In Huckel's rule (4n+2), based on what do we choose the number that replaces "n" ?

This is one of the beauties of math and science.  In this case, essentially, you pick.In organic chemistry, Huckel's rule pertains to aromatic molecules, meaning:- it must be cyclic- it must be conjugated (all atoms much be able to have a pi-bond)- it must be flat

Also, it must have a certain number of pi-electrons.  That what Huckel's Rule gives, those number of pi-electrons in aromatic molecules.  Given the structures of...

This is one of the beauties of math and science.  In this case, essentially, you pick.

In organic chemistry, Huckel's rule pertains to aromatic molecules, meaning:
- it must be cyclic
- it must be conjugated (all atoms much be able to have a pi-bond)
- it must be flat

Also, it must have a certain number of pi-electrons.  That what Huckel's Rule gives, those number of pi-electrons in aromatic molecules.  Given the structures of aromatic molecules, the number of pi-electrons has to be 2, 6, 10, 14, 18, 22, etc.  That series of numbers pertains to a mathematical sequence 4n+2.  So, the "n" only pertains to whichever number of pi-electrons you are looking at.

For instance, if n = 0, 4*0+2 = 2, so 2 pi-electrons
If n = 1, 4*1+2 = 6 pi-electrons
If n = 2, 4*2+2 = 10 pi-electrons
And so on.

So, n has essentially nothing to do with anything specific.  It is simply a whole number (0, 1, 2, 3, 4, etc.)

So, a sample problem could be, "Can an aromatic molecule have 12 pi-electrons?"  You would either need to remember the sequence 2, 6, 10, 14, 22, etc.  Or, you remember 4n+2 and start to plug in the numbers like above.  When 12 doesn't show up, the answer is "no".

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