Page 166 - Mechanism and Theory in Organic Chemistry

Substituent Effects on Strengths of Br~lnsted Acids and Bases 155

Table 3.8 lists comparative gas-phase acidities for a variety of compounds.
Comparison of the relative gas-phase acidities with the solution pKa values given
in Table 3.2 reveals a number of changes in order. The most striking difference
is the position of water, which, in comparison with other compounds, is a very
much weaker acid in the gas phase than in the liquid phase. One may conclude
that the strong propensity for water to solvate ions and polar molecules, parti-
cularly through hydrogen bonding, influences its acid-base properties so strongly
as to overshadow other effects arising from the internal structure and bonding.
The conclusion that should be drawn from this discussion is that there are
two kinds of acidity that must not be confused: (1) an intrinric acidity, which is
best approximated by gas-phase measurements and which reflects the properties
of the ions and molecules in isolation, and (2) a practical liquid-phase acidity in
which solvation effects may play the dominant role. In interpretation of structure-
reactivity relationships, the liquid-phase acidity will probably be misleading
unless the structures being compared are very similar; for thinking about chemical
behavior in solution, however, the liquid-phase acidities are clearly the important
ones.

Acidities of Amines
Acidities of amines in solution are less well known than those of alcohols. Streit-
wieser and co-workers report that cyclohexylamine is somewhat less acidic than
triphenylmethaneYBB but there is little information available about the effects of
structural variation on acidity. In the gas phase, Brauman and Blair found the
order (most acidic to least acidic) (c,H,),NH > (CH,),CCH,NH, 2 (CH,),
CNH, 2 (CH,),NH 2 (CH,),CHNH, > CH3CH2CH2NH2 > C2H5NH2 >
CH,NH, > NH3.89 Water falls between diethylamine and ammonia. T,h_e
-
~
owd order is generally c o ~ t d t h that the ch-d -__ conju~ate
-
-
bapp is better stahihed bme a&argczalJky~~~ups. In the gas phase the
amines are apparently of comparable acidity to the alcohols, whereas in solution
they are much weaker acids.
Acidities of Carbon Acids
Another class of acids of interest in organic chemistry is the group of carbon
acids. Here we may discern three kinds of effects on acidity. The first of these is
illustrated by the acidity of methane (pKa x 48) compared with that of cyclo-
hexane (pKa x 52) (Table 3.1). It would appear that the trend is in the direction
of decreasing acid strength with substitution of hydrogen by alkyl. Note that the
tendency here is in the direction opposite to the effect in alcohols if we take
Brauman's gas-phase results to be the more accurate indication of intrinsic acid
strength. The hydrocarbon data are from solution measurements subject to
considerable uncertainty, and the differences are small. It seems risky to inter-
pret the results in terms of intrinsic molecular properties.
A second effect of structure on acidity is evident from the data in Table 3.9.
Here the differences are considered to be due primarily to the change in hybrid-

A. Streitwieser, Jr., J. I. Brauman, J. H. Hammons, and A. H. Pudjaatmaka, J. Amer. Chem. Soc.,
87, 384 (1965).
B9 J. I. Brauman and L. K. Blair, J. Amer. Chem. Soc., 91, 2126 (1969).