Page 472 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

be so close as to prevent a clear distinction as to stability. (2) The molecule may 453
adopt a geometry that is intermediate between a classical geometry and a symmetrical
∗∗
bridged structure. Computational studies (MP2/6-311G ) have been carried out on SECTION 4.4
several nonclassical carbocations. 153 The results show structural features similar to the Structure and Reactions
of Carbocation
norbornyl cation, with relatively long ∼ 1 8Å bonds to the bridging carbon and a Intermediates
much shorter (1 39Å) bond between the bridged carbons. The bond path from the
bridging carbon is directed between the two bridged carbons, with a bond order of
∼ 0 48, whereas the bridged bond order is ∼ 1 2.
+
The C H cation shown as the first entry in Scheme 4.5 is a particularly interesting
4 7
case. It can be described as a bridged structure that is isomeric with cyclopropylmethyl
and cyclobutyl ions.
H
H H H exo
+ H +
H + +
+ H H
H H H H endo
H
NMR studies show that all three methylene groups are equivalent, but the exo and
endo sets of hydrogen do not exchange. The barrier for exchange among the three CH 2
groups is < 2kcal. MO calculations at the MP4SDTQ/6-31G level indicate that both
∗
the cyclopropylmethyl and the bridged (bicyclobutonium) cations are energy minima,
differing by only 0.26 kcal. The secondary cyclobutyl cation is about 12 kcal higher in
energy. 154 The bridged structure is a tetracyclic cation in which each of the methylene
groups is pentacoordinate. 155
To summarize, bridged structures are readily attainable intermediates or transition
structures for many cations and are intimately involved in rearrangement processes.
In some cases, such as the norbornyl cation, the bridged structure is the most
stable one. As a broad generalization, tertiary cations are nearly always more
stable than related bridged ions and therefore have classical structures. Primary
carbocations can be expected to undergo rearrangement to more stable secondary
or tertiary ions, with bridged ions being likely transition structures (or interme-
+
diates) on the rearrangement path. Recall that the ethylium ion, C H ,isanH-
2 5
bridged structure in the gas phase. Unlike other primary carbocations, it cannot
rearrange to a more stable secondary structures. The energy balance between
classical secondary structures and bridged structures is close and depends on the
individual system. Bridged structures are most likely to be stable where a strained
bond can participate in bridging or where solvation of the positive charge is
difficult. Because of poor solvation, bridged structures are particularly likely to
be favored in superacid media and in the gas phase. In the cases examined
so far, proximity to anions favors classical structures in relation to bridged
structures.

153 I. Alkorta, J. L. M. Abboud, E. Quintanilla, and J. Z. Davalos, J. Phys. Org. Chem., 16, 546 (2003).
154 M. Saunders, K. E. Laidig, K. B. Wiberg, and P. v. R. Schleyer, J. Am. Chem. Soc., 110, 7652 (1988);
S. Sieber, P. v. R. Schleyer, A. H. Otto, J. Gauss, F. Reichel, and D. Cremer, J. Phys. Org. Chem., 6,
445 (1993).
155
For further discussion of this structure see R. F. W. Bader and K. E. Laidig, Theochem, 261, 1 (1992).