Page 164 - Mechanism and Theory in Organic Chemistry

Substituent Effects on Strengths of Brcansted Acids and Bases 153

Table 3.6 SOLUTION ACIDITIES WATER AND THE SIMPLE ALCOHOLS
OF
Compound PK~ Kea

" J. Hine and M. Hine, J. Amer. Chem. Sac., 74, 5266 (1952). In isopropyl alcohol,

[A-I
K. =
[HA] [i-pro- ]
The value for isopropyl alcohol is determined by the definition of K,
Calculated for

using K, = lo-'* and [H,O] = 55.5 M.
A. Unmack, Z. Phys. Chem., 129, 349 (1927); 131, 371 (1928); 133,45 (1928).
W. K. McEwen, J. Amer. Chem. Sac., 58, 1124 (1936). Measured in benzene using Unmack's value
for CH30H as standard.
P. Ballinger and F. A. Long, J. Amer. Chem. Sac., 82, 795 (1960). Measured by conductivity in water.
' Ref. d, by extrapolation of a correlation with Taft's a* parameters (see text).

Table 3.7 RELATIVE GAS-PHASE ACIDITIES ALCOHOLS
OF
Acidity Ordera ApKa (gas)
Strongest acid +OH>>
(CH,),COH >
(CH,) ZCHOH >
1
CH,CH,OH > 0.5
CH,OH >
Weakest acid Hz0
' Determined by J. I. Brauman and L. K. Blair, J. Amer. Chem. Sac., 92, 5986 (1970) using ion cyclo-
tron resonance spectroscopy.
D. K. Bohme, E. Lee-Ruff, and L. B. Young, J. Amer. Chem. Sac., 93, 4608 (1971). The technique
for the quantitative measurements was the flowing afterglow method. See D. K. Bohme and L.
B. Young, J. Amer. Chem. Sac., 92, 3301 (1970).

are somewhat uncertain. Before the advent of the gas-phase measurements, these
data were the only ones available and were generally interpreted in terms of the
inductive effects of the alkyl groups. It is well known, for example, that increasing
alkyl substitution stabilizes carbocations (see Section 5.3), and so it was presumed
that an alkyl group, being evidently electron-donating, should destabilize a
negative charge. Hence it was reasonable that the alcohols with more or larger
groups should have less tendency to form a negative ion by loss of a proton and
hence should be less acidic. This interpretation was apparently supported by the
establishment of a correlation between alcohol acidity and the Taft o* inductive
parameter (see Sektion 2.2, p. 67), although it should be pointed out that the