Organic chemistry: Arenes

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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Organic chemistry: Arenes

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