Question

In Huckel’s $(4n + 2)\pi $ rule for aromaticity, $n$ represents ?
A.Number of carbon atoms
B.Number of rings
C.Whole number
D.Fractional number or integer or zero

Answer 1

Hint: Huckel gave the number of electrons / pi-bonds in an aromatic compound by this formula $(4n + 2)\pi $ . Here, $'n'$ can be any positive natural number starting from zero .

Complete answer:
A compound is said to be aromatic if follows the following conditions –
-It is cyclic.
-If it is planar i.e. if all the atoms or bonds lie in the same plane.
-It has delocalised pi- electrons ( un-hybridised p-orbital) .
-Follows Huckel’s rule: which says that if a compound has $(4n + 2)\pi $ electrons and satisfies above three conditions ,it is an aromatic compound .

Here , $n = 0,1,2,3,4......$
-This implies that $n$ can be any number starting from zero .
When $n = 0$, number of electrons $ = 2$
When $n = 1$ , number of electrons $ = 6$ and so on .
-So the number of pi-electrons is $2,6,10,14,18......$ and it follows the above three conditions, it falls into the category of aromatic compounds.
-If any compound doesn’t follow the given conditions ,it is either non-aromatic or anti-aromatic . If a compound is anti- aromatic , it has $4n\pi $ electrons where the $n$ is the whole number.
We can conclude that $n$ is simply the whole number .
Hence, the correct option is C.

Note:
We have to keep in mind that $n$ can never be any fractional value or negative value. It always has to be positive,where n is the number of electrons.