Page 194 - Mechanism and Theory in Organic Chemistry

The Solvent, Substrate, Nucleophile, and Leaving Group 183
decrease and~we have the situation already discussed above in which electron-
withdrawing groups increase the rate; but,iffXjs an excellent leaving group and
.
.
-.
.
..
.
,.
withdraqv~.c!e_ctr.ons more thhaann-Y .su.ppl.!es them,then the partial positive chaEge
on the reacting carbon.should increase ad. elec~on-withdra~in~substituents
-....
.
..
.
should decrease the rate.36 (Of course, if the situation just described becomes very
.
.
.
...
..
~
pronounced, the mechanism will change to SN1 .) The nature of the substrate also
plays a role in determining the relative extents of bond making and breaking. If
it can form a relatively stable carbocation, bond breaking is likely to have pro-
ceeded further. Thus it is not surprising that the rates of SN2 reactions that differ
in nucleophile, leaving group, and/or substrate show a variable dependence on
the polar influence of substituents.
Another reason for the apparent inconsistencies in experimental data is that,
depending on the nature and position of the substituent, steric and conjugative
effects often outweigh polar influences. In order to study pure polar effects, Holtz
and Stock have carried out rate studies of displacements by thiophenoxide ion on
4-Z-bicyclo-[2.2.2]-octylmethyl toluenesulfonate (Equation 4.17) .37

This system has the virtues of (1) being completely rigid so that changing Z
does not change the steric environment of the transition state and (2) having the
substituent so far removed from the reaction site through saturated bonds that
conjugation is impossible. They found that although alkyl substituents had little
effect on the reaction rate, electron-withdrawing groups in general did increase

Table 4.3 RELATIVE RATES OF ~-Z-B~C~C~O[~.~.~]~CTYLMETHYL
ION
TOLUENESULFONATE THIOPHENOXIDE
WITH
Relative Rate

SOURCE: H. 0. Holtz and L. M. Stock, J. Amr. Chem. Soc., 87, 2404 (1965). Reproduced by per-
mission of the American Chemical Society.

'' C. G. Swain and W. P. Langsdorf, Jr., J. Amr. Chem. Soc., 73, 2813 (1951). See, for example, the
calculations of R. F. W. Bader, A. J. Duke, and R. R. Messer, J. Amer. Chem. Soc., 95, 7715 (1973).
See note 24 (a), p. 180.