Page 256 - Mechanism and Theory in Organic Chemistry

Mechanisms Intermediate Between SN 1 and SN2 245

vent.lo5 Shiner and his co-workers have also found evidence for rate-determining
attack on ion pairs.lo6
In analyzing their data, Sneen and Larsen had to correct for salt effects,
since they were comparing rate with azide present to rate without.lo7 Schleyer
and co-workers have criticized Sneen's conclusions by pointing out the uncer-
tainties involved in such corrections,lo8 and Sneen has replied, justifying his
earlier conclusions and presenting similar evidence for a-phenylethyl systems,log
and for an allylic system.l10 The question is far from settled, and will continue to
be a subject of investigation.ll1

Solvent Nucleophilicity
A point of key importance in study of solvolysis is the nucleophilicity of the sol-
vent. Whereas the Y and other scales have been available for measuring ionizing
power for some years, there has been no satisfactory scale for nucleophilicity.
Swain, Mosely, and Bown attempted to set up an equation for correlation
of solvolysis rates that included both nucleophilicity and ionizing power;l12
their system did not prove particularly helpful for understanding mechanism.l13
The Swain-Scott equation, discussed in Chapter 4 (p. 185), was not evaluated for
solvents.
For lack of a better system, the ratio of rate in an ethanol-water mixture of
the same Y value as acetic acid to rate in the much less nucleophilic acetic acid,
(k,,oH/k,,oH)y, has served as a measure of sensitivity to solvent nucleophilicity.
More recently, the problem has received renewed attention, and two groups have
proposed possible approaches.l14 Of the two proposals, that of Bentley, Schadt,
and Schleyer is easier to apply. Their scheme defines the solvent nucleophilicity,
N, by Equation 5.21, where k is the solvolysis rate constant of methyl tosylate in

the solvent of interest, ko is the solvolysis rate constant of methyl tosylate in the
reference solvent, and the term -0.3Y corrects for susceptibility to ionizing
power. (The value 0.3 is m for methyl tosylate in the Winstein-Grunwald mY
correlation for the very non-nucleophilic solvents acetic acid and formic acid.)
Methyl tosylate solvolysis thus serves as the standard reaction for determining
nucleophilicity, just as t-butyl chloride solvolysis (now replaced by 2-adamantyl
tosylate solvolysis) does for finding Y.

lo6 C. G. Swain and M. M. Kreevoy, J. Amer. Chem. Soc., 77, 1122 (1955).
loe (a) V. J. Shiner, Jr., R. D. Fisher, and W. Dowd, J. Amer. Chem. Soc., 91, 7748 (1969); (b) V. J.
Shiner, Jr., S. R. Hartshorn, and P. C. Vogel, J. Org. Chem., 38, 3604 (1973).
lo7See note 104(a).
loB D. J. Raber, J. M. Harris, R. E. Hall, and P. v. R. Schleyer, J. Amer. Chem. Soc., 93,4821 (1971).
log R. A. Sneen and H. M. Robbins, J. Amer. Chem. Soc., 94, 7868 (1972).
110 R. A. Sneen and W. A. Bradley, J. Amer. Chem. Soc., 94, 6975 (1972).
'I1 See, for example, W. T. Bentley, S. H. Liggero, M. A. Imhoff, and P. v. R. Schleyer, J. Amer.
Chem. Soc., 96, 1970 (1974) ; (b) F. G. Bordwell and G. A. Pagani, J. Amer. Chem. Soc., 97, 1 18 (1975),
and following papers.
lla C. G. Swain, R. B. Mosely, and D. E. Bown, J. Amer. Chem. Soc., 77, 3731 (1955).
llS S. Winstein, A. H. Fainberg, and E. Grunwald, J. Amer. Chem. Soc., 79, 4146 (1957).
11' (a) P. E. Peterson and F. J. Waller, J. Amer. Chem. Soc., 94, 991 (1972); (b) T. W. Bentley, F. L.
Schadt, and P. v. R. Schleyer, J. Amer. Chem. Soc., 94, 992 (1972).