Page 168 - Mechanism and Theory in Organic Chemistry

Substituent Effects on Strengths of Brcansted Acids and Bases 157

Table 3.10 ACIDITIES CARBON ACIDS CONTAINING
OF
ELECTRON-WITHDRAWING GROUPS
Compound PK~

0
I I
CH(C-CH3)3 6
CH3S02CH3 29"
CHz(SOzCH3)z 14
CH(SO2CH3)3 0
CH3C-N 2 5
CH2 (CGN)~ ' 12
CH(C=N) 0
SOURCE: Reprinted with permission from R. G. Pearson and R. L. Dillon, J. Amer. Chem. SOG., 75,
2439 (1953). Copyright by the American Chemical Society.
a See Ref. 15 in Table 3.2.
precision and compared with considerable confidence, despite the fact that the
differences are small.90 The pK,'s of a very large number of carboxylic acids
have been determined; we list in Table 3.1 1 only a few representative values to
illustrate trends.91 The data indicate that the effect of electron-witldmmng
substituents is to increase acid strcqgh,-euwati~cke:
localization of charge is nott-m$.smbLe+ as it is in the substituted carbon acids
considered above. Again, one must be very careful to keep solvation factors
constant in making comparisons if interpretations in terms of molecular structure
are to be made; the most careful and informative studies of the carboxylic acids
have been carried out in series in which the structural changes occur as far from
the reaction site as possible, as for example in ring systems such as 7.92

The greatly increased acidity of the carboxylic acids over water and the alcohols is accounted for
by delocalization of charge in the conjugate base, as indicated by Structures a.

References to compilations are given in Table 3.2, note a.
O2 (a) H. D. Holtz and L. M. Stock, J. Amer. Chem. SOG., 86, 5188 (1964); (b) R. Golden and L. M.