Aromatic Chemistry

AQA Unit 4 syallabus notes of Aromatic chemistry. All the important info!

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Bonding in benzene ring.

Arenes are hydrocarbons --> C6H6

first isolated from sweet smelling compounds

  • 'aromatic compounds' now refers to structure
    • each corner has 1 carbon and 1 hydrogen
    • unsaturated structure so very stable
    • H group can be replaced by substituting it for other functional groups.
  • Bonding in the structure
    • doesn't really form addition compounds
    • all C atoms equivalents - suggesting all bond lengths were the same.
      • intermediate between a single and a double bond.
      • Explained by saying some electrons are de-localised.
    • Each carbon has three covalent bonds
      • one to H
      • 2 to C's
      • 1 to p-orbital which overlap and are de-localised.
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Electron de-localisation is the reason for stabili

Electrons from a region of electron density over benzene ring.

This given benzene aromatic stability.

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thermochemical evidence can be used to demonstrate

  • 'Cyclohexene' - (hydrogenation)
    • C6H4 + H2 --> C6H6 [cyclohexene]
    • enthalpy = -120kj mol-1
  • 'Kelvie -type' benzene
    • C6 + 3H2 --> C6H6
    • enthalpy = -360 kj mol-1 [tripled previous]
  • Enthalpy change for benzene
    • C6H6 + 2H2 --> C6H6
    • enthalpy = -208 kj mol-1 [152kj mol-1 more stable than unsaturated rind structure.
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Physical properties of arenes.

  • benzene is a colourless liquid at room temp
  • boils at 353 K
  • freezes at 279K
    • benzene is a flack hexagonal molecule that packs well together in a solid state
      • they are therefore harder to separate.
  • non-polar
  • arenes don't mix with water
  • mix with hydrocarbons and other non-polar substances.
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Naming benzene compounds

  • One substitute:
    • C6H5Cl --> chloro- benzene
    • C6H5CH3 -->methyl benzene
  • If two number the ring, using the lowest possible numbers
    • CClCClCHCHCHCH --> 1, 2 dichloro benzene
    • CClCHCHCClCHCH --> 1,4 - dichloro benzene
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Important factors in reactivity of benzene rings

  • Ring is an area of high electron density, attractive to electrophiles
  • Aromatic ring is very stable, so a certain amount of energy 'de-localisation energy' needs to be supplied before it becomes reactive. Ring almost remains intact in the reaction of arenes
  • Reaction is always substitution
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Nitration

  • when an NO2+ group is subbed for a H on the arene ring
  • NO2+ is generated in the reaction mix of conc nitric acid and conc sulphuric acid
    • Step 1: H2SO4 + HNO3 --> H2NO3+ + HSO4-
      • H2SO4 is a stronger acid than HNO3 so donates proton
    • Step 2: H2NO3+ --> H2O + NO2+ [nitronium ion]
    • You need to know the mechanism for this reaction.
  • Uses:
    • important step in making explosives.
    • first step in making arenes used for making dyes.
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Fredel craft acylation reactions

  • Calalyst = AlCl3 = aluminium chloride = reformed by reaction of ACl4- ion with H+
    • AlCl4- + H+ --> AlCl3 + HCl
  • mechanism = substutions = RCO+ subbed onto aromatic ring.
  • RCO+ is provided by acyl chlorides.
    • reacts with ALCL3 to form ALCL4- and RCO+
      • RCOCl + ALCl3 --> RCO+ + ALCL4-
  • Reaction:
    • Step 1: RCOCl + AlCl3 [Catalyst] -->RCO+ [nucleophile] + AlCl4-
    • Step 2: C6H6 + RCO+ --> C6H6RCO+
    • Step 3: C6H6RCO+ --> C6H5RCO + H+
    • Step 3: AlCl4- + H+ --> AlCl3 [catalyst regenerated] + HCl
    • You need to know the mechanism for this!!!
  • Acylation is a useful step in the synthesis of new subbed aromatic compounds.
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