Page 447 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

428 derived from thermodynamic cycles that relate pK and electrochemical potentials.
a
The hydride affinity, − G, for the reaction
CHAPTER 4
−
+
Nucleophilic Substitution R +H → R–H
is a measure of carbocation stability. This quantity can be related to an electrochemical
potential by summation with the energy for hydrogen atom removal, i.e., the homolytic
bond dissociation energy.

+
R −H → R +H − G − H
.
.
R +H → R −H G hom
.
.
.
.
o
+
+
−
R +H → R +H − −F E H /H – R /R
so
.
.
o
+
−
G - = G −−F E H /H –R /R
H hom
. . . 71
−
where H /H and R /R are one-electron oxidation potentials for H and R .
−
+
.
The former potential is about −0 55V in DMSO. Measurement of R /R can be
+
accomplished by cyclic voltammetry for relatively stable carbocations and by other
methods for less stable cations. The values obtained range from 83 kcal/mol for the
aromatic tropylium ion to 130 kcal/mol for a benzylic cation destabilized by a EWG
substituents. Some of these data are included in Table 4.16. Note that these values
are considerably smaller than the corresponding gas phase values, which range from
200 kcal/mol for tropylium ion to 239 kcal/mol for the benzyl cation, although the
difference in stability is quite similar. This is the result of solvent stabilization.
It is possible to obtain thermodynamic data for the ionization of alkyl chlorides
by reaction with SbF , a strong Lewis acid, in the nonnucleophilic solvent SO ClF. 72
5
2
The solvation energies of the carbocations in this medium are small and do not differ
much from one another, which makes comparison of nonisomeric systems reasonable.
As long as subsequent reactions of the carbocation can be avoided, the thermodynamic
characteristics of the ionization reactions provide a measure of the relative ease of
carbocation formation in solution. There is good correlation between these data and the
Table 4.16. Solution Hydride Affinity of Some Carbocations a
Carbocation H (kcal/mol) H gas (kcal/mol)
Tropylium ion 83 200 b
Ph 3 C + 96 215
Ph 2 C H 105 222
+
PhCH + 2 118 238
p-CH 3 OC 6 H 4 CH + 2 106 227
p-NCC 6 H 4 CH + 122 247
2
a. J.-P. Cheng, K. L. Handoo, and V. D. Parker, J. Am. Chem. Soc., 115, 2655 (1993).
b. See Table 3.10.
71 J.-P. Cheng, K. L. Handoo, and V. D. Parker, J. Am. Chem. Soc., 115, 2655 (1993).
72
E. M. Arnett and N. J. Pienta, J. Am. Chem. Soc., 102, 3329 (1980); E. M. Arnett and T. C. Hofelich,
J. Am. Chem. Soc., 105, 2889 (1983).