arenes textbook p4-5. explain the terms: arene and aromatic. describe and explain the models used to...
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• Explain the terms: arene and aromatic.
Describe and explain the models used to describe the structure of benzene.
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Structures of some common benzene derivatives
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Three isomers of C7H7Br
So arene or aromatic means the compound contains a benzene ring
Is aspirin - on the next slide an arene?
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Structure of aspirin
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Benzene is classified as a carcinogen
Most arenes are not carenogenic – just as sodium in sodium chloride does not behave like sodium metal on its own.
models used to describe the structure of benzene
• Benzene has the molecular formula C6H6
• What is its emprical formula?
• Answer Q1 in your pack
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Suggested linear structure for benzene, with several double bonds
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Kekulé structure of benzene
STRUCTURE OF BENZENESTRUCTURE OF BENZENE
Primary analysis revealed benzene had...
an empirical formula of CH and
a molecular mass of 78 and
a molecular formula of C6H6
STRUCTURE OF BENZENESTRUCTURE OF BENZENE
Primary analysis revealed benzene had...
an empirical formula of CH and
a molecular mass of 78a molecular formula of C6H6
Kekulé suggested that benzene was...
PLANARCYCLIC and
HAD ALTERNATING DOUBLE AND SINGLE BONDS
STRUCTURE OF BENZENESTRUCTURE 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 BENZENESTRUCTURE 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 oscillatedbetween the two Kekulé forms but was represented by neither ofthem. It was a RESONANCE HYBRID.
THERMODYNAMIC EVIDENCE FOR STABILITYTHERMODYNAMIC 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 STABILITYTHERMODYNAMIC EVIDENCE FOR STABILITY
2 3
- 120 kJ mol-1
When 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.
THERMODYNAMIC EVIDENCE FOR STABILITYTHERMODYNAMIC EVIDENCE FOR STABILITY
2 3
- 120 kJ mol-1
Theoretical- 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 cyclohexane
C6H6(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.
THERMODYNAMIC EVIDENCE FOR STABILITYTHERMODYNAMIC EVIDENCE FOR STABILITY
2 3
Experimental- 208 kJ mol-1- 120 kJ mol-1
Theoretical- 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 cyclohexane
C6H6(l) + 3H2(g) ——> C6H12(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 STABILITYTHERMODYNAMIC EVIDENCE FOR STABILITY
2 3
MORE STABLE THAN EXPECTED
by 152 kJ mol-1
Experimental- 208 kJ mol-1- 120 kJ mol-1
Theoretical- 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 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.
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 STABILITYTHERMODYNAMIC EVIDENCE FOR STABILITY
2 3
MORE STABLE THAN EXPECTED
by 152 kJ mol-1
Experimental- 208 kJ mol-1- 120 kJ mol-1
Theoretical- 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 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.
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.
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• Compare the Kekulé and delocalised models of benzene in terms of p-orbital overlap forming π-bonds.
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Bonding around a carbon atom in a section of a benzene ring or alkene
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 - STRUCTURE OF ALKENES - REVISIONREVISION
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The delocalised structure of benzene forms when the p-orbitals overlap sideways
STRUCTURE OF BENZENE - STRUCTURE OF BENZENE - DELOCALISATIONDELOCALISATION
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 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
STRUCTURE OF BENZENE - STRUCTURE OF BENZENE - DELOCALISATIONDELOCALISATION
6 single bonds one way to overlapadjacent 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 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.
STRUCTURE OF BENZENE - STRUCTURE OF BENZENE - DELOCALISATIONDELOCALISATION
6 single bonds one way to overlapadjacent p orbitals
anotherpossibility
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 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.
STRUCTURE OF BENZENE - STRUCTURE OF BENZENE - DELOCALISATIONDELOCALISATION
6 single bonds one way to overlapadjacent p orbitals
delocalised piorbital system
anotherpossibility
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 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.
STRUCTURE OF BENZENE - STRUCTURE OF BENZENE - DELOCALISATIONDELOCALISATION
6 single bonds one way to overlapadjacent p orbitals
delocalised piorbital system
anotherpossibility
This 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 () 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.
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• 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.
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Equation for the preparation of nitrobenzene
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Preparing nitrobenzene by heating benzene
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Benzene reacts with chlorine to produce chlorobenzene
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Equation for the preparation of bromobenzene
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• Outline the mechanism of electrophilic substitution in arenes.
• Outline the mechanism for the mononitration and monohalogenation of benzene.
Electrophilic substitution mechanism (Nitration)
2. Electrophilic attack on benzene
NO2
NO2+
3. Re-forming the catalyst
+
NO2
H
the nitronium ion
HNO3
+ H2SO4 + HSO4- + H2O
1. Formation of NO2+
NO2+
reaction equation
+ H+
HSO4- + H+ H2SO4
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Electrophilic substitution in benzene
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Curly arrows are used to represent the movement of electron pairs
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 - ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATIONHALOGENATION
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 electrophilethe 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
The Reactivity of Alkenes and Benzene
• Textbook p14-15
• Explain the relative resistance to bromination of benzene compared with alkenes.
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Reaction of cyclohexene with bromine
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A double bond in cyclohexene induces a dipole in a bromine molecule
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Electrophilic addition in cyclohexene
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 → C6H5BrElectrophilic 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.