ABSTRACT
Carbocations are molecules with a formal positive charge at carbon and an even number of electrons. Although the first examples of such ions were reported in 1902, when Baeyer [1,2] recognized the salt-like charac ter of the compounds formed from triphenylmethanol and sulfuric acid [3], the concept of carbocation chemistry essentially was developed by Meerwein and Ingold in the 1920s [4], Meerwein rationalized the rear rangement of camphene hydrochloride to isobomyl chloride by suggesting the ionization of the C-Cl bond and successive rearrangement of the cationic intermediate [5]. Other molecular rearrangements were inter preted analogously [6 ]. Kinetic investigations led Ingold to the conclusion that nucleophilic aliphatic substitutions follow two different mechanisms, one of which (the so-called SN1 mechanism) involves the intermediacy of carbocations [7]. Winstein’s kinetic and stereochemical studies of solvolytic displacement reactions revealed the importance of ion pairing [8 ] and anchimeric assistance [9] in reactions with a stepwise mechanism. These investigations furthermore led to the formulation of nonclassical carbo cations, i.e., carbocations which cannot be properly described by twoelectron two-center bonds [1 0 ].
