Page 291 - Mechanism and Theory in Organic Chemistry

lysis ofthreo-3-aryl-2-butyl brosylate (16) with a number of different X substituents.
Their goal was to calculate from both product and kinetic data the amount of
aryl participation in ionization and then to compare the results. If both methods
gave the same answer, that would be convincing evidence for the phenonium

They analyzed the product data by assuming that all the threo-acetate
formed arises from a phenonium ion and all the eythro-acetate from backside
assistance of the solvent to ionization. Then the percentage of aryl participation
is synonymous with the percentage of threo-acetate in the product. The product
analyses for acetolysis at 75OC are in the sixth column of Table 6.1.
The rate data were analyzed by assuming Equation 6.17. In this equation
kt is the titrametric rate constant for product formation, k, is the solvent-assisted
ionization constant, and FkA is the fraction of the aryl-assisted rate constant that
gives rise to product (as opposed to the fraction that gives starting material
through internal return).
kt = k, + Fk, (6.1 7)
The constant kt is determined experimentally, and k, can be calculated by use of a
Hammett plot as described below. Then Fk, can be calculated by simply sub-
tracting k, from kt. If FkA is the rate of formation of anchimerically assisted
solvolysis product, it should lead to threo-acetate. The product arising from
k, should lead to erythro-acetate. Therefore the fraction of threo-acetate ex-
pected can be calculated by Fk,/(Fk, + k,) or FkA/kt. Column five of Table 6.1
shows the calculated fraction of threo-acetate.
In order to find k, for each compound, the logs of the kt's for acetolysis of 16
were determined and plotted against the Hammett u constants of the X sub-
stituents (Figure 6.4). For electron-withdrawing X the plot is a straight line
with a negative slope. This is just what would be expected from an aryl-non-
assisted pathway in which a negative inductive effect from the phenyl ring de-
creases the rate of ionization (cf. the effect of substituents on the ionization of

Table 6.1 RATES AND PRODUCTS OF ACETOLYSIS SUBSTITUTED
OF
~~~~?~-~-PHENYL-~-BuTYL (16) AT 75.0°C
BROSYLATES
16 kt x 1 O5 k, x 1 O5 Fk, x 1 O5 FkA/k, x 100 Percent threo-
X = 17-acetate

SOURCE: H. C. Brown, C. J. Kim, C. J. Lancelot, and P. v. R. Schleyer, J. Amer. Chem. SOG., 92,5244
(1970). Reprinted by permission of the American Chemical Society.
as See note 28.