Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. Although this sp3 center disrupts the continuous overlap of p-orbitals, traditionally thought to be a requirement for aromaticity, considerable thermodynamic stability and many of the spectroscopic, magnetic, and chemical properties associated with aromatic compounds are still observed for such compounds. This formal discontinuity is apparently bridged by p-orbital overlap, maintaining a contiguous cycle of π electrons that is responsible for this preserved chemical stability. The concept of homoaromaticity was pioneered by Saul Winstein in 1959, prompted by his studies of the “tris-homocyclopropenyl” cation. Since the publication of Winstein's paper, much research has been devoted to understanding and classifying these molecules, which represent an additional class of aromatic molecules included under the continuously broadening definition of aromaticity. To date, homoaromatic compounds are known to exist as cationic and anionic species, and some studies support the existence of neutral homoaromatic molecules, though these are less common. The 'homotropylium' cation (C8H9+) is perhaps the best studied example of a homoaromatic compound. Overview Naming The term "homoaromaticity" derives from the structural similarity between homoaromatic compounds and the analogous homo-conjugated alkenes previously observed in the literature. The IUPAC Gold Book requires that Bis-, Tris-, etc. prefixes be used to describe homoaromatic compounds in which two, three, etc. sp3 centers separately interrupt conjugation of the aromatic system. History The concept of homoaromaticity has its origins in the debate over the non-classical carbonium ions that occurred in the 1950s. Saul Winstein, a famous proponent of the non-classical ion model, first described homoaromaticity while studying the 3-bicyclo[3.1.0]hexyl cation. In a series of acetolysis experiments, Winstein et al. observed that the solvolysis reaction occurred empirically faster when the tosyl leaving group was in the equatorial position. The group ascribed this difference in reaction rates to the anchimeric assistance invoked by the "cis" isomer. This result thus supported a non-classical structure for the cation. Winstein subsequently observed that this non-classical model of the 3-bicyclo[3.1.0]hexyl cation is analogous to the previously well-studied aromatic cyclopropenyl cation. Like the cyclopropenyl cation, positive charge is delocalized over three equivalent carbons containing two π electrons. This electronic configuration thus satisfies Huckel's rule (requiring 4n+2 π electrons) for aromaticity. Indeed, Winstein noticed that the only fundamental difference between this aromatic propenyl cation and his non-classical hexyl cation was the fact that, in the