Introduction to Huckel’s rule
Huckel formulated a rule to identify aromatic compounds that is known as Huckel’s rule. The conditions of aromaticity are as follows:
a) Aromatic compounds should be cyclic.
b) The compound should have a planar structure.
c) There should be delocalized pi electrons in the compound.
d) There should be (4n + 2) pi electrons in the compound. The value of n =0, 1, 2, 3…
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The rule d) given here is the Huckel’s rule.
Any compound that does not follow one or more than one rules as stated above, are not aromatic – they are non-aromatic.
Substituting n=0, 1, 2, 3 … in (4n+2), we get 2, 6, 14 … respectively. So, in short any planar cyclic system should contain 2, 6, 14 … pi electrons, with at least one delocalized pi electron.
Application of Huckel’s Rule
Let us try a few examples in finding whether the compound is aromatic or non-aromatic.
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Solutions of problems on huckel’s rule:
a) There are 3 double bonds. So, we have 3 pairs of pi electrons there. We can also see 2 other lone pairs of electrons there. So, totally, we have
(3×3) + (2×2) pi electrons
9 + 4 pi electrons
It is not aromatic because it is not in accordance with the Huckel’s rule.
b) (2×1) electrons from the lone pair present + (3×2) electrons from the 3 double bonds present
2+6 pi electrons
The compound is non aromatic because it does not follow the Huckel’s rule.
c) There are 3 double bonds which implies there are
(2×3) pi electrons
6 pi electrons
This is an aromatic compound as it is accordance with the Huckel’s rule.
d) (3×2)+ (2×1) pi electrons
6+2 = 8 pi electrons
This is a non- aromatic compound.
e) There are (2×2) pi electrons
= 4 pi electrons
This a non-aromatic compound
f) (3×2) + (3×2)
= 12 pi electrons
This is a non-aromatic compound.
Some Practice Questions on Huckel’s Rule
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