Aromaticity
Aromaticity
Formula
Hückel's rule: 4n + 2 π electrons (n = 0, 1, 2…)
Real World
The aromatic porphyrin ring in haemoglobin (with 18 π electrons, n = 4) gives blood its red colour and enables oxygen binding — its exceptional stability comes directly from satisfying Hückel's rule.
Exam Focus
Check all four criteria — cyclic, planar, fully conjugated, and 4n+2 π electrons — before labelling any molecule aromatic.
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