Question

Which of the following compounds are aromatic

• A.
• B.
  
• C.
  
• D.

Answer 1

Answer: (A), (B), (D)

The correct options are
A
B

D

Aromatic hydrocarbons are hydrocarbons containing sigma bonds and pi electrons between the carbon atom in a ring. According to the Huckel rule, ring having planarity, complete delocalization of the pi electrons, conjugation of double bonds, and the presence of $4n+2\pi$ electrons in the ring where n is an integer are aromatic compounds.

Thus,


are aromatic in nature. In option (b) and (d) lone pairs on $N$ atom and $O$ atom respectively takes part in the conjugation.
(a) For $n=1$, $4n+2\pi =6\pi$ and benzene is having $6\pi$ electrons as can be seen from its structure. All double bonds are in conjuation and are in one plane. Hence, it is aromatic.

(b) For $n=1=6\pi$ electrons as the lone pair on nitrogen atom participate in conjugation. Hence, aromatic.

(c)
For this compound, there are only $4\pi$ electrons, as per huckel's rule ($4n+2\pi$ )electrons, keeping $n=0$ we get, $2\pi$ electrons and keeping $n=1$ we get $6\pi$ electrons but in the system , only $4\pi$ electrons are present. Hence, it is non-aromatic.

(d) Compound (d) contains $6\pi$ electrons as lone pair of oxygen takes part in conjugation, hence it is aromatic.