Arenes Textbook p4-5

Explain the terms: arene and aromatic Explain the terms: arene and aromatic. Describe and explain the models used to describe the structure of benzene.

Some examples of arenes or aromatic compounds and how they are named

Structures of some common benzene derivatives Week 1 Structures of some common benzene derivatives © Pearson Education Ltd 2009 This document may have been altered from the original 4

Week 1 Three isomers of C7H7Br 5 © Pearson Education Ltd 2009 This document may have been altered from the original 5

So arene or aromatic means the compound contains a benzene ring Is aspirin - on the next slide an arene?

Week 1 Structure of aspirin 7 © Pearson Education Ltd 2009 This document may have been altered from the original 7

Benzene is classified as a carcinogen Week 1 Benzene is classified as a carcinogen Imagine putting sodium on your chips if sodium chloride if it did react like sodium metal! Most arenes are not carenogenic – just as sodium in sodium chloride does not behave like sodium metal on its own. © Pearson Education Ltd 2009 This document may have been altered from the original 8

models used to describe the structure of benzene Benzene has the molecular formula C6H6 What is its emprical formula? Answer Q1 in your pack

Suggested linear structure for benzene, with several double bonds Week 1 Suggested linear structure for benzene, with several double bonds © Pearson Education Ltd 2009 This document may have been altered from the original 10

Kekulé structure of benzene Week 1 Kekulé structure of benzene © Pearson Education Ltd 2009 This document may have been altered from the original 11

Now try Q1-5 on p5 of the text

The Structure of Benzene Textbook p6-7 Review the evidence for a delocalised model of benzene.

a molecular formula of C6H6 STRUCTURE OF BENZENE Primary analysis revealed benzene had... an empirical formula of CH and a molecular mass of 78 and a molecular formula of C6H6

a molecular formula of C6H6 STRUCTURE OF BENZENE Primary analysis revealed benzene had... an empirical formula of CH and a molecular mass of 78 a molecular formula of C6H6 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

To explain the above, it was suggested that the structure oscillated 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 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. When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C6H10(l) + H2(g) ——> C6H12(l) 2 3 - 120 kJ mol-1

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. 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 cyclohexane C6H6(l) + 3H2(g) ——> C6H12(l) Theoretical - 360 kJ mol-1 (3 x -120) 2 3 - 120 kJ mol-1

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. 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 cyclohexane C6H6(l) + 3H2(g) ——> C6H12(l) Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale Theoretical - 360 kJ mol-1 (3 x -120) 2 3 Experimental - 208 kJ mol-1 - 120 kJ mol-1

THERMODYNAMIC EVIDENCE FOR STABILITY MORE STABLE THAN EXPECTED 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. 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 cyclohexane C6H6(l) + 3H2(g) ——> C6H12(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. MORE STABLE THAN EXPECTED by 152 kJ mol-1 Theoretical - 360 kJ mol-1 (3 x -120) 2 3 Experimental - 208 kJ mol-1 - 120 kJ mol-1

THERMODYNAMIC EVIDENCE FOR STABILITY MORE STABLE THAN EXPECTED 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. 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 cyclohexane C6H6(l) + 3H2(g) ——> C6H12(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. MORE STABLE THAN EXPECTED by 152 kJ mol-1 Theoretical - 360 kJ mol-1 (3 x -120) 2 3 Experimental - 208 kJ mol-1 - 120 kJ mol-1

Now try textbook p7 Q1-3

Week 1 Compare the Kekulé and delocalised models of benzene in terms of p-orbital overlap forming π-bonds. © Pearson Education Ltd 2009 This document may have been altered from the original 25

The delocalised model of benzene p8-9

Bonding around a carbon atom in a section of a benzene ring or alkene Week 1 Bonding around a carbon atom in a section of a benzene ring or alkene © Pearson Education Ltd 2009 This document may have been altered from the original 27

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.

Week 1 The delocalised structure of benzene forms when the p-orbitals overlap sideways © Pearson Education Ltd 2009 This document may have been altered from the original 29

STRUCTURE OF BENZENE - 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 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 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 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 one way to overlap adjacent p orbitals

STRUCTURE OF BENZENE - 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 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 one way to overlap adjacent p orbitals another possibility

STRUCTURE OF BENZENE - 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 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 one way to overlap adjacent p orbitals another possibility delocalised pi orbital system

STRUCTURE OF BENZENE - 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 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 one way to overlap adjacent p orbitals another possibility delocalised pi orbital system 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.

STRUCTURE OF BENZENE

STRUCTURE OF BENZENE ANIMATION

Now try Textbook p9 Q1-4

Benzene and its reactions Textbook p10-11

Week 1 Describe the electrophilic substitution of arenes with concentrated nitric acid in the presence of concentrated sulfuric acid. Describe the electrophilic substitution of arenes with halogens in the presence of a suitable halogen carrier. © Pearson Education Ltd 2009 This document may have been altered from the original 39

Equation for the preparation of nitrobenzene Week 1 Equation for the preparation of nitrobenzene © Pearson Education Ltd 2009 This document may have been altered from the original 40

Preparing nitrobenzene by heating benzene Week 1 Preparing nitrobenzene by heating benzene © Pearson Education Ltd 2009 This document may have been altered from the original 41

Benzene reacts with chlorine to produce chlorobenzene Week 1 Benzene reacts with chlorine to produce chlorobenzene © Pearson Education Ltd 2009 This document may have been altered from the original 42

Equation for the preparation of bromobenzene Week 1 Equation for the preparation of bromobenzene © Pearson Education Ltd 2009 This document may have been altered from the original 43

Substitution reactions of benzene Textbook p12-13

Outline the mechanism of electrophilic substitution in arenes. Week 1 Outline the mechanism of electrophilic substitution in arenes. Outline the mechanism for the mononitration and monohalogenation of benzene. © Pearson Education Ltd 2009 This document may have been altered from the original 45

Electrophilic substitution mechanism (Nitration) 1. Formation of NO2+ the nitronium ion HNO3 + H2SO4 NO2+ + HSO4- + H2O 2. Electrophilic attack on benzene NO2+ + NO2 H NO2 + H+ 3. Re-forming the catalyst HSO4- + H+ H2SO4 reaction equation

Electrophilic substitution in benzene Week 1 Electrophilic substitution in benzene © Pearson Education Ltd 2009 This document may have been altered from the original 47

Curly arrows are used to represent the movement of electron pairs Week 1 Curly arrows are used to represent the movement of electron pairs © Pearson Education Ltd 2009 This document may have been altered from the original 48

Halogenation C6H6 + Cl2 → C6H5 Cl + HCl Substitution of a halogen in the presence of a halogen carrier; Halogen carriers include iron, iron halides and aluminium halides. (FeCl3 + Cl2 → FeCl4- Cl+ ) Cl+ is the electrophile

ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATION Reagents chlorine and a halogen carrier (catalyst) Conditions reflux in the presence of a halogen carrier (Fe, FeCl3, AlCl3) chlorine is non polar so is not a good electrophile the halogen carrier is required to polarise the halogen Equation C6H6 + Cl2 ———> C6H5Cl + HCl Mechanism Electrophile Cl+ it is generated as follows... Cl2 + FeCl3 FeCl4¯ + Cl+

Keywords Arene Addition reaction Mononitration Aromatic Substitution reaction Monohalogenation Delocalisation Electrophile Benzene π-electron Halogen carrier Orbital overlap π-bond Reaction mechanism Catalyst Curly arrow Electrophilic substitution

Now try Textbook p13 Q1-3

The Reactivity of Alkenes and Benzene Textbook p14-15 Explain the relative resistance to bromination of benzene compared with alkenes.

Reaction of cyclohexene with bromine Week 2 Reaction of cyclohexene with bromine © Pearson Education Ltd 2009 This document may have been altered from the original 54

A double bond in cyclohexene induces a dipole in a bromine molecule Week 2 A double bond in cyclohexene induces a dipole in a bromine molecule © Pearson Education Ltd 2009 This document may have been altered from the original 55

Electrophilic addition in cyclohexene Week 2 Electrophilic addition in cyclohexene © Pearson Education Ltd 2009 This document may have been altered from the original 56

Relative resistance to bromination of benzene compared with cyclohexene Alkenes - add Br2 - room temp C6H10 + Br2 → C6H10Br2 Electrophilic addition Arenes – add Br2 – halogen carrier needed C6H6 + Br2 → C6H5Br Electrophilic substitution

Benzene needs a halogen carrier to produce a more powerful electrophile Br+ – alkenes do not need a halogen carrier for reaction. Benzene has a lower electron density between two carbon atoms compared to alkenes. There is insufficient pi electron density between and two carbon atoms to polarise non-polar molecules – unlike an alkene.

Now try Textbook p15 Q1-2