Page 455 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

436 H O 3 SCF 3 CH 3 O 2 CCH 3 H O 2 CCH 3
C C CH 3 CO 2 H CH 3 CH C CH 2 + CH 3 C CCH 3 + C C + C C
5% 77% H CH 3 CH 3
CHAPTER 4 CH 3 CH 3 CH 3
16% 1%
Nucleophilic Substitution
Ref. 99

The phenyl cation is a very unstable cation, as is reflected by the high hydride
affinity shown in Figure 3.18. In this case, the ring geometry resists rehybridization so
2
the vacant orbital retains sp character. Since the empty orbital is in the nodal plane
of the ring, it receives no stabilization from the electrons.

+

Phenyl cations are formed by thermal decomposition of aryldiazonium ions. 100 The
cation is so reactive that under some circumstances it can recapture the nitrogen
generated in the decomposition. 101 Attempts to observe formation of phenyl cations
by ionization of aryl triflates have only succeeded when especially stabilizing groups,
such as trimethylsilyl groups are present at the 2- and 6-positions of the aromatic
ring. 102

4.4.2. Direct Observation of Carbocations

A major advance in the study of carbocations occurred during the 1960s when
methods for generation of carbocations in superacid media were developed. The term
superacid refers to media of very high proton-donating capacity, e.g., more acidic
than 100% sulfuric acid. A convenient medium for these studies is FSO H −SbF −
5
3
SO . The fluorosulfonic acid acts as a proton donor and antimony pentafluoride is
2
a powerful Lewis acid that assists ionization. This particular combination has been
dubbed “magic acid” because of its powerful protonating ability. The solution is
essentially nonnucleophilic, so carbocation of even moderate stability can be generated
and observed by NMR spectroscopy. 103 Some examples of these studies are given in
Scheme 4.4. Alkyl halides and alcohols, depending on the structure of the alkyl group,
react with magic acid and give rise to carbocations. Primary and secondary alcohols
are protonated at −60 C, but do not ionize. Tertiary alcohols do ionize, giving rise to

the corresponding cation. As the temperature is increased, carbocation formation also
occurs from secondary alcohols. sec-Butyl alcohol ionizes with rearrangement to the

99
R. H. Summerville, C. A. Senkler, P. v. R. Schleyer, T. E. Dueber, and P. J. Stang, J. Am. Chem. Soc.,
96, 1100 (1974).
100 C. G. Swain, J. E. Sheats, and K. G. Harbison, J. Am. Chem. Soc., 97, 783 (1975).
101
R. G. Bergstrom, R. G. M. Landells, G. W. Wahl, Jr., and H. Zollinger, J. Am. Chem. Soc., 98, 3301
(1976).
102 Y. Apeloig and D. Arad, J. Am. Chem. Soc., 107, 5285 (1985); Y. Himeshima, H. Kobayashi, and
T. Sonoda, J. Am. Chem. Soc., 107, 5286 (1985).
103
A review of the extensive studies of carbocations in superacid media is available in G. A. Olah,
G. K. Surya Prakash, and J. Sommer, Super Acids, John Wiley & Sons, New York, 1985.