Cyclopropenium

Other names: cyclopropenium cation

C₃H₃⁺

32/50 uncommon

The cyclopropenium cation (C₃H₃⁺) is the smallest possible aromatic system, containing just three carbon atoms in a ring. First synthesized by Breslow and colleagues in 1957, this remarkably stable carbocation defied expectations that such a small, strained ring would be too reactive to isolate. Its stability stems from having 2π electrons (a 4n+2 system, where n=0), making it aromatic according to Hückel's rule. This aromaticity distributes positive charge across all three carbon atoms, significantly enhancing stability despite the ring's extreme angular strain (≈60° bonds rather than the ideal 120° for sp² hybridized carbon). The cyclopropenium ion serves as the foundation for numerous stable salts and derivatives used in organocatalysis and ionic liquids. Its existence validated theoretical predictions about aromaticity in minimal systems and demonstrated that electronic effects can sometimes overcome seemingly prohibitive geometric constraints.