Aromatic Chemistry

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.

Electrons from a region of electron density over benzene ring.

This given benzene aromatic stability.

• '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.

• 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.

• 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

• 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

• 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.

• 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.