Page 137 - Mechanism and Theory in Organic Chemistry

any of a variety of charges, a fact that is evident from the equilibria shown in
Equations 3.6-3.9.

In general we might write:

where m and n can each be a positive or negative integer or zero.
Next, we must recognize that many molecules that we ordinarily think of as
exhibiting neither acidic nor basic behavior are in fact acids or bases, or, fre-
quently, both. For example, acetone, which is neutral in water solution, reacts as
a base in sulfuric acid according to the equilibrium 3.1 1 ; and in dimethylsulf-
oxide containing sodium methoxide, acetone is an acid (Equation 3.12).5
:o: :OH+

Logical extension of these ideas leads to the conclusion that acetic acid is a base
as well as.an acid, and that aniline, a substance ordinarily considered as a base,
can also act as an acid (Equations 3.13 and 3.14).

. . . .
C~H,-NH, + NH,- ,--' C,H,NH- + NH, (3.14)
Indeed, one may conclude that any molecule containing hydrogen is a
potential Brcansted acid, whereas any molecule at all is a potential Br~nsted base.

Acid and Base Strength
F_or acids that can be studied in aqueous solution, we measure the strength
by the magnitude of the equilibrium constant for dissociation, K,. This quantity
is defined by first writing the equilibrium constant Kh for on 3.15, using

Later in this chapter we shall consider the experimental methods of detecting reactions like 3.11
and 3.12 and the problem of measurement of their equilibrium constants.