Carbocation
In 1891, G. Merling reported that he added bromine to tropylidene (cycloheptatriene) and then heated the product to obtain a crystalline, water soluble material, . He did not suggest a structure for it; however Doering and Knox convincingly showed that it was tropylium (cycloheptatrienylium) bromide. This ion is predicted to be aromatic by the Hückel Rule.