Return to Contents Chapter 11 : Arenes and Aromaticity Ch 11 contents
Implications

Aromaticity typically has a profound effect on the properties systems. Here are a few examples that illustrate some important possibilities:
   

Electrophilic aromatic susbtitution Aromatics tend to undergo substitution and retain the aromatic unit rather than lose it as would be the case if electrophilic addition occurred as is the case with alkenes.
keto-enolo tautomerism The ketone form of the tautomerisation is usually preferred. Not so here where the stability of the aromatic product makes the enol of the phenol form more favourable. 
Cyclopentadienyl anion Hydrocarbons are not usually very acidic (pKa > 50). But cyclopentadiene has a much lower pKa due to the aromatic stability of its aromatic conjugate base. 
What other systems have a pKa = 16 ? ANSWER
Basicity of pyridine Pyridine is like benzene but an N has replaced one CH. The N atom is weakly basic since the lone pair is in an sp2 hybrid orbital. 
Is the conjugate acid aromatic ?  ANSWER
Basicity of pyrrole Pyrrole is a much weaker base than pyridine (see above). This is because the lone pair on the N atom is already involved in the aromatic array of p electrons. Protonation results in loss of aromaticity and is therefore unfavourable.

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary