Page 231 - Mechanism and Theory in Organic Chemistry

in the ion pair is undoubtedly less than that required for racemization (Scheme
4), where the cation must turn over. Indeed, in various substituted benzhydryl
systems, racemization of unreacted substrate is slower than equilibration of the
oxygen^.^^ The a-phenylethyl systems yield similar results.23 Oxygen equilibra-
tion thus provides a more sensitive test for ionization followed by return than
does loss of chirality. It should nevertheless be pointed out that even this tech-
nique is not definitive. It is still possible that, after the ionization but before the
two oxygens become equivalent, there might be time for the oxygen originally
bonded to carbon to return.
Allylic systems have also provided fertile ground for investigation of ion-
pair phenomena. Young, Winstein, and Goering established the importance of
ion pairs in solvolysis of these compounds. They showed that ion pairs are
responsible for the rearrangement of a,a-dimethylallyl chloride to y,y-dimethyl-
ally1 chloride (Equation 5.8).24 Goering's labeling methods have subsequently
supplied a number of details about allylic ion-pair structure.25
_ _ One further detail of the ion-pair mechanism remains. Winstein's work
demonstrated th&in some systems at least. there is more than Fnexgi_ofion
- --
pas an the: sol~-. The evidence originates mainly with the effect 2
added salts on rates. Nearby ions affect the free energy of an ion in solution;
hence, a change in the concentration of dissolved salt will alter the rate of any
elementary step in which ions form or are destroyed. For S,1 solvdysex,therate
. .
i-,f. In the usual solvolysis solvents,
for example acetic acid, aqueous acetone, and ethanol, the increase follows the
linear Equation 5.8, where k,,,, is the rate constant with added salt and k, is the
rate constant in the absence of salt.26

Certain systems depart from this behavior. Addition of a low concentration
of a n&lcommon ionsalt such as lithium perchlorate causes a -;;iqc;,
but-aaa66eld&l~c~~l~e &rff ad-finallw expected
lineax- relation.-This s#ecial salt @ect27 is illustrated in Figure 5.1 for solvolys~s
of the rearranging system 3. Note that k, exhibits only the normal linear salt

aa See note 2 1, p. 219.
a3 H. L. Goering, R. G. Briody, and G. Sandrock, J. Amer. Chem. Soc., 92, 7401 (1970).
a4 W. G. Young, S. Winstein, and H. L. Goering, J. Amer. Chem. Soc., 73, 1958 (1951).
as (a) H. L. Goering and E. C. Linsay, J. Amer. Chem. Soc., 91, 7435 (1969); (b) H. L. Goering,
G. S. Koermer, and E. C. Linsay, J. Amer. Chem. Soc., 93, 1230 (1971); (c) H. L. Goering, M. M.
Pombo, and K. D. McMichael, J. Amer. Chem. Soc., 85, 965 (1963).
ae (a) A. H. Fainberg and S. Winstein, J. Amer. Chem. Soc., 78, 2763, 2780 (1956). Salt effects in
nonpolar solvents such as ether are of dramatic magnitude, and follow a more complex relationship.
See S. Winstein, E. C. Friedrich, and S. Smith, J. Amer. Chem. Soc., 86, 305 (1964). In water, the
relationship is logarithmic. See note 13, Chapter 7. (b) C. L. Perrin and J. Pressing, J. Amer. Chem.
Soc., 93, 5705 (1971) discuss the mechanism of the linear salt effect.
a7 S. Winstein and G. C. Robinson, J. Amer. Chem. Soc., 80, 169 (1958).