Arenes

Benzne

Contents

• Arenes and Benzene• Kuklé

model + Problems with it– Low reactivity– Carbon-Carbon bond

lengths– Hydrogenation of

Benzene

• Delocalised model

• Reactions– Electrophilic

substitution– Halogenation of

Benzene

• Definitions• Summary

– Reactions– Electrophilic substitutio

n

Arenes and Benzene

• Arenes are aromatic hydrocarbons containing one or more benzene rings

• Aromatic comes from the Latin ‘aroma’ meaning fragrance

• Arenes occur in crude oil and coal

• Benzene has a molecular formula of C₆H₆ and has a molecular weight of 78

• The ring is classified as an aromatic compound

• It is a carcinogen which can cause cancer or anaemia

• It is a colourless liquid with a sweet smell and highly flammable

• Benzene is used in detergents, explosives, pharmaceuticals and dyes

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Kuklé model

• Kuklé was the first person to come up with the idea of a ring

• He suggested that it had a planar, cyclic structure

• He also said there were alternating double and single bonds

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Problems: Low Reactivity

• If there were C=C bonds then it would act like an alkene

• It did not react with bromine water though which is the test for double bonds

• Benzene also takes part is substitution reactions not addition reactions at expected with double bonds

• It was thought that the bonds create an equilibrium which is represented in the resonance hybrid

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Problems: Carbon-Carbon Bond Lengths

• Kuklé supposed there were 3 C–C bonds and 3 C=C bonds

• When X-ray studies revealed that C–C bonds and C=C bonds had different lengths

• However, it was found that all the bond lengths in benzene were equal and between the two lengths (0.139nm)

• So benzene looked like this:

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Problems: Hydrogenation of Benzene

• Benzene was compared with cyclohexene

• When looking at the enthalpy change of cyclohexene it was -120 kJmol⁻¹

• Thus benzene should have been 3 times this

• However it was found to be -208 kJmol⁻¹ rather than -360 kJmol⁻¹

• This shows that it is more stable than thought

• This explains why it is less reactive than it was previously thought

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Delocalised model

• The theory suggests that there are 3 localised bonds and 6 delocalised electrons

• This forms p-orbitals and these overlap to create pi (π) bonds

• This would give the planar structure and no double bonds

• The structure is stable and resists attempts to break it down by addition reactions

• But substitution of hydrogens would not affect the delocalised theory

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Reactions: Electrophilic substitution

• Reagents: conc. Nitric acid and conc. Sulphuric acid (catalyst)

• Conditions: reflux at 55C• Equation:

• Mechanism:

• Restoring the Catalyst:

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Reactions: Halogenation of Benzene

• Reagents: Chlorine and a halogen carrier (catalyst)• Conditions: reflux and halogen carrier• Equation:• Mechanism:

• Halogen Carriers: iron, iron (III) chloride, iron (III) bromide, aluminium chloride

• Chlorine is non polar so anhydrous aluminium chloride can act as a catalyst

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Definitions• Delocalised electrons – shared between more than two atoms• Addition reaction – a reactant is added to an unsaturated molecule

to make a saturated molecule• Substitution reaction – an atom or group of atoms is replaced with a

different atom or group of atoms• Electrophile – an atom (or group of atoms) that is attracted to an

electron-rich centre, where it accepts a pair of electrons to form a new covalent bond

• Electrophilic substitution – a type of substitution reaction in which an electrophile is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond

• Reaction mechanism – a series of steps that, together, make up the overall reaction

• Curly arrow – a symbol used in reaction mechanisms to show the movement of an electron pair in the breaking or formation of covalent bonds

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Summary - Reactions

Nitrobenzne Chlorobenzene

Bromobenzene

Benzene

Br₂

FeBr₃Conc. HNO₃

Conc. H₂SO₄

50°C

Cl₂

AlCl₃

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Summary – Electrophilic Substitution

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