Page 233 - Mechanism and Theory in Organic Chemistry

return to RX.,O The net rate of disappearance of RX therefore increases more
SCHEME 531
RX R+X- R+ 11 X- R+ + X-
Intimate Solvent- Free ions
ion pair separated
(external)
ion pair SOH

v
ROS + H+ ROS + H+ ROS + H4
than it would if the new salt were exerting only the normal linear salt effect.
Racemizationd oxygen qilfirgtion occur through the intim-and
the s p e ~ i The s ~ ~
~origin of the induced common ion
R+ 1) X- + Y- + R+ 1) Y- + X- (5.9)
effect lies in the production through Equation 5.9 of free X- ions; added X-
reverses 5.9 by the usual mass law mechanism, fewer ioiipme diverted, more
can return to RX, and part of the acceleration caused by Y - is canceled. Benz-
hydryl systems show special salt effects with added azide through Equation 5.10;
in this case the R+ 11 N,- ion pair collapses to RN,, which is stable and accum-

ulates as one of the products.33
It should be pointed out that not all solvolyzing systems will exhibit the
phenomena associated with Scheme 5 ; except for those systems that yield relative-
ly stable carbocations, capture by the solvent at an early stage will preclude
observation of some or all of these subtle effects.

5.2 LIMITING UNIMOLECULAR NUCLEOPHILIC REACTIONS.
EFFXCTS OF STRUCTURE AND SOLVENT

In the previous section ye_ defined a limiting unimolecular substitu_fian-as one in
wh&hhekmkg groupkprbxith naa~ist_a_n~_from solveGt-other thaa non-
---
c-&ation of the incipient ions. We shall return in %ctTo~ ~4to prob-
the
lem of how to de<ide experimentally whether a reaction is following the limiting
pathway; here we want to look at predictions of the limiting S,1 mechanism
concerning the influence of structures and conditions on rates and on products.

30 See note 28, p. 221.
31 Adapted with permission from S. Winstein and G. C. Robinson, J. Amer. Chem. Soc., 80, 175
(1958). Copyright by the American Chemical Society.
In certain favorable cases of rearranging systems, the occurrence of internal return distinct from
external ion-pair return can be demonstrated without recourse to optical rotation or isotopic
labeling experiments. See S. Winstein and A. H. Fainberg, J. Amer. Chem. Soc., 80, 459 (1958);
S. Winstein, P. E. Klinedinst, Jr., and E. Clippinger, J. Amer. Chem. Soc., 83, 4986 (1961).
See note 17(b), p. 218.