Aromatic Compounds

Aromatic Compounds

1. Benzene

   • History
     • Kekule: Snake - planar, cyclic
       • BUT
         • Thermodynamically more stable than expected
           • Releases 208 kJ per mole when reduced instead of 3*120. Resonance energy = 152 kJ per mole
         • Bond lengths all similar
         • Did not undergo electrophilic addition → no true double bond
         • SO
           • Resonance Hybrid
             • Each bond is between single and double
             • Lower in energy than Kekule → thermochemically stable
           • Delocalised pi orbital system, planar
     • C6H6
       • X Aliphatic
     • First isolated in 1825
   • Electrophilic substitution
     • Does not affect delocalisation
     • Need stronger electrophiles than those with alkene
     • Nitration
       • Conc HNO3, conc H2SO4 (catalyst)
       • 55 degrees
       • Electrophile: NO2+ (nitronium)
       • Nitro benzene
         • → Phenylamine → diazonium dyes
         • Not safe, side effects
     • Halogenation
       • Chlorine, Halogen carrier (catalyst)
       • Reflux
     • Friedel-Crafts Reaction
       • Acylation
         • Reflux, 50 degrees, dry ether
         • RCOX, anhydrous AlCl3
         • Electrophile: RC+=O
       • Alkylation
         • RX, anhydrous AlCl3 (catalyst)
         • Electrophile: R+
         • Room temp, dry ether
     • Further substitution
     • Sulfonation
       • + Fuming sulphuric acid → Benzenesulfonic acid
       • Room temp
   • Addition
     • Bromine in UV light
       • Free radical reaction
     • Hydrogenation with Raney nickel catalyst, 150 degrees C
       • Cyclohexane
   • Burns with yellow smoky flame

2. Phenol

   • C6H5OH
   • Aromatic alcohol
   • Preparation
     • Nitration of benzene
       • Reduction of nitro benzene with Tin, Conc HCl, Reflux
         • Diazotisation of phenyl amine: nitrous acid and hydrochloric acid, below 10 degrees
           • Substitution of diazogroup by OH: water, warm above 10 degrees
   • More reactive than benzene as OH group activates benzene ring
   • Weak acid
   • Electrophilic substitution
     • Without catalyst
     • Further Substitution
       • With Bromine water
       • With dilute nitric acid
   • Antiseptic, used to make polymers

3. Benzene diazonium chloride

   • C6H5N2+Cl-
   • Preparation
     • Phenylamine, nitrous acid (made in situ from sodium nitrite), hydrochloric acid
     • Kept below 10 degrees to slow down decomposition of salt
   • Reactions
     • Substitution of OH (hydrolysis) → phenol
     • Substitution with I using KI solution, warm
     • Coupling
       • + NaOH + Phenol → Azo dye (N=N)

4. Any hydrocarbon system which has a stabilised ring of delocalised electrons