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Aromatic compounds are highly unsaturated, cyclic molecules that
show extra stability as compared to their open chain counterpart.
Huckel's rule for aromaticity states that a molecule must meet all
of the following four conditions in order to be aromatic:
1) cyclic -- the molecule must be a cyclic polyene;
2) planar -- the geometry of the cyclic part must be planar;
3) each atom in the cyclic system must have a p orbital perpendicular
to the ring;
4) the cyclic system has 4n+2 pi electrons.
Benzene is an example of aromatic compounds. It meets all the above
four conditions. In this tutorial, we will examine "live"
molecules for their meeting geometry requirement (Condition 2).
This is the "live" molecule of benzene. Examine its
structure for planarity. Benzene can also be called -annulene.
Shown below are some other cyclic polyenes that may or may not be
aromatic. The number in the brackets indicates the number of double
bond carbons. Examine their structures carefully to see if they meet
all the four conditions for aromaticity. Which one(s) is(are) aromatic?