Chapter 11 : Arenes and Aromaticity |
Aromaticity typically has a profound effect on
the properties systems. Here are a few examples that illustrate some important
possibilities:
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. | |
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. | |
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 |
|
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 |
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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. |
© Dr. Ian Hunt, Department of Chemistry |