Page 250 - Mechanism and Theory in Organic Chemistry

Mechanisms Intermediate Between SN1 and S,2 239
Solvolysis Mechanisms
The problem of how to classify and account for this intermediate behavior
continues to plague chemists interested in mechanism. The greatest difficulty
arises for solvolysis, because the kinetic behavior with respect to solvent cannot
be determined; we shall be concerned here primarily with reactions of this type.
An obvious possibility is that in some cases SN2 and limiting SN1 processes
occur simultaneously. This idea does not seem to have been particularly fruit-
f~l.~l Most discussions of the problem assume that there is a range of mechanism
possible between the extremes, and that even in intermediate cases some particu-
lar mechanism prevails.92
Three central themes are important in the mechanistic investigations. The
first possibility is that there is an experimentally detectable distinction between
cases in which a particular solvent molecule assists departure of the leaving
group by forming a covalent bond to carbon at the transition state, and cases in
which the solvent stabilizes the transition state and resulting ion pair only by
nonspecific electrostatic solvation interaction^.^^ This hypothesis allows for a
range of behavior by postulating that bonding to the leaving group, and, in the
solvent-assisted cases, to solvent, may be strong or weak, and by allowing the
intervention of ion-pair intermediates. Figure 5.6 summarizes the argument in
the form of reaction coordinate diagrams.
The second alternative is that there is only one mechanism; specific bonding
to some nucleophile always assists the breaking of the C-X bond, even if only
slightly. SN2 behavior arises from a "tight" transition state with both entering
and leaving groups close and strongly interacting; SN1 behavior is the result of a
"loose" transition state, C-X bond nearly completely broken, and S-C bond
only just starting to form.94 Again, the initial product can be an ion pair. We
outline this proposal in Figure 5.7. Comparison of Figures 5.6 and 5.7 will show
that the only real distinction between alternatives 1 and 2 is in their description
of the SN1 process.
Finally, a third idea, not a separate mechanism but a concept that can be
applied to either of the other two, is that an ion pair is always formed first, so that
even the "pure" SN2 reaction has an intermediate. This possibility is shown in
Figure 5.8.
The tools used in investigating the mechanistic problem are those we have

@' For a contrary view, see (a) G. Kohnstam, A. Queen, and B. Shillaker, Proc. Chem. Soc., 157 (1959) ;
(b) B. J. Gregory, G. Kohnstam, M. Paddon-Row, and A. Queen, Chem. Commun., 1032 (1970); and
for a refutation of their interpretation, (c) R. A. Sneen and J. W. Larsen, J. Amer. Chem. SOC., 91,6031
(1969); (d) R. A. Sneen and H. M. Robbins, J. Amer. Chem. SOC., 94, 7868 (1972).
The concepts associated with mechanism are statistical; a mechanism is an average path for a
large number of molecules. On a molecular levcl, individual molecules follow different paths
across the potential energy surface. (Refer to the discussion in Section 2.6, p. 99). By a "single
mechanism" we mean z valley across the potential energy surface with a high point lower than the
high points of other valleys by an energy large compared to kT. Two simultaneous mechanisms
would occur if there were two valleys leading from reactant to product with high points of nearly the
same energy but separated from each other by hills high compared to kT.
O3 (a) C. K. Ingold, Structure and Mechanism in Organic Chemistry, 2nd ed., Cornell University Press,
Ithaca, N.Y., 1969, chap. VII; (b) W. v. E. Doering and H. H. Zeiss, J. Amer. Chem. Soc., 75, 4733
(1953).
84 (a) E. R. Thornton, J. Amer. Chem. Soc., 89, 2915 (1967); (b) G. J. Frisone and E. R. Thornton,
J. Amer. Chem. SOC., 90, 12 1 1 (1 968).