Hello Mina,

I was thinking on "Why would Pyrrole, which has more aromatic stability be more reactive than Furan, which is less stable aromatically?" as a follow up question but I think you addressed it with your answer in saying it should be the resultant effect of both the aromatic stability and dipole moment.

Thank you very much for the answer!

Hi   Dilhan.   Yes you must consider more than resonance energy  when trying to rationalise the chemical reactivity of aromatic molecules. When pyrrole and furan react with an electrophile the resultant intermediate will have a positive charge on the heterocyclic ring. Nitrogen is a better electron donor than oxygen as it is less electronegative  (nitrogen is a better nucleophile than oxygen for the same reason) and thus the nitrogen will stabilise the cation intermediate better than oxygen. In conjunction with the Hammond-Leffler postulate the stability of the cation intermediate would be expected to be more important than the ground state dipole moment of the aromatic molecule although the latter would be expected to contribute as well to the final experimental observation. This requirement to stabilise the cationic intermediate is the reason why the reactivity order is different from the order of aromatic stability (resonance energy). Note also that we can now explain Aromaticity in terms of the classical Electronic theory that was developed by Robert Robinson, Gilbert Lewis and others during the 1910-1920's. (The Electronic Theory is the so called 'electron pushing' approach[curved arrows]). This has become possible since 2006 when I published the Cplex-isoelectronic theory which is an expanded Electronic theory. I regard it as the 'missing theory' as nobody could explain Aromaticity via the Electronic theory. Even though I have now restored the power of the Electronic theory, which was the first major theory in organic chemistry and is still being used to some degree (but poorly explained and  with  little emphasis in modern textbooks), there still continues to be a major emphasis on quantum chemical theories to explain all chemistry despite its many well documented limitations (stated by some of its founders) and in many cases incorrect predictions as well as no absolute experimental proof of its validity. Furthermore it is in antithesis to Complexity Theory in its expectations. They continue to put all their 'eggs into one basket' including only accepting quantitative theories or at best regarding them as being more important than qualitative theories!!!!!!!!! A bad, dangerous and non objective approach when nobody knows the final answer to all things!!!!! Who, on this latter point, can argue or prove that maths will reveal the final scientific knowledge!!!!! Objectively we need a multi approach method! 

Dear Sir,

It feels really great to see such a lengthy and a very understandable answer from an expert in the field! This is the very first time I came across the theory you've mentioned and am going to read more about it.

And many would love to see more of the classical theories coming up! It is much easier to understand a particle which can be only at one place at once! Congratulations on your great work and thank you again for responding.

Hi Dilhan.

Your welcome. It is great to find someone who supports and appreciates the Electronic Theory! I plan in the future to publish a paper explaining the aromaticity of 5-membered heterocycles (pyrrole, furan, thiophene etc)  using the Cplex-isoelectronic theory. I will put the reference on Researchgate.

We can now explain all Organic Chemistry, including Pericyclic Chemistry, without invoking Quantum Chemistry!

Best of luck.

Martin