From idea to long-lived
At the beginning of the 20th
century it was not known that reaction mechanisms
make use of short-lived reactive intermediates, but
the idea was born at about that time. In 1922 the
German chemist H. Meerwein explained his results from
studies of the kinetics of a rearrangement reaction
by suggesting that carbocations were formed as
During the 1930s C.-K. Ingold
and E.D. Hughes in England and later S. Winstein in
the USA performed detailed kinetic investigations of
substitution and elimination reactions. They found
that in many of these reactions carbocations were
intermediates. But they were so short-lived and
appeared in such low concentrations that they could
not be directly observed with physical methods.
The reason the carbocations
are short-lived is their extreme reactivity towards
surrounding molecules e.g. solvent molecules or
negative ions (nucleophiles).
During the 1950s Olah and his
co-workers, building on results obtained by Meerwein
in the 1940s, studied complexes which acylfluorides
and alkylfluorides form with borontrifluoride,
BF3, and other Lewis acids.
At the beginning of the1960s
Olah made the pioneering discovery that he could
prepare long-lived stable alkyl cations if he used
even stronger acids than BF3. He chose the
superacid antimon-pentafluoride SbF5.