Page 445 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

426 that the carbocation not be prohibitively high in energy. Carbocations are inherently
high-energy species. The ionization of t-butyl chloride is endothermic by 153 kcal/mol
CHAPTER 4 in the gas phase. 67
Nucleophilic Substitution
(CH ) C Cl (CH ) C + + Cl –
3 3
3 3
An activation energy of this magnitude would lead to an unobservably slow reaction at
normal temperature. Carbocation formation in solution is feasible because of solvation
of the ions that are produced. It is also important to understand the effect structure and
substituents have on carbocation stability. We introduced this subject in Section 3.4.1,
where we emphasized inherent structural effects in the gas phase, but owing to the
important role of solvation in ionization reactions, we have to consider carbocation
stability in solution as well. A method that is applicable to highly stabilized cations
is to determine the extent of carbocation formation from the parent alcohol in acidic
solution. The triarylmethyl cations are stabilized by the conjugation that delocalizes the
positive charge. In acidic solution, equilibrium is established between triarylcarbinols
and the corresponding carbocation:

ROH + H + R + + H 2 O

The relative stability of the carbocation can be expressed in terms of its pK +, which
R
is defined as
R
+
pK + = log +H R
R
ROH
where H is an acidity function defined for the medium. 68 In dilute aqueous solution,
R
H is equivalent to pH, and pK + is equal to the pH at which the carbocation and
R
R
alcohol are present in equal concentrations. The values shown in Table 4.15 were
determined by measuring the extent of carbocation formation at several acidities and
applying the definition of pK +.
R
The pK + values allow for a comparison of the stability of relatively stable carbo-
R
cations. The data in Table 4.15 show that ERG substituents on the aryl rings stabilize
the carbocation (less negative pK + , whereas EWGs such as nitro are destabilizing.
R
This is as expected from the electron-deficient nature of carbocations. The diarylmethyl
cations listed in Table 4.14 are 6–7 pK + units less stable than the corresponding
R
triarylmethyl cations. This indicates that the additional aryl groups have a cumulative,
although not necessarily additive, effect on the stability of the carbocation. Primary
R
benzylic cations are generally not sufficiently stable for direct determination of pK +
values. A value of ≤ 20 has been assigned to the benzyl cation based on rate measure-
ments for the forward and reverse reactions. 69 A particularly stable benzylic ion, the
R R
2,4,6-trimethylphenylmethyl cation has a pK + of −17 4. t-Alkyl cations have pK +
values around −15.
Several very stable carbocations are included in the “Other Carbocations” part
of Table 4.15. The tricyclopropylmethyl cation, for example, is more stable than the

67 D. W. Berman, V. Anicich, and J. L. Beauchamp, J. Am. Chem. Soc., 101, 1239 (1979).
68 N. C. Deno, J. J. Jaruzelski, and A. Schriesheim, J. Am. Chem. Soc., 77, 3044 (1955).
69
T. L. Amyes, J. P. Richard, and M. Novak, J. Am. Chem. Soc., 114, 8032 (1992).