F324 MOD1: Arenes

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  • Created on: 28-03-17 10:53
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  • F324 MODULE 1: Arenes
    • Benzene
      • Structure & Bonding
        • Delocalised Model
          • *sideways double overlap of p-orbitals between adjacent carbon atoms both above and below the plane of the ring
          • *results in one pi bonding region with delocalised (lowers e- density) pi electrons.
          • *sigma bonds between C-H
        • Kekule model/ resonance structure
          • Localised electrons between c=c ( pi bond)
          • sideways double overlap of p-orbitals between two carbon atoms
        • Evidence against kekule
          • X-ray diffraction shows that all bonds in benzene have an equal length
            • Benzene does not decolourise bromine water at rm temp,it can only react with Br2 in the presence of a halogen carrier and a high temperature.
              • enthalpy of hydrogenation ( -152kjmol-1 delocalisation enthalpy) is less exothermic than expected, when compared to cyclohexene, benzene is more stable
      • Reactions: Electrophilic substitution
        • Nitration
          • electrophile generation: H2SO4 + HNO3 -> HSO4- + NO2+ + H2O
          • Overall eq.: C6H6 + NO2+ ->C6H5NO2 + H2O
          • Catalyst regenerationHSO4- + H+-> H2SO4
        • Halogenation
          • Electrophile generation : FeBr3 + Br2 -> FeBr4- + Br+
          • Overall eq. : C6H6 + Br2 -> C6H5Br + HBr
          • Catalyst regeneration : FeBr4- + H+ -> FeBr3 + HBr
    • Cyclohexene
      • Reactions
        • Bromine
          • pi bond contains localised electrons above and below the C=C
            • producesa region of high electron density; allowing cyclohexene to react with a non-polar molecule such as bromine
              • electrons in pi bond repels those in Br-Br bond; inducing adipole in the Br2 molecule
                • pielectron pair in C=C is attracted to slightly positive Br; double bond breaksto form a new bond with Br, creating a positive carbocation
                  • Br- formed from the heterolytic fission of Br2, is attracted to the carbocation and forms a covalent bond
        • Comparison with Benzene
          • benzene has 6 delocalised pi electrons spread over 6 carbons, whereas cyclohexene has localised pi electrons in C=C meaning, it has a higher e- density than benzene
            • benezene's insufficient pi electron density means it cannot polarise a non-polar molecule such as Br2 without the aid of a halogen carrier to turn the bromine molecule a more powerful electrophile.
    • Phenol
      • Structure & Bonding
        • pink crystalline solid at rm. temp
          • weak acid in which the hydroxyl group is attached directly to the benzene ring
            • soluble in H2O, as OH froms hydrogen bonds with water molecules
      • Reactions
        • Bromination
          • phenol being a weak acid; partially dissociates and is reactive in positions: 2,4,6
            • forms 2,4,6 tribromophenol; white ppt
        • Bases
          • bases such as NaOH, Na causes a neutralisation reaction to form a salt e.g. C6H5O-Na+
            • the reaction can be reversed upon the addition of a strong acid (proton donor) e.g. HCL
      • Uses
        • alkyl phenols: detergents
          • chloro phenols: antiseptics
            • Salicyclic acid, a derivative of phenol, is used in the preparation of Aspirin
      • Reactivity
        • lone pair of electrons occupying the p-orbital on oxygen are partially dolcalised into the pi region of the benzene ring
          • this increases the electron density, therefore the ring is activated and becomes more nucleophilic
            • as a result phenol is able to polarise the electrophile and attract it more readily


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