Page 136 - Mechanism and Theory in Organic Chemistry

Bronsted Acids and Bases 125

but rather is always solvated by at least one molecule of some other species. Thus
in water protons exist as hydronium ions, H30+ ; in ammonia as ammonium,
NH4+; in alcohols as ROH,+.2,3 Clearly, in each of these cases the solvating
molecule has acted as a base according to the Brransted definition. The small size
and consequent large electrostatic field of the proton makes it seem very likely
that in solution association of H+ with a base is a general phen~menon.~ It is
therefore more reasonable to represent the actual process of dissociation of an
acid (acetic acid, for example) in a solvent such as water as shown in Equation 3.3.
Here the acid donates a proton and thc base accepts it; this chemical change
constitutes an acid-base reaction in the Brransted sense.

If we look again at Equation 3.3, we can see that we should consider the
reverse process as an acid-base reaction just as the forward process is. The acetate
ion is a base that can accept a proton from the acid H30 +. This reciprocal rela-
tionship is emphasized by the terminology applied to processes like that in
. .
Equation 3.3 : &etate ion is ca lled CH,COOH, and
_H,Od the con_iu.~ate acid of the base H,O.
-
Il I I
H,C-C-OH + H20 H,O+ + H3C-c--0- (3.4)
acid base conjugate conjugate
acid base
In considering an acid-base reaction, it is important to realize that the choice of
-acid the coniugate acid is completely arbitrary. In Equation
3.4 we could just as well have decided to call H20 the conjugate base of the acid
H30+ and CH3COOH the conjugate acid of the base CH3COO-. It would
perhaps be better to emphasize the fundamental symmetry of the situation by
writing Equation 3.5 :

acid, base, acid, base,
However, the conjugate acid-conjugate base nomenclature is convenient, and we
shall continue to use.it.
We now need to generalize our ideas in various ways. First, it is clear that
it is not always necessary that the molecules and ions involved be of the charge
types shown in the example that has been used so far. Acids and bases can haye

a See (a) R. P. Bell, The Proton in Chemistry, 2nd ed., Cornell University Press, Ithaca, N.Y., 1973,
p. 13 ; (b) M. Eigen, Agnew. Chem. Int. Ed., 3, 1 ( 1964).
The exact degree of solvation is not known; we use these designations for convenience, although
some formula such as H(H20),+ might in fact be more accurate. The symbol H+ is also commonly
used to represent the proton; this notation should be understood as an abbreviation for the solvated
species that is actually present.
G. A. Olah, A. M. White, and D. H. O'Brien, Chem. Rev., 70, 561 (1970), review evidence for co-
ordination of protons with a large number of substances.