Page 245 - Mechanism and Theory in Organic Chemistry

the carbonium ions in whkh the positive nr carb ons hasmination
_-
-b-------
numTe?-EZ 5, as in the brid~ed structure 19. The former structures, for many
years designated classical ions, have ordinary two-electron bonds; the latter,
known earlier as nonclassical ions, have a three-center, two-electron bond.69
Carbenium ions derived from alkenes by protonation may also be called alkenium
ions.
The Existence of Carbocations
Most carbocations are too reactive to be directly observable in ordinary solvents,
and until relatively recently evidence has been obtained indirectly, primarily
through the study of reaction kinetics and trapping processes, experiments dis-
cussed in Sections 5.1, 5.2, and 5.4. Nevertheless, a few types of compounds have
long been known to produce observable concentrations of positive ions relatively
easily. The triarylmethyl derivatives were the first of this type to be investigated;
the halides ionize readily in non-nucleophilic solvents such as sulfur dioxide,I0
and the alcohols yield solutions of the ions in concentrated sulfuric acid. Early
observations by the freezing-point depression technique (see Section 3.2, p. 130)
established that each mole of triphenyl carbinol yields 4 moles of ions in sulfuric
acid, the reaction presumably being by way of Equation 5.14.11 Results in
methane-sulfonic acid are similar.I2

The cryoscopic method is also applicable to other triarylmethyl systems, to
some diarymethyl and allylic ions, and, when ortho substituents are present, to
aryl acylium ions (20) (Equation 5.15) ;I3 unfortunately, side reactions frustrate
most attempts to generate carbocations in sulfuric acid.

More recently, development of the superacid solvent systems has permitted
the preparation at low temperature of stable solutions of carbocations of many
structural types. The solvents ordinarily used consist of the strong Lewis acid
antimony pentafluoride with or without an added protonic acid, usually hydro-

See note 68.
'O N. N. Lichtin, hg. Phys. Osg. Chem., 1, 75 (1963).
" (a) A. Hantzsch, 2. Physik. Chem., 61, 257 (1907); (b) L. P. Hammett and A. J. Deyrup, J. Amer.
Chem. Soc., 55, 1900 (1933).
72 R. A. Craig, A. B. Garrett, and M. S. Newman, J. Am. Chem. Soc., 72, 163 (1950).
73 (a) H. P. Treffers and L. P. Hammett, J. Amr. Chem. Soc., 59, 1708 (1937); (b) M. S. Newrnan
and N. C. Deno, J. Arner. Chem. SOC., 73, 3644 (1951); (c) N. C. Deno, H. G. Richey, Jr., J. D.
Hodge, and M. J. Wisotsky, J. Amr. Chem. Sod., 84, 1498 (1962).