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3.3.10 Aromatic Chemistry (Bonding (benzene ring (structure (each carbon…
3.3.10 Aromatic Chemistry
Bonding
benzene ring
structure
each carbon has three covalent bonds
delocalised ring cause electron density above and below the plane of the molecule
bond lengths: intermediate between single and double bond length
theoretical cyclohexa-1,3,5-triene
structure
bond lengths: alternate between single and double bond length
stability
delocalisation of p electrons makes benzene ring more stable than cyclohexa-1,3,5-triene
thermochemical evidence to account for stability;
enthalpies of hydrogenation:
substitution reactions occur in preference to addition reactions as delocalised ring causes extra stability of benzene ring; delocalised system has high electron density that attracts eletrophiles
Electrophilic substitution
Nitration
formation of nitronium ion overall equation:
H₂S0₄ + HNO₃ --> NO₂⁺ + H₂0 + HSO₄⁻
important step in synthesis, including manufacture of explosives (TNT) and formation of amines
substitution of NO₂ for one of hydrogen atoms on an arene ring
electrophile is NO₂⁺; generated in reaction mixture of concentrated nitric acid and concentrated sulfuring acid:
H₂S0₄ + HNO₃ --> H₂NO₃⁺ + HSO₄⁻
H₂NO₃⁺ loses water to give nitronium ion
H₂NO₃⁺ --> NO₂⁺ + H₂0
Mechanism:
Acylation
AlCl₃ as a catalyst:
RCOCl + AlCl₃ --> RCO+ + AlCl₄⁻
AlCl₄⁻+ H⁺ --> AlCl₃ + HCl
Friedel-Crafts acylation reactions are important steps in synthesis
RCO group from acyl chloride substitutes for a hydrogen on the benzene ring
products are acyl substitutes arenes
Mechanism:
Sammer Sheikh