Determining Aromaticity

by James

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.

{ 4 comments… read them below or add one }

Deepesh

Really appreciate your hard work….
A question :
Why is the lone pair not in conjugation with the pi system in pyridine and is not being counted for purposes of aromaticity??

Reply

Jonathan Potthast

I believe it is because, since aromaticity is the most stable / desired state for the molecule, atoms with lone pairs that can be either sp2 or p lone pairs will choose to be in whichever orbital makes aromaticity possible. So in a purine (5-membered nitrogen-containing ring), the nitrogen lone pair will choose to be in a p orbital to create aromaticity (6 Pi electrons) but in a pyrimidine (such as above), the lone pair is in an sp2 orbital so as not to interfere with the aromaticity.

Reply

Jonathan Potthast

Correction: they are not called purines and pyrimidines. I meant 5-membered and 6-membered rings containing nitrogen. I forget what those are properly called.

Reply

James

I know you meant to say Pyrroles and pyridines : – )

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