Question

C4H4+ aromaticity

Answer 1

Non-aromatic

Answer 2

The molecule C4H4+ in its most stable cyclic form, 3-cyclobuten-1-yl cation, is a 4-membered ring. It contains one double bond and one carbocation. For a molecule to be aromatic, it must be cyclic, planar, fully conjugated, and have (4n+2) π electrons. In 3-cyclobuten-1-yl cation, one carbon atom (C1, the one not directly involved in the double bond or carbocation) is sp3 hybridized, breaking the continuous conjugation around the ring. Thus, it fails the "fully conjugated" criterion of Hückel's rule and is classified as non-aromatic.