Aromatic Compounds

Benzene
1. History
  1. Kekule: Snake - planar, cyclic
    1. BUT
      1. Thermodynamically more stable than expected
        Releases 208 kJ per mole when reduced instead of 3*120. Resonance energy = 152 kJ per mole
      2. Bond lengths all similar
      3. Did not undergo electrophilic addition → no true double bond
      4. SO
        Resonance Hybrid
        Each bond is between single and double
        Lower in energy than Kekule → thermochemically stable
        Delocalised pi orbital system, planar
  2. C6H6
    1. X Aliphatic
  3. First isolated in 1825
2. Electrophilic substitution
  1. Does not affect delocalisation
  2. Need stronger electrophiles than those with alkene
  3. Nitration
    1. Conc HNO3, conc H2SO4 (catalyst)
    2. 55 degrees
    3. Electrophile: NO2+ (nitronium)
    4. Nitro benzene
      1. → Phenylamine → diazonium dyes
      2. Not safe, side effects
  4. Halogenation
    1. Chlorine, Halogen carrier (catalyst)
    2. Reflux
  5. Friedel-Crafts Reaction
    1. Acylation
      1. Reflux, 50 degrees, dry ether
      2. RCOX, anhydrous AlCl3
      3. Electrophile: RC+=O
    2. Alkylation
      1. RX, anhydrous AlCl3 (catalyst)
      2. Electrophile: R+
      3. Room temp, dry ether
  6. Further substitution
  7. Sulfonation
    1. + Fuming sulphuric acid → Benzenesulfonic acid
    2. Room temp
3. Addition
  1. Bromine in UV light
    1. Free radical reaction
  2. Hydrogenation with Raney nickel catalyst, 150 degrees C
    1. Cyclohexane
4. Burns with yellow smoky flame
Phenol
1. C6H5OH
2. Aromatic alcohol
3. Preparation
  1. Nitration of benzene
    1. Reduction of nitro benzene with Tin, Conc HCl, Reflux
      1. Diazotisation of phenyl amine: nitrous acid and hydrochloric acid, below 10 degrees
        Substitution of diazogroup by OH: water, warm above 10 degrees
4. More reactive than benzene as OH group activates benzene ring
5. Weak acid
6. Electrophilic substitution
  1. Without catalyst
  2. Further Substitution
    1. With Bromine water
    2. With dilute nitric acid
7. Antiseptic, used to make polymers
Benzene diazonium chloride
1. C6H5N2+Cl-
2. Preparation
  1. Phenylamine, nitrous acid (made in situ from sodium nitrite), hydrochloric acid
  2. Kept below 10 degrees to slow down decomposition of salt
3. Reactions
  1. Substitution of OH (hydrolysis) → phenol
  2. Substitution with I using KI solution, warm
  3. Coupling
    1. + NaOH + Phenol → Azo dye (N=N)
Any hydrocarbon system which has a stabilised ring of delocalised electrons

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Aromatic Compounds

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	Created by Cindy Lam about 11 years ago