Question

The ring systems having following characteristics are aromatic.

(i) Planar ring containing conjugated π bonds.

(ii) Complete delocalisation of the π−electrons in ring system i.e. each atom in the ring has unhybridised p-orbital, and

(iii) Presence of (4n + 2) π−electrons in the ring where n is an integer (n = 0, 1, 2,...........) [Huckel rule].

Using this information classify the following compounds as aromatic/non-aromatic.

(A)	(B)	(C)	(D)
(E)	(F)	(G)

Answer 1

A = Planar ring, all atoms of the ring sp² hybridized, has six delocalised π electrons, follows Huckel rule. It is aromatic.

B = Has six π electrons, but the delocalisation stops at sp³ hybridized CH₂ – carbon. Hence, not aromatic.

C = Six delocalised π-electrons (4π electrons + 2 unshared electrons on negatively charged carbon) in a planar ring, follows Huckel's rule. It is aromatic.

D = Has only four delocalised π-electrons. It is non-aromatic.

E = Six delocalised π-electrons follows Huckel's rule. π electrons are in sp² hybridised orbitals, conjugation all over the ring because of positively charged carbon. The ring is planar hence is aromatic.

F = Folloes Huckel's rule, has 2π electrons i.e. (4n + 2) π-electrons where (n = 0), delocalised π-electrons. It is aromatic.

G = 8π electrons, does not follow Huckel's rule i.e. (4n + 2) π-electrons rule. It is not aromatic.