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determine whether aliphatic or aromatic

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Given an atom in a molecule, how do I know whether it is aliphatic/aromatic? I know that stuff like benzene becomes more stable due to delocalized pi-bonds and is thus called ‘aromatic’.

But from a molecular graph perspective, given a bunch of atoms connected together how do I know whether a node is aliphatic/aromatic? 


Also, is Not(aromatic) equal to (aliphatic)?
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baoilleach
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wdiwdi [ Editor ] from Frankfurt am Main, Deutschland

Rajarshi already gave some good advice in his post.

Basics are ring detection, electron assignment (formal charges, bond electrons, pi electrons, free electron pairs, relative orientation of pi systems if you want to be fancy) and the Huckel rule.

Whether not(aromatic) is the same as aliphatic, depends on the definition of your system. There is also anti-aromaticity as a separate concept from non-aromaticity,and of course in the real world, it is all a question of energies.

And no, aromaticity determination algorithms vary quite a lot in what kind of corner-case systems they perceive as aromatic. The Daylight system is actually quite an evil one, but if you are doing SMILES/SMARTS, you cannot simply replace it by a more sane definition for reasons of compatibility.



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