Physical and Chemical Properties of Aromatic Compounds

• Physical Properties of Aromatic Compounds


  • Nonpolar
  • Immiscible in water
  • Solvent for nonpolar compounds
  • Less dense than water
• Solubility


  Nonpolar, Immiscible in water, Solvent for nonpolar compounds
• Density


  Less dense than water
• Alkylation of Benzene


  1. Friedel-Crafts Alkylation

  2. Electrophilic aromatic substitution

  3. Replacement of hydrogen in the aromatic compound with an alkyl group

  4. Formation of C=C bonds when treated with alkyl halides in the presence of a strong lewis acid

  
• Electrophilic attack with aromatic ring of carbo ion, Substitution, Aromaticity is restored as final product

  
• Intramolecular Friedel-Crafts alkylation involves the nucleophile (aromatic ring) and the electrophile (alkyl halide) in the same compound, attached through a tether

  
• Biphenyl can be used as the starting reagent for Friedel-crafts alkylation


  • Reacts with tert-butyl chloride to give 4,4' di-tert-butylphenyl

  
• Friedel-Crafts Alkylation Mechanism

  1. Lewis acid catalyst reacts with alkyl halide to form carbo cation

  2. Carbo cation reacts with benzene ring to form cyclohexadienyl, temporarily losing aromaticity

  3. Hydrogen is considered as proton, protonation removes positive charge and restores aromaticity

  4. H+ reacts with AlCl4 to restore Lewis acid catalyst

  
• Friedel-Crafts Acylation


  Synthesizes a monoacylated aromatic ring, occurs when a carboxylic acid chloride reacts with an aromatic ring in the presence of a Lewis acid

  
• Friedel-Crafts Acylation Mechanism


  1. Chlorine forms a Lewis acid-base complex with AlCl3, acting as a leaving group to form a resonance-stabilized acyl cation

  2. Acyl cation reacts with benzene, a benzene π-bond acts as a nucleophile forming a new C-C bond

  3. AlCl4- abstracts the hydrogen to re-form the aromatic double bond, regenerating the AlCl3 catalyst and generating HCl

  
• Halogenation of Benzene


  Reacting benzene with halogens to form aryl halides, using a Lewis acid catalyst

  
• Halogenation of Benzene Mechanism

  1. Halogen acts as a Lewis base, reacting with the Lewis acid catalyst

  2. Electrophilic species reacts with benzene ring, temporarily losing aromaticity

  3. Addition of electrophile, then protonation to restore aromaticity

  
• Ortho-para directors


  1. 4 directors, activating groups, major product
• Meta directors


  3 directors, deactivating groups, minor product
• Resonance effect


  Conjugation between the rings and the substituent, delocalization of pi electrons

  
• Inductive effect


  Withdrawal of sigma electrons away from the ring toward the substituent due to higher electronegativity
• Electron Donating Group (EDG)


  Activating group, increases electron density and reactivity of benzene

  
• Electron Withdrawing Group (EWG)


  Deactivating group, decreases electron density and reactivity of benzene

  
• Halogens are an exception, they deactivate the ring by inductive effect not by resonance
• Halogens have an unpaired pair of electrons that gets donated to the ring, but the inductive effect pulls away the sigma electrons from the ring due to their high electronegativity
• Halogens are mild deactivators that direct substitution to the ortho or para positions