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Resonance and Aromaticity

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Huckel’s Rule

Simply put, Huckel’s rule for aromaticity states that a monocyclic system will be aromatic if there are 4n + 2 delocalised electrons, (n an integer) contained within it. e.g.: 2, 6, 10, 14 etc.


The most well-known aromatic molecule is benzene, so we shall start with a brief discussion of this molecule. It must be made clear at this point that “aromatic” here refers to a molecule with certain electronic properties, it does not mean a molecule that smells (although that is in fact where the name first came from).


Resonance is a useful concept in chemistry. Earlier, we looked at the following carbocation, and explained it in terms of a spreading of charge. An alternative (and sometimes more useful) way of descibing this phenomenon of the lowering of the energy of this species is by use of resonance.


A carbocation is a positively charged species. The positive charge is localised on an sp2 hybridised carbon atom. Hence the carbocation itself is planar around this carbon.