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/nonaromatic.

Answer 1

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

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

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

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

E = Six delocalised π-electrons follows Huckel’* 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 = Follows Huckel’* rule, has 2 π electrons i.e. (4n+2) π-electrons where (n=0), delocalised π-electrons. It is aromatic.

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