Organic chemistry: Arenes

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  • Created by: Gemma
  • Created on: 07-01-14 09:19
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  • Arenes
    • Phenol
      • Contains a hydroxyl, -OH group
      • Reactions
        • undergoes similar substituion reactions to benzene
        • Why more reactive than benzene?
          • Because of the -OH group. It directs incoming groups to particular positions around the ring
          • Oxygen form the OH has two pairs of non-bonding electrons
            • One pair is drawn to delocalised pi electron system
              • Ring better at attracting electrophiles so reactions occur more easily
        • Substituion reactions
          • With bromine water
            • multi-electrophilic substituion occurs immediately, Bromine water is decolourised
              • product= white ppt of 2,4,6-tribroophenol (smells antiseptic)
          • With dil Nitric acids
            • white ppt of 2,4,6-trinitrophenol
            • multi-electrophilic substituion
            • Does not need conc sulphuric acid to assist (unlike benzene)
  • Benzene
    • Arenes
      • Phenol
        • Contains a hydroxyl, -OH group
        • Reactions
          • undergoes similar substituion reactions to benzene
          • Why more reactive than benzene?
            • Because of the -OH group. It directs incoming groups to particular positions around the ring
            • Oxygen form the OH has two pairs of non-bonding electrons
              • One pair is drawn to delocalised pi electron system
                • Ring better at attracting electrophiles so reactions occur more easily
          • Substituion reactions
            • With bromine water
              • multi-electrophilic substituion occurs immediately, Bromine water is decolourised
                • product= white ppt of 2,4,6-tribroophenol (smells antiseptic)
            • With dil Nitric acids
              • white ppt of 2,4,6-trinitrophenol
              • multi-electrophilic substituion
              • Does not need conc sulphuric acid to assist (unlike benzene)
    • Kekule
      • six-membered carbon ring with alternate double and single bonds between carbon atoms
      • Evidence against kekule
        • double bonds would show a tendency to undergo addition reactions- They do not occur easily! Most are substituion reactions
        • Thermochemical evidence-Theoretical value of enthalpy change= +252KJmol-1. Actual= 49KJmol-1= Benzene is more energetically stable than kekule model
    • New model
      • Each bond is between a double and a single bond
        • Evidence for new model
          • Enthalpy value very close to observed value
          • enthalpy for hydrogenation of kekule benzene would be 3x-120KJmol-1. Actual is -208KJmol-1. Suggests an addition reaction with hydrogen is not occuring across a normal double bond.
          • X-ray diffraction used to measure bond length
            • A C-C bond length is 0.154nm. A C=C bond length is 0.133nm.
            • Bond length= 0.139- Shows there is only one bond length in benzene- inbetween a single and a double bond
              • Supported by the electron density map of benzene
          • All bond angles= 120
    • Arrangement of electrons
      • Each carbon atom forms a single sigma bond with each of the two carbon atoms joined to it
        • Planar hexagonal ring
      • Each carbon has one p orbital containing a single electron that fuse to form a ring above and below the sigma bonded skeleton
        • The electrons are free to move around the ring (delocalised) which causes all the C-C bonds to be identical in benzene and makes it stable
    • Reactions
      • Combustion
        • 6C6H6 (l) + 15O2 (g) --> 12CO2 (g) + 6H2O (l)
          • Needs plenty of oxygen
      • substitutuion Reactions
        • Involve attack on the ring by electrophiles
        • Reaction of benzene with fuming sulphuric acid
          • Sulfonation
            • Room temp with conc sulphuric acid that contains extra sulfur trioxide as an effective electrophile
            • Reversible reaction
            • Used in the manufacture of soapless detergents
          • SO3HC6H5
        • With halogens
          • In the dark- doesn't react without catalyst (Ironbromide, FeBr3)
            • Catalyst is a hydrogen carrier. The bezene and hydrogen carrier are refluxed together
              • Bromobenzene is formed
            • Iron (III) polarises the bromine which didn't have a charge
        • Nitation
          • doesn't react with conc nitric acid but with a mixture of conc HNO3 and conc H2SO4
            • Product= nitrobenzene
            • temp below 55 degrees C
              • If temp is higher- multiple substitution occurs resulting in a mixture of products
        • Alkylation
          • Refluxing benzene with a halogenoalkane in the presence of AlCl3 (halogen carrier catalyst)
            • AlCl3 polarises the halogenoalkane molecule promoting the fomration of an electrophilic alkyl cation which is attracted to the ring
        • Acylation
          • refluxed with acyl chloride
          • Forms a ketone which can be reduced to a secondary alcohol
  • Properties
    • Freezes as 6 degrees C
    • Benzene
      • Kekule
        • six-membered carbon ring with alternate double and single bonds between carbon atoms
        • Evidence against kekule
          • double bonds would show a tendency to undergo addition reactions- They do not occur easily! Most are substituion reactions
          • Thermochemical evidence-Theoretical value of enthalpy change= +252KJmol-1. Actual= 49KJmol-1= Benzene is more energetically stable than kekule model
      • New model
        • Each bond is between a double and a single bond
          • Evidence for new model
            • Enthalpy value very close to observed value
            • enthalpy for hydrogenation of kekule benzene would be 3x-120KJmol-1. Actual is -208KJmol-1. Suggests an addition reaction with hydrogen is not occuring across a normal double bond.
            • X-ray diffraction used to measure bond length
              • A C-C bond length is 0.154nm. A C=C bond length is 0.133nm.
              • Bond length= 0.139- Shows there is only one bond length in benzene- inbetween a single and a double bond
                • Supported by the electron density map of benzene
            • All bond angles= 120
      • Arrangement of electrons
        • Each carbon atom forms a single sigma bond with each of the two carbon atoms joined to it
          • Planar hexagonal ring
        • Each carbon has one p orbital containing a single electron that fuse to form a ring above and below the sigma bonded skeleton
          • The electrons are free to move around the ring (delocalised) which causes all the C-C bonds to be identical in benzene and makes it stable
      • Reactions
        • Combustion
          • 6C6H6 (l) + 15O2 (g) --> 12CO2 (g) + 6H2O (l)
            • Needs plenty of oxygen
        • substitutuion Reactions
          • Involve attack on the ring by electrophiles
          • Reaction of benzene with fuming sulphuric acid
            • Sulfonation
              • Room temp with conc sulphuric acid that contains extra sulfur trioxide as an effective electrophile
              • Reversible reaction
              • Used in the manufacture of soapless detergents
            • SO3HC6H5
          • With halogens
            • In the dark- doesn't react without catalyst (Ironbromide, FeBr3)
              • Catalyst is a hydrogen carrier. The bezene and hydrogen carrier are refluxed together
                • Bromobenzene is formed
              • Iron (III) polarises the bromine which didn't have a charge
          • Nitation
            • doesn't react with conc nitric acid but with a mixture of conc HNO3 and conc H2SO4
              • Product= nitrobenzene
              • temp below 55 degrees C
                • If temp is higher- multiple substitution occurs resulting in a mixture of products
          • Alkylation
            • Refluxing benzene with a halogenoalkane in the presence of AlCl3 (halogen carrier catalyst)
              • AlCl3 polarises the halogenoalkane molecule promoting the fomration of an electrophilic alkyl cation which is attracted to the ring
          • Acylation
            • refluxed with acyl chloride
            • Forms a ketone which can be reduced to a secondary alcohol

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