Page 229 - Mechanism and Theory in Organic Chemistry

Table 5.2 RATE PROCESSES SOLVOLYSIS REACTIONS
IN
Reactiona Rate Constant Notation

RX + SOH + ROS + HX kt
d-RX or I-RX - ofitically inactive products k,
d-RX or I-RX + dl-RX
RX + *X- R-*X + X-

*O 0
11 I1
\
R-0-C-Ar - R-*O-C-Ar ke,
*
' denotes isotopically labeled atom; d- or 1- indicates a chiral and dl- a racemic substance.
and preference for backside attack (Chapter 4) means that net inversion will
occur. Ion pairs in which the anion remains closer to the carbon to which it was
originally attached could likewise accommodate the allylic product spread.
In Scheme 3 we modify the simple S,1 mechanism to include the ion-pair
idea. Change of substrate structure so as to make the carbocation less reactive,
3
SCHEME
kl
RX - RCX-
k-1
RCX- + SOH A ROS + X- + H+

R+ + SOH A ROS + Ht
or change of the solvent so as to aid ion separation, will favor the dissociation
step k, relative to direct reaction of the ion pair (k,), and will give results more
characteristic of the free ions. Elimination can occur in competition with substitu-
tion either at the ion-pair stage or at the free-ion stage.
Winstein and his research group elaborated the ion-pair mechanism in a
series of experiments reported during the 1950s and 1960s.16 Several kinds of
kinetic information about solvolysis reactions can be obtained in addition to the
rate of product formation, kt, determined by titration of acid formed. These
various rates are summarized in Table 5.2. Winstein's group found that for
substrates that have the leaving group bound to a chiral center, k, frequently
exceeds kt by substantial factors.17 For example, with p-chlorobenzhydryl
chloride in acetic acid, the ratio ka/kt is between 30 and 70, and is about 5 in
aqueous acetone.18 The excess of rate of loss of optical activity, k,, over rate of
product formation, kt, means that some process racemizes the substrate more
rapidly than the substrate can form products. The observation that in these same
systems k, is also larger than k,, rules out the possibility that racemization occurs

la For a summary and complete references, see P. D. Bartlett, "The Scientific Work of Saul Win-
stein," J. Amer. Chem. Soc., 94, 2161 (1972).
I' (a) S. Winstein and D. Trifan, J. Amer. Chem. Soc., 74, 1154 (1952) ; (b) S. Winstein and K. C.
Schreiber, J. Amer. Chem. Soc., 74, 2165 (1952).
Is S. Winstein, J. S. Gall, M. Hojo, and S. Smith, J. Amer. Chem. Soc., 82, 1010 (1960).