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Arenes -  Benzene
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Arenes - Benzene

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Revision of Benzene

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    Arenes -  Benzene Arenes - Benzene Presentation Transcript

    • Arenes Benzne
    • Contents
      • Arenes and Benzene
      • Kukl é 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 substitution
    • 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 C ₆ H ₆ 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
      Back to Contents
    • Kukl é model
      • Kukl é 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
      Back to Contents
    • 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
      Back to Contents
    • Problems: Carbon-Carbon Bond Lengths
      • Kukl é 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:
      Back to Contents
    • 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
      Back to Contents
    • 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
      Back to Contents
    • Reactions: Electrophilic substitution
      • Reagents: conc. Nitric acid and conc. Sulphuric acid (catalyst)
      • Conditions: reflux at 55C
      • Equation:
      • Mechanism:
      • Restoring the Catalyst:
      Back to Contents
    • 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
      Back to Contents
    • 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
      Back to Contents
    • Summary - Reactions Br ₂ FeBr ₃ Conc. HNO ₃ Conc. H ₂ SO ₄ 50 ° C Cl ₂ AlCl ₃ Back to Contents Nitrobenzne Chlorobenzene Bromobenzene Benzene
    • Summary – Electrophilic Substitution Back to Contents