After completing this section, you should be able to
- use the Hückel 4n + 2 rule to explain the stability of the cyclopentadienyl anion, the cycloheptatrienyl cation and similar species.
- use the Hückel 4n + 2 rule to determine whether or not a given unsaturated cyclic hydrocarbon anion or cation is aromatic.
- draw the resonance contributors for the cyclopentadienyl anion, cation and radical, and similar species.
Charged Aromatic Compounds
Carbanions and carbocations may also show aromatic stabilization. Some examples are:
The three-membered ring cation has 2 \(\pi\)-electrons and is surprisingly stable, considering its ring strain. Cyclopentadiene is as acidic as ethanol, reflecting the stability of its 6 π-electron conjugate base. Salts of cycloheptatrienyl cation (tropylium ion) are stable in water solution, again reflecting the stability of this 6 \(\pi\)-electron cation.
Draw the resonance structures for cycloheptatriene anion. Are all bonds equivalent? How many lines (signals) would you see in a H1 C13 NMR?
All protons and carbons are the same, so therefore each spectrum will only have one signal each.
The following reaction occurs readily. Propose a reason why this occurs?
The ring becomes aromatic with the addition of two electrons. Thereby obeying the 4n+2 rule.