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Arenes Textbook p4-5. Explain the terms: arene and aromatic. Describe and explain the models used to...

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Arenes Textbook p4-5
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Arenes

Textbook p4-5

• 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

Week 1

© Pearson Education Ltd 2009This document may have been altered from the original

Structures of some common benzene derivatives

Week 1

© Pearson Education Ltd 2009This document may have been altered from the original

Three isomers of C7H7Br

So arene or aromatic means the compound contains a benzene ring

Is aspirin - on the next slide an arene?

Week 1

© Pearson Education Ltd 2009This document may have been altered from the original

Structure of aspirin

Week 1

© Pearson Education Ltd 2009This document may have been altered from the original

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

Week 1

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Suggested linear structure for benzene, with several double bonds

Week 1

© Pearson Education Ltd 2009This document may have been altered from the original

Kekulé structure of benzene

Now try Q1-5 on p5 of the text

The Structure of BenzeneTextbook p6-7

• Review the evidence for a delocalised model 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.

• Now try textbook p7 Q1-3

Week 1

© Pearson Education Ltd 2009This document may have been altered from the original

• Compare the Kekulé and delocalised models of benzene in terms of p-orbital overlap forming π-bonds.

The delocalised model of benzene p8-9

Week 1

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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

Week 1

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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.

STRUCTURE OF BENZENESTRUCTURE OF BENZENE

STRUCTURE OF BENZENESTRUCTURE OF BENZENE

ANIMATIONANIMATION

• Now try Textbook p9 Q1-4

Benzene and its reactions

• Textbook p10-11

Week 1

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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.

Week 1

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Equation for the preparation of nitrobenzene

Week 1

© Pearson Education Ltd 2009This document may have been altered from the original

Preparing nitrobenzene by heating benzene

Week 1

© Pearson Education Ltd 2009This document may have been altered from the original

Benzene reacts with chlorine to produce chlorobenzene

Week 1

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Equation for the preparation of bromobenzene

Substitution reactions of benzene

• Textbook p12-13

Week 1

© Pearson Education Ltd 2009This document may have been altered from the original

• 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

Week 1

© Pearson Education Ltd 2009This document may have been altered from the original

Electrophilic substitution in benzene

Week 1

© Pearson Education Ltd 2009This document may have been altered from the original

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

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.

Week 2

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Reaction of cyclohexene with bromine

Week 2

© Pearson Education Ltd 2009This document may have been altered from the original

A double bond in cyclohexene induces a dipole in a bromine molecule

Week 2

© Pearson Education Ltd 2009This document may have been altered from the original

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.

• Now try Textbook p15 Q1-2


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