See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Huckel's rule states that a planar, fully conjugated cyclic compound is aromatic if it contains (4n+2) pi electrons, where n is a non-negative integer (n = 0, 1, 2, ...). Step 1 - Analyze each option for pi electron count: (a) Cyclopentadienyl anion (5-membered ring, negative charge): The ring has 4 pi electrons from two double bonds plus 2 pi electrons from the lone pair on the carbanion, giving 6 pi electrons total. 6 = 4(1)+2, so n=1. This satisfies Huckel's rule and is AROMATIC. (b) Cyclopentadienyl cation (5-membered ring, positive charge): The ring has 4 pi electrons from two double bonds, and the carbocation contributes an empty p orbital (0 pi electrons), giving 4 pi electrons total. 4 = 4(1), which satisfies the anti-aromatic condition (4n rule, n=1). This compound is ANTI-AROMATIC, not aromatic. (c) Cyclopropenyl cation (3-membered ring, positive charge): The ring has 2 pi electrons from one double bond, and the carbocation contributes an empty p orbital, giving 2 pi electrons total. 2 = 4(0)+2, so n=0. This satisfies Huckel's rule and is AROMATIC. (d) Cycloheptatrienyl cation (7-membered ring, positive charge): The ring has 6 pi electrons from three double bonds, and the carbocation contributes an empty p orbital, giving 6 pi electrons total. 6 = 4(1)+2, so n=1. This satisfies Huckel's rule and is AROMATIC. Step 2 - Identify the non-aromatic compound: Option (b), the cyclopentadienyl cation, has 4 pi electrons (4n, n=1), making it anti-aromatic, not aromatic. Step 3 - Why other options fail to be the answer: Options (a), (c), and (d) all satisfy Huckel's (4n+2) pi electron rule and are therefore aromatic compounds. Therefore, the correct answer is B.