Page 423 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

404 with the dioxane solvent, which would react with water to give product of retained
configuration. When azide ion is present, dioxane does not effectively compete for the
CHAPTER 4 19
ion pair intermediate and all of the alcohol arises from the inversion mechanism.
Nucleophilic Substitution
CH 3 CH 3 CH 3 CH 3
N 3 – or
R C OTs R C+ – OTs R C N 3 or R C OH
H 2 O
H H H inversion H
CH 3 CH 3
H 2 O
O O C OH
R C O+ O R
H H
inversion net retention

Entry 5 shows data for a tertiary chloride in several solvents. The results range
from nearly complete inversion in aqueous dioxane to slight net retention in TFE.
These results indicate that the tertiary carbocation formed does not achieve symmetrical
solvation but, instead, the stereochemistry is controlled by the immediate solvation
shell. Stabilization of a carbocation intermediate by benzylic conjugation, as in the
1-phenylethyl system shown in Entry 6, leads to substitution with extensive racem-
ization. A thorough analysis of the data concerning stereochemical, kinetic, and isotope
effects on solvolysis reactions of 1-phenylethyl chloride in several solvent systems has
20
been carried out. The system was analyzed in terms of the fate of the contact ion pair
and solvent-separated ion pair intermediates. From this analysis, it was estimated that
for every 100 molecules of 1-phenylethyl chloride that undergo ionization, 80 return
to starting material of retained configuration, 7 return to inverted starting material, and
13 go on to the solvent-separated ion pair in 97:3 TFE-H O. A change to a more nucle-
2
ophilic solvent mix (60% ethanol-water) increased the portion that solvolyzes to 28%.
13 0
+
R Cl R Cl – R + Cl – R + Cl –
+
80
SOH
6 1 SOH
–
Cl R Cl R + Cl – R + ROS + SOR
The results in Entry 7 show that even for the tertiary benzylic substrate
2-phenyl-2-butyl p-nitrobenzoate, the expectation of complete racemization is not
realized. In moderately nucleophilic media, such as potassium acetate in acetic acid,
this ideal is almost achieved, with just a slight excess of inversion. The presence of
the better nucleophile azide ion, however, leads to product with a significant (56%)
degree of inversion. This result is attributed to nucleophilic attack on an ion pair
prior to symmetrical solvation. More surprising is the observation of net retention of
configuration in the hydrolysis of 2-phenyl-2-butyl p-nitrobenzoate in 90% aqueous
acetone. It is possible that this is the result of preferential solvent collapse from the
front side at the solvent-separated ion pair stage. The bulky tertiary system may hinder
solvation from the rear side. It is also possible that hydrogen bonding between a water

19 H. Weiner and R. A. Sneen, J. Am. Chem. Soc., 87, 292 (1965).
20
V. J. Shiner, Jr., S. R. Hartshorn, and P. C. Vogel, J. Org. Chem., 38, 3604 (1973).