Hypercoordinate Carbocations and Their Borane Analogs

GENERAL CONCEPT OE CARBOCATIONS CARBENIUM VERSUS CARBONIUM IONS 

Hypercarbon Chemistry, Second Edition. George A. Olah, G. K. Surya Prakash, Kenneth Wade, Arpad Molnar, Robert E. Williams. 

It was Meerwein and van Emster who, in 1922, while studying the kinetics of the rearrangement of camphene hydrochloride (1) to isobomyl chloride (2) [Eq. (5.1)], noticed that the reaction rate increased in a general way with the dielectric constant of the solvent. Further, they found that metalhc chlorides such as SbCh, SnCb, FeCb, AlCb, and SbCf (but not BCh or SiCh), as well as dry HCl (all of which promote ionization of triphenylmethyl chloride by the formation of ionized complexes), also considerably accelerate the rearrangement of camphene hydrochloride. Meerwein concluded that the conversion of camphene hydrochloride to isobornyl chloride actually does not proceed by way of migration of the chlorine atom, but by a rearrangement of a cationic intermediate. Thus, the modern concept of carbocation intermediates was born. 

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