Question: what are necessary conditions for any system?
Answer: (iii) It should be completely conjugated i.e, Aromatic compounds must have a p orbital on every atom and each must overlap with adjacent p orbitals. (iv) The total number of π–electrons present in the ring should be equal to (4n + 2), where n = 0, 1, 2 … etc. This is known as Huckel's rule.