Page 457 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

438 tert-butyl cation. At −30 C the protonated primary isomer, iso-butyl alcohol, ionizes,
also forming the tert-butyl cation. Protonated n-butanol is stable to 0 C, at which

CHAPTER 4 point it, too, gives rise to the t-butyl cation. It is typically observed that ionizations
Nucleophilic Substitution in superacids give rise to the most stable of the isomeric carbocations that can be
derived from the alkyl group. The t-butyl cation is generated from C systems, whereas
4
C and C alcohols give rise to the t-pentyl and t-hexyl ions, respectively. Some
5 6
examples of these studies are given in Scheme 4.4. Entries 6 to 9 and 10 to 12 further
illustrate the tendency for rearrangement to the most stable cation to occur. The tertiary
1-methylcyclopentyl cation is the only ion observed from a variety of five- and six-
membered ring derivatives. The tertiary bicyclo[3.3.0]octyl cation is formed from all
bicyclooctyl C H precursors. The tendency to rearrange to the thermodynamically
+
8 13
stable ions by multiple migrations is a consequence of the very low nucleophilicity of
the solvent system. In the absence of nucleophilic capture by solvent, the carbocations
undergo extensive skeletal rearrangement and accumulate as the most stable isomer.
Another important development in permitting structural conclusions from NMR
studies on carbocations resulted from the use of theoretical computations of 13 C and
1 H chemical shifts. Known as the MP2-GIAO method, 104 it has also been applied
successfully to allylic, cyclopropylmethyl, and phenonium ions. 105
Carbocations can also be studied by X-ray crystallography. 106 Early studies
involved strongly stabilized cations such as triphenylmethyl 107 and cyclopropylmethyl
cations. 108 More recently, the structure of less stable ions, including the t-butyl cation,
have been obtained. 109 The structure is planar with C−C bonds averaging 1 442Å.
2
3
This is substantially less than the sp –sp bond length in neutral compounds, which
is about 1 50Å. This finding is consistent with C−H hyperconjugation, although the
structure determination did not permit assignment of C−H bond lengths.

4.4.3. Competing Reactions of Carbocations
The product of a substitution reaction that follows the limiting S 2 mechanism
N
is determined by the identity of the nucleophile. The nucleophile replaces the leaving
group and product mixtures are obtained only if there is competition from several
nucleophiles. Product mixtures from ionization mechanisms are often more complex.
For many carbocations there are two competing processes that lead to other products:
elimination and rearrangement. We discuss rearrangements in the next section. Here
we consider the competition between substitution and elimination under solvolysis
conditions. We return to another aspect of this competition in Section 5.10, when
base-mediated elimination is considered.
The fundamental nature of the substitution-versus-elimination competition is illus-
trated in Figure 4.10, which is applicable to carbocations such as tertiary alkyl and
secondary benzylic that have lifetimes on the order of 10 −12 −1 in hydroxylic solvents
s
(SOH). The carbocation is at a relatively high energy, with very small barriers to either
solvent capture (k , substitution product) or proton loss (k , elimination product.) The
s
e
104
J. Gauss, J. Chem. Phys., 99, 3629 (1993).
105 P. v. R. Schleyer and C. Maerker, Pure Appl. Chem., 67, 755 (1995).
106 T. Laube, Acc. Chem. Res., 28, 399 (1995).
107
A. H. Gomes de Mesquita, C. H. MacGillavry, and K. Eriks, Acta Cryst.18, 437 (1965).
108 R. F. Childs, R. Faggiani, C. J. L. Lock, M. Mahendran, and S. D. Zweep, J. Am. Chem. Soc., 108,
1692 (1986).
109
S. Hollenstein and T. Laube, J. Am. Chem. Soc., 115, 7240 (1993).