Page 76 - Mechanism and Theory in Organic Chemistry

Linear Free-Energy Relationships 65

because the COO- group in the benzoate anion cannot come into direct reso-
nance interaction with any ring substituent. Similarly, a P-methoxide group is
much more effective at increasing the rate of ionization of triphenylmethyl
chloride than would be predicted from the a, - .,,= constant (see Equation 2.13).

Several investigators found8 that rate and equilibrium constants can be
better correlated by the Hammett equation if two new types of a constants are
.-. site,that be-
introduced. When there is through-resonance between a . reaction .~
.
. . .
.
.
.
.~
.
.
- - . . A - .
.
.
-. -
. .
comes electron-rich and. a-.&stjtuea. electron-withdra,wing by resonance, _. _. . the
.
a- constant should be used, The standard reactions for the evaluation of a-con-
-.
stants are the ionizations of para-substituted phenols and of para-substituted
a
anilinium ions.g The _L_.___.__ a+ . constant .... shobld be v~edd~henever subshtue9_t electron-
donating by resonance is para-taa rea_cti~~sctte~hat bec~messlsctron-deficient,
and when through-resonance-is. possible. between the two groups. The standard
-
___.-
of
rGc~th~~~valuation the solvolysis of para-substituted t-cumyl
is
a+
chlorides in 90 percent aqueous acetone (Equation 2.14).1° Table 2.1 lists a
number of a+ and a- constants.
CH3
I
I
acetone -XeC-OH (2.14)
CH3
Figure 2.2, in which a constants are plotted against log klk,, for the bromina-
tion of monosubstituted benzenes, shows an example of the usefulness of these
new parameters. As can be seen from Structures 12 and 13-which are represen-
tations of the intermediates in the ortho and para bromination of anisole-sub-
stituents electron-donating by resonance ortho or para to the entering bromine
can stabilize the positive charge in the intermediate and therefore also in the
transition state by through-resonance.
In Figure 2.2a, in which Hammett a constants are plotted, there is only a scatter
of points; but in Figure 2.2b a+ parameters are used and a straight line is
obtained.
For reviews see (a) Note 4(d), p. 60; (b) I,. M. Stock and H. C. Brown, Advan. Phys. Org. Chcm.,
I, 35 (1963).
See note 4(f), p. 60.
lo H. C. Brown and Y. Okamoto, J. Amcr. Chcm. SOL, 80, 4979 (1958).