Page 77 - Mechanism and Theory in Organic Chemistry

k k
log - log -
ko ko

Figure 2.2 Bromination of monosubstituted benzenes in acetic acid. From H. C. Brown and
Y. Okamoto, J. Amer. Chem. Soc., 80, 4979 (1958). Reprinted by permission of
the American Chemical Society.

an and a0 Constants
a+ and a- constants have been widely and successfully used. However, they have
also been strongly criticized by Wepster" and by Taft.12 Both investigators pre-
dicted that the ability of a substituent to interact with a reaction site by resonance
should depend on the exact nature of the reaction as well as on the substituent
and there should, therefore, be a whole spectrum of a values for every substituent.
Wepster introduced the an constant as the "normal" a value, representative only
of inductive effects and free of all resonance effects. The constants for m-C1,
m-CH,, and m-NO, were taken by Wepster as primary an values.13 The a,,,,
constants were rejected because, even in the absence of through-resonance,
resonance can effect the electron supply at the site of reaction. (Consider, for
example, the contribution from Structure 14 to the anion ofp-nitrobenzoic acid.

There is no through-resonance between the nitro group and the negative charge
on the carboxylate group. However, the partial positive charge, which results
from electron-withdrawing resonance by the nitro group, on the ring carbon that
bears the carboxylate does stabilize the negative charge.) Using only the primary
an values, p for any reaction of interest is calculated. Once p for a reaction is
known, the a constants of all other substituents for that reaction can be found.

l1 (a) H. van Bekkum, P. E. Verkade, and B. M. Wepster, Rec. Trav. Chim. Pays-Bar, 78, 815 (1959);
(b) A. J. Hoefnagel,.J. C. Monshouwer, E. C. G. Snorn, and B. M. Wepster, J. Amer. Chem. SOC., 95,
5350 (1973).
l2 R.'w. aft, ~r. and I. C. Lewis, J. Amer. Chem. Soc., 80,2436 (1958); 81,5343 (1959); R. W. Taft,
Jr. and I. C. Lewis, Tetrahedan, 5, 2 10 (1959).
l3 The u constants obtained for substituents electron-donating by resonance that are para to a
reaction site that becomes electron-rich, and for substituents electron-withdrawing by resonance that
are para to a reaction site that becomes electron-deficient, were also taken as un values, since in these
cases there would be no stabilization by resonance.