Page 230 - Mechanism and Theory in Organic Chemistry

Kinetics and Stereochemistry 219

through free ions.lg If free ions were forming, the X- anions would become
equivalent to any other X- ions present; if isotopically labeled *X- ions are
added, the process shown in Equation 5.6 would have to occur. Since the experi-
mental results show that racemization is faster than this process, there must be

present some intermediate or intermediates, presumably ion pairs, in which the
C--X bond is broken, but in which the X- ion is still closely associated with the
particular C+ ion to which it was bonded and to which it can return. lt is im-
portant to note that form of ion pairs and their return to th_e_ covalen9
As
bonded state does not ~ive -. rise - to the com-m-o-nien rate depres~ion.~~ long as
--
the X- ion remains associated with a particular carbocation in the ion pair, it is
part of a species chemically distinct from the free X- ions.
Excess of rate of racemization over rate of product formation supports the
idea of ion pairs that can return to substrate; nevertheless, as it is possible that
some, or even most, of the ion pairs return without racemizing (Scheme 4),
considerable doubt remains about k,, the rate of formation of the ion pairs. The
difficulty is that there is no way to detect the event represented by k, if it is fol-
lowed immediately by k -,. We shall know that something has happened only when
k, is followed by k, or by k,. If k-, competes with these processes, some ionization
will go undetected.
SCHEME 4
a a
\ kt \
C-X c+x-
,/'A k- I ,,'A
c b
d-RX cb products

a a "\ /;6
\ \ kt a\
C-X + 9-X 4 C+X- + F+X-
//'i // A k-1 /,'A ;
A
cb bc c b b c
dl-RX
Goering and his collaborators developed a method for finding a better
approximation to the ionization rate.21 Their technique uses as leaving group
an aryl ester, usually a p-nitrobenzoate. Equation 5.7 illustrates that if one
oxygen of the carboxyl group in the substrate is labeled, ionization and return
to the covalent state may bring about equilibration of the label between the two
oxygens. The structural change necessary to make the two oxygens equivalent

'@ See note 18.
S. Winstein, E. Clippinger, A. H. Fainberg, R. Heck, and G. C. Robinson, J. Amer. Chem. Soc.,
78, 328 (1956).
'' (a) H. L. Goering, R. G. Briody, and J. F. Levy, J. Amer. Chm. Soc., 85, 3059 (1963); (b) H. L.
Goering and H. Hopf, J. Amer. Chem. Soc., 93, 1224 (197 1).