Page 255 - Mechanism and Theory in Organic Chemistry

sions.lOl Evidently, earlier arguments in the literature based on the assumption
that secondary systems are limiting or close to it must be reevaluated in the light
of these findings. It also seems quite clear that, despite earlier reports,lo2 primary
systems do not follow a limiting mechanism, even in acidic solvents.
As we shall see in more detail in Chapter 6, there are structures in which an
-
internal nucleoph;1., in the form of a neighboring group such as phenyl, 3-
the ionization. In these instances, the neighboring group takes up the space at
t w the reacting carbon opposite the leavi~group and so blowyept
. . ---
participa&&g-amPtlmP~~l_oping electron
deficiency _and SO rahu5nq the need of nucleophi~~~~olvent. For
molecules of this kind, then, secondary and even primary substrates can solvolyze
without solvent participation, by a mechanism we may call SN1 with internal
assistance. This process may occur in competition with a path like that for ordinary
primary and secondary substrates, in which solvent participates and the internal
group does not.lo3
Returning, then, to the two alternatives for solvolysis mechanisms with
which we began this section, it appears that it is indeed possible to construct
systems that solvolyze without nucleophilic assistance from solvent. For solvent-
assisted reactions, the two alternatives are essentially equivalent; we can there-
fore choose the first alternative as being more consistent with current informa-
tion.
The Ion-Pair Mechanism
The results we have cited do not bear on the ion-pair question. It is still possible
that the reactions occurring with participation of nucleophile are attacks on a
reversibly formed ion pair rather than on the covalent substrate. Sneen and
Larsen found that 2-octyl methanesulfonate reacts in aqueous dioxane containing
added azide ion to yield a mixture of 2-octyl alcohol and 2-octyl azide.lo4 The
water and azide ion are competing, as we would expect on the basis of the dis-
cussion above. But the ratio of the rate of disappearance of the methanesulfonate
in the presence of azide to the rate in the absence of azide was that expected
if there were an intermediate that could react in any one of three ways: return to
substrate, combination with water, or combination with azide. (The derivation
of the rate expressions is left to the reader in Problem 15.) Sneen and Larsen pro-
posed that the intermediate is an ion pair. This finding, in a system that would
have been expected to react by an SN2-like process, led them to propose that all
nucleophilic substitutions, SN2 and SN1 alike, react through ion pairs. The charac-
teristic SN2 kinetic behavior would be the consequence of rate-determining attack
by nucleophile on ion pair rather than on covalent substrate (Figure 5.8).
Earlier, Swain and Kreevoy had suggested the possibility of rate-determining
attack by methanol on ion pairs from triphenylmethyl chloride in benzene sol-

lol J. M. Harris, D. J. Raber, R. E. Hall, and P. v. R. Schleyer, J. Amer. Chem. Soc., 92,5729 (1970).
lo2 See C. K. Ingold, Structure and Mechanism in Organic Chemistry, p. 436.
lo3 (a) F. L. Schadt and P. v. R. Schleyer, J. Amer. Chem. Soc., 95, 7860 (1973); (b) G. A. Dafforn
and A. Streitwieser, Jr., Tetrahedron Lett., 3159 (1970); (c) P. C. Myhre and E. Evans, J. Amer.
Chem. Soc., 91, 5641 (1969).
lo4 (a) R. A. Sneen and J. W. Larsen, J. Amer. Chem. Soc., 91, 362, 6031 (1969); (b) R. A. Sneen,
Accts. Chem. Res., 6, 46 (1973).