Aromaticity Criteria

Aromaticity Criteria

Qu: How can we recognise systems that are aromatic ?Ans: By applying the criteria for aromaticity outlined below.

Based on the properties of aromatic compounds, there are FOUR criteria about the π system that need to be met inorder for the "special" aromatic stabilisation to be observed:

1. Conjugated : there needs to one "p" orbital from each atom in the ring, so each atom must be either sp² or sp hybridised.
2. Cyclic : linear systems are not aromatic, all atoms in the ring must be involved in the π system (i.e. no sp³ atoms)
3. Planar : if the ring is planar flat then this means there is good overlap / interaction between the "p" orbitals....not always easy to consider.
4. The Huckel Rule..... 4n+2 π electrons in the cyclic conjugated π system (n = 0, 1, 2, 3 etc.) This is equivalent to an odd number of π-electrons pairs).
5. The most important and well known aromatic system is benzene. It consists of a set of 6 sp² C arranged in a ring. This creates a set of 6 p orbitals, no gaps in the π system so we have a cyclic, conjugated π system. The planarity of the ring means that the interaction of the adjacent p orbitals is optimal. The 3 C=C in benzene mean that we have 3 pairs of π electrons = 6 π electrons = a 4n+2 number where n=1 .