Page 225 - Mechanism and Theory in Organic Chemistry

The observed kinetics of the solvolysis is first-order in the benzhydryl chloride.
This fact alone tells little about the mechanism because
soJcen& is alway_s_present inlarge excess, & we have seen in Section 2.5 (p. 92),
-
-
its concentration therefore undergms a negligible proportiodchange and will
not-e-intnkrate~x~region even if it is involveddgt-_or before the rate-
determinhgsteiteed nucleophiles have relatively small effects on therate,
and ~~arbu~c_le~ghdic sahdfk~
,- onlywfromher.
Most added salts accelerate the pjocess, but common ion salts_(chlor_ides iq the
-- ---- . .
of
case -- benzhydryl chloride) mak e it slower, azide causes
formation of some benzhydryl aide, but affects th_e_r_af.e-in>he ~ame-_~ay?is
&
oThe
~ r ~ n ~ ah2 ~stereochemistry of the substitution can be
c
investigated by starting with a chiral analog such. as p-chlorobenzhydryl chlo-
ride. The salv01ysis products are almost completely racem-i~,Although these
results have been known for many years,3 a number of questions about details of
mechanism remain unanswered.
The SN1 Mechanism
Early investigators, notably Hughes, Ingold, and co-workers, accounted for the
a rate-
solvolysis results by proposing the S,1 mexhan-s --
.
. .
..
- d tn a carbocatio~ed by rap-cdlheion
b w Equations 5.2 and 5.3 delineate the SN1 route for the case of ion
capture by solvent.
kt
RCl - + C1-
R+
k-I
The reverse of the second step, although it should be included to be rigorously
correct, is frequently omitted because the final product in many cases is suffi-
ciently unreactive that no experimentally significant amount will return to
carbocation during the time the reaction is under observation.
This simple two-step mechanism, when combined with the stationary-state
assumption for the presumably highly reactive positive ion (see Section 2.5,
p. 93), leads to the prediction given in Equation 5.4 for the rate of product
formation. (See Problem 1.) The term in-paredxses in Equam.4 -. will

----
reduce to unity whenever k,[SOH] >> k-,[Cl-] ; in that case a simple firskwder

(a) L. C. Bateman, E. D. Hughes, and C. K. Ingold, J. Chem. Soc., 974 (1940); (b) C. G. Swain,
C. B. Scott, and K. H. Lohmann, J. Amer. Chem. Soc., 75, 136 (1953); (c) D. Kovatevid, Z. Majerski,
S. BorEid, and D. E. Sunko, Tetrahedron, 28, 2469 (1972).
For early work on benzhydryl solvolysis, see L. C. Bateman, M. G. Church, E. D. Hughes, C. K.
Ingold, and N. A. Taher, J. Chem. Soc., 979 (1940), and references cited therein.
* The mechanism was proposed by S. C. J. Olivier and G. Berger, Rec. Trav. Chim., 45, 712 (1926);
A. M. Ward, J. Chem. Soc., 2285 (1927); and C. K. Ingold, Ann. Reps. Chem. Soc., 24, 156 (1927). It
was set out in detail by E. D. Hughes, C. K. Ingold, and C. S. Patel, J. Chem. Soc., 526 (1933); the
SN1 terminology was introduced by J. L. ~leaie, E. D. Hughes, and C. K. Ingold, J. Chm. Soc.,
236 (1935).