a tetrahedral arrangement. In ALKENES, the three sp 2 orbitals repel each other into a planar arrangement and the 2p orbital lies at right angles to them STRUCTURE OF ALKENES - REVISION

Covalent bonds are formed by overlap of orbitals. An sp 2 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

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

two sp 2 orbitals overlap to form a sigma bond between the two carbon atoms ORBITAL OVERLAP IN ETHENE - REVIEW two 2p orbitals overlap to form a pi bond between the two carbon atoms s orbitals in hydrogen overlap with the sp 2 orbitals in carbon to form C-H bonds the resulting shape is planar with bond angles of 120º

STRUCTURE OF BENZENE - DELOCALISATION The theory suggested that instead of three localised (in one position) double bonds, the six p (  ) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure. 6 single bonds

STRUCTURE OF BENZENE - DELOCALISATION 6 single bondsone way to overlap adjacent p orbitals The theory suggested that instead of three localised (in one position) double bonds, the six p (  ) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure.

STRUCTURE OF BENZENE - DELOCALISATION 6 single bondsone way to overlap adjacent p orbitals another possibility The theory suggested that instead of three localised (in one position) double bonds, the six p (  ) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure.

STRUCTURE OF BENZENE - DELOCALISATION 6 single bondsone way to overlap adjacent p orbitals delocalised pi orbital system another possibility The theory suggested that instead of three localised (in one position) double bonds, the six p (  ) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure.

STRUCTURE OF BENZENE - DELOCALISATION 6 single bondsone way to overlap adjacent p orbitals delocalised pi orbital system another possibility This final structure was particularly stable and resisted attempts to break it down through normal electrophilic addition. However, substitution of any hydrogen atoms would not affect the delocalisation. The theory suggested that instead of three localised (in one position) double bonds, the six p (  ) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure.

STRUCTURE OF BENZENE

ANIMATION

WHY ELECTROPHILIC ATTACK? TheoryThe high electron density of the ring makes it open to attack by electrophiles HOWEVER... Because the mechanism involves an initial disruption to the ring, electrophiles will have to be more powerful than those which react with alkenes. A fully delocalised ring is stable so will resist attack.

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 DELOCALISED AROUND THE WHOLE RING - LESS STABLE STABLE DELOCALISED SYSTEM

ELECTROPHILIC SUBSTITUTION Theory 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

ELECTROPHILIC SUBSTITUTION Theory 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

ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION Reagentsconc. nitric acid and conc. sulphuric acid (catalyst) Conditionsreflux at 55°C Equation C 6 H 6 + HNO 3 ———> C 6 H 5 NO 2 + H 2 O nitrobenzene

ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION Reagentsconc. nitric acid and conc. sulphuric acid (catalyst) Conditionsreflux at 55°C Equation C 6 H 6 + HNO 3 ———> C 6 H 5 NO 2 + H 2 O nitrobenzene Mechanism

ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION Reagentsconc. nitric acid and conc. sulphuric acid (catalyst) Conditionsreflux at 55°C Equation C 6 H 6 + HNO 3 ———> C 6 H 5 NO 2 + H 2 O nitrobenzene Mechanism Electrophile NO 2 +, nitronium ion or nitryl cation; it is generated in an acid-base reaction... 2H 2 SO 4 + HNO 3 2HSO 4 ¯ + H 3 O + + NO 2 + acid base

ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION Reagentsconc. nitric acid and conc. sulphuric acid (catalyst) Conditionsreflux at 55°C Equation C 6 H 6 + HNO 3 ———> C 6 H 5 NO 2 + H 2 O nitrobenzene Mechanism Electrophile NO 2 +, nitronium ion or nitryl cation; it is generated in an acid-base reaction... 2H 2 SO 4 + HNO 3 2HSO 4 ¯ + H 3 O + + NO 2 + acid base Use The nitration of benzene is the first step in an historically important chain of reactions. These lead to the formation of dyes, and explosives.

ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATION Reagentschlorine and a halogen carrier (catalyst) Conditionsreflux in the presence of a halogen carrier (Fe, FeCl 3, AlCl 3 ) chlorine is non polar so is not a good electrophile the halogen carrier is required to polarise the halogen Equation C 6 H 6 + Cl 2 ———> C 6 H 5 Cl + HCl Mechanism Electrophile Cl + it is generated as follows... Cl 2 + FeCl 3 FeCl 4 ¯ + Cl + a Lewis Acid

FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION OverviewAlkylation involves substituting an alkyl (methyl, ethyl) group Reagents a halogenoalkane (RX) and anhydrous aluminium chloride AlCl 3 Conditionsroom temperature; dry inert solvent (ether) Electrophilea carbocation ion R + (e.g. CH 3 + ) EquationC 6 H 6 + C 2 H 5 Cl ———> C 6 H 5 C 2 H 5 + HCl

FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION OverviewAlkylation involves substituting an alkyl (methyl, ethyl) group Reagents a halogenoalkane (RX) and anhydrous aluminium chloride AlCl 3 Conditionsroom temperature; dry inert solvent (ether) Electrophilea carbocation ion R + (e.g. CH 3 + ) EquationC 6 H 6 + C 2 H 5 Cl ———> C 6 H 5 C 2 H 5 + HCl Mechanism GeneralA catalyst is used to increase the positive nature of the electrophile and make it better at attacking benzene rings. AlCl 3 acts as a Lewis Acid and helps break the C—Cl bond.

FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION Catalystanhydrous aluminium chloride acts as the catalyst the Al in AlCl 3 has only 6 electrons in its outer shell; a LEWIS ACID it increases the polarisation of the C-Cl bond in the haloalkane this makes the charge on C more positive and the following occurs RCl + AlCl 3 AlCl 4 ¯ + R +

FRIEDEL-CRAFTS REACTIONS - INDUSTRIAL ALKYLATION IndustrialAlkenes are used instead of haloalkanes but an acid must be present Phenylethane, C 6 H 5 C 2 H 5 is made by this method Reagentsethene, anhydrous AlCl 3, conc. HCl ElectrophileC 2 H 5 + (an ethyl carbonium ion) EquationC 6 H 6 + C 2 H 4 ———> C 6 H 5 C 2 H 5 (ethyl benzene) Mechanismthe HCl reacts with the alkene to generate a carbonium ion electrophilic substitution then takes place as the C 2 H 5 + attacks the ring Useethyl benzene is dehydrogenated to produce phenylethene (styrene); this is used to make poly(phenylethene) - also known as polystyrene

FRIEDEL-CRAFTS REACTIONS OF BENZENE - ACYLATION OverviewAcylation involves substituting an acyl (methanoyl, ethanoyl) group Reagents an acyl chloride (RCOX) and anhydrous aluminium chloride AlCl 3 Conditionsreflux 50°C; dry inert solvent (ether) ElectrophileRC + = O ( e.g. CH 3 C + O ) EquationC 6 H 6 + CH 3 COCl ———> C 6 H 5 COCH 3 + HCl Mechanism ProductA carbonyl compound (aldehyde or ketone)

FURTHER SUBSTITUTION OF ARENES TheoryIt 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 goes Group ELECTRON DONATING ELECTRON WITHDRAWING Example(s) OH, CH 3 NO 2 Electron density of ring Increases Decreases Ease of substitution Easier Harder Position of substitution 2,4,and 6 3 and 5

FURTHER SUBSTITUTION OF ARENES ExamplesSubstitution of nitrobenzene is... more difficult than with benzene produces a 1,3 disubstituted product Substitution of methylbenzene is… easier than with benzene produces a mixture of 1,2 and 1,4 isomeric products Some groups (OH) make substitution so much easier that multiple substitution takes place

STRUCTURAL ISOMERISM 1,3-DICHLOROBENZENE meta dichlorobenzene RELATIVE POSITIONS ON A BENZENE RING 1,2-DICHLOROBENZENE ortho dichlorobenzene 1,4-DICHLOROBENZENE para dichlorobenzene Compounds have similar chemical properties but different physical properties

THE CHEMISTRY OF ARENES THE END © 2009 KNOCKHARDY PUBLISHING © 2009 KNOCKHARDY PUBLISHING