Page 320 - Mechanism and Theory in Organic Chemistry

Carbonium Ions 307
There are abundant examples that show that classical nonbridged 2-
norbornenium ions certainly do exist if there is an alkyl or other electron-releasing
group on C2.110 For example, optically active 89 solvolyses with partial retention
of configuration. Thus the carbenium ion must be formed in spite of the fact that
C, is an excellent bridging group.ll1

When 2-norbornyl fluoride is dissolved in superacid solution, a carbo-
cation is obtained as shown in Equation 6.42. This ion has been examined by a
number of physical methods, and the data are consistent with its structure being
the bridged ion, 82.112 For example, the ion has been examined by ESCA

(electron spectroscopy for chemical analysis). By this method one can determine
the energy required to remove inner shell electrons from around the nucleus.
A sample is exposed to high-energy X-rays of known wavelength, which cause
electrons to be ejected from the molecule. The energy conservation expression
for the photoemission process can be expressed by

where E,,, E,,,, and Eb are the X-ray energy, the kinetic energy of the electron
emitted, and the binding energy of the electron emitted, respectively. Ed is a
constant for a given system and can be determined. An electron multiplier de-
tector counts the emitted electrons, and an electron energy analyzer determines
the kinetic energies of the emitted electrons. Thus Eb can be determined from
Equation 6.43.113
The energy required to remove a 1s electron from a hydrocarbon is almost a
constant. For example, by ESCA one cannot distinguish between benzene and
neopentane. In classical, nonresonance-stabilized carbocations, the positive
charge is usually centered on a single atom, and thus more energy must be applied
to remove an electron from this atom than from its uncharged neighbors. Figure
6.15 shows the carbon 1s electron spectrum for the t-butyl cation. The positive
carbon is well separated from the carbons of the methyl groups. Figure 6.16

110 For a study of how electron-releasing a 2-substituent must be for a 2-norbornyl derivative to
ionize to the unbridged derivative, see D. G. Farnum and A. D. Wolf, J. Amer. Chem. Soc., 96, 5166
(1974).
ll1 H. L. Goering, C.-S. Chan, and J. V. Clevenger, J. Amer. Chem. Soc., 96, 7602 (1974).
112 G. A. Olah, G. Liang, G. D. Mateescu, and J. L. Riernenschneider, J. Amer. Chem. Soc., 95, 8698
11973) and references therein.
\- z
113 For a reivew of ESCA studies, see J. M. Hollander and W. L. Jolly, Accfs. Chem. Res., 3, 193
(1970).