Huckel rule aromatic ions

Molecular Orbital Derivation of Huckel s Rule 725 16-8 Aromatic Ions 726... 

Benzylic Side Chains and Retrosynthetic Analysis 431 Cyclobutadiene and Cyclooctatetraene 431 Huckel s Rule 433 Annulenes 435 Aromatic Ions 437... 

From among the molecules and ions shown all of which are based on cycloundecapentaene identify those which satisfy the criteria for aromaticity as prescribed by Huckel s rule... 

Use Huckel s rule (and the criteria for its application), rather than resonance, to determine which annulenes and ions are aromatic, antiaromatic, and nonaromatic. 

Aromatic systems that obey Huckel s 4 + 2 rule where = 0 and so possess two 7i-electrons do exist and are indeed stable. The smallest possible ring is three membered and the derived unsaturated structure is cyclopropene. The theoretical loss of a hydride ion from this molecule leads to the cyclopropenyl cation, which contains two 7C-electrons distributed over the three carbon atoms of the planar cyclic system (Figure 1.8). 

---