Which of the following is not aromatic?

Step-by-step solution

Aromatic compounds must satisfy Huckel's rule, which states that a planar, cyclic, conjugated system must have (4n + 2) electrons to be aromatic, where n is an integer. Cyclopentadienyl anion has 6 electrons ( 4 from two double bonds and 2 from the lone pair on the carbon atom), which satisfies 4n + 2 = 6 for n = 1 . Thus, it is aromatic. Pyridine is a heterocyclic aromatic compound with 6 electrons ( 6 from the ring atoms), satisfying Huckel's rule. Thus, it is aromatic. Cyclopropenyl cation has 2 electrons from t