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Determining Aromaticity

At some point you’ll be asked to answer the following question: “is this molecule aromatic?”

I really like making tables. It’s a great way to systematize things. So when I’m trying to figure out if a molecule is aromatic or not, I’ll build a table like this. It helps make decisions much easier.

aromatic

Aromatic molecules have the following characteristics:

  1. They are cyclic.
  2. They are conjugated all around the ring. The easiest way to  put it is this: every atom in the ring must be able to participate in resonance. So atoms that have pi-bonds, carbocations, or lone pairs are OK; but atoms with four single bonds are out. 
  3. the molecule must have (4n+2) Pi electrons. This is just a different way of expressing the series (2,6,10,14, 18, etc. )**. Huckel’s Rule states that all aromatic molecules must satisfy this condition. Now, Pi electrons can either come from double bonds, or from lone pairs that can align themselves with the pi system. [What does 4n+2 mean?]
  4. The molecule must be flat. This is generally assumed to be true for most molecules that meet the previous 3 conditions, except in cases where steric hindrance prevents it. An example is 

Note how pyridine (the third example) has a lone pair but it doesn’t “count” for purposes of aromaticity! More on that next time.

Thanks for reading! James

PS Yet another way of expressing (4n+2) I’ve heard is “odd numbers of electron pairs” i.e. 1,3,5,7… electron pairs representing 2, 6, 10, 14 pi electrons.