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Conjugated and aromatic systems (Read 2571 times)
Gerrit-Jan Linker
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Conjugated and aromatic systems
16.03.11 at 13:06:43
 
Conjugated and aromatic systems
 
Conjugated systems
A conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the system. The compound may be cyclic, acyclic, linear or mixed.
 
Aromatic compounds
Conjugated, planar, cyclic compounds that follow Hückel's rule (number of pi electrons in an aromat is 2n+2; n=0,1,2,...)are aromatic and exhibit an unusual stability.  
 
example: benzene
 
Criteria for aromaticity:
  • Molecule must be cyclic.
  • Every atom in the ring must have a p orbital, which overlaps with p orbitals on either side (completely conjugated).
  • Molecule must be planar.
  • It must contain an odd number of pairs of pi electrons; must satisfy Huckel's rule: (4n+2) pi electrons, where n is an integer starting at zero.
     
     
    Non aromatic compounds
    Not all compounds with alternating double and single bonds are aromatic. When they are not planar they are non-aromatic.
     
    example: cyclohexane  
     
    Anti-aromatic compounds
    Antiaromatic molecules are cyclic systems containing alternating single and double bonds, where the pi electron energy of antiaromatic compounds is higher than that of its open-chain counterpart. Antiaromatic compounds usually fail Hückel's rule of aromaticity.
     
    example: cyclobutadiene
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    « Last Edit: 31.12.12 at 23:19:57 by Gerrit-Jan Linker »  

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