Page 165 - Mechanism and Theory in Organic Chemistry

Table 3.8 ORDER OF ACIDITY IN THE GAS PHASE FOR SELECTED COMPOUNDS~
Most acidic: CH3(CH2)3SH
CH3N02
'K'

Least acidic: H2C=CH2, 0.
//,
c.4

a D. K. Bohme, E. Lee-Ruff, and L. B. Young, J. Amer. Chem. Soc., 94, 5153 (1972).

correlation was made up of alcohols of the structure RCHzOH and include&-
only one of the substances in Table 3.6, namely methanol.87
This interpretation is again one that attributes the observed effects solely
to the intrinsic properties of the acid and conjugate base and ignores solvation.
The solvation might in fact be expected to be quite important, since in all likeli;
hood the bulky (CH,),CO- ion will be much less well solvated than the OH-
ion. The intrinsic bacisity of OH- will thus be reduced by solvation more than
will that of (CH,),CO-. The gas-phase results of Brauman and Blair (Table 3.7))
show that in the absence of solvent, water is the weakest acid (OH- the strongest
base) and tert-butyl alcohol the strongest acid [(CH,) ,CO - the weakest base].
If we assume that the gas-phase order reflects intrinsic molecular properties, we
must conclude that preferential solvation is indeed reversing the order in solution.
Brauman and Blair feel that the inductive effects have been misinterpreted in the
past, and that alkyl groups are better able to stabilize both positive and negative
charge than is hydrogen. They attribute this ability to the increasing polarizability
of the alkyl groups as they become larger, and they give the outline of a theoretical
interpretation of the effect.

87 P. Ballinger and F. A. Long, J. Amr. Chem. Soc., 82, 795 (1960).