Chemistry Reference
In-Depth Information
Figure 2.8 Henderson-Hesselbach equation.
Figure 2.9 In a perfect equilibrium, pK
a
ΒΌ
pH.
2.4 RELATIVE ACIDITIES
Having explored the relationships between solution pH and pK
a
values, we can now explore
the relative acidities of various hydrogen atoms and how these values are influenced by
neighboring functional groups and heteroatoms. In this arena, it is important to remember
that how a reaction proceeds is largely dependent upon the relative acidities of protons
(hydrogen atoms) compared to one another and not on the absolute acidity of a
given proton.
Considering a compound that produces a solution pH greater than 7, that compound is
generally referred to as basic. However, a proton of interest (proton A) on this compound
may be considered acidic compared to another proton (proton B) if the pK
a
of proton A is
lower than the pK
a
of proton B. In other words, the lower the pK
a
for a given proton, the
more acidic that proton is. Consequently, in order to predict the mechanistic course of a
given organic reaction, it is extremely important to be able to recognize the most acidic
proton in a given molecule.
As previously stated, acidities of various protons are dependent upon their associated
functional groups and nearby heteroatoms. Furthermore, these protons may be either com-
ponents of relevant functional groups or adjacent to relevant functional groups. Figure 2.10
illustrates examples of compounds possessing acidic protons associated with representative
functional groups, and Figure 2.11 illustrates examples of compounds possessing acidic
protons adjacent to representative functional groups. In both figures, the acidic protons
are highlighted in bold.
In studying the relationships between functional groups and proton acidities, we will first
look at carboxylic acids. As illustrated in Scheme 2.2, carboxylic acids dissociate to form
protons and carboxylate anions. Furthermore, as shown in Scheme 2.3, the carboxylate
anion is stabilized through two resonance forms. It is this resonance stabilization that
serves as the primary driving force behind the acidic nature of carboxylic acids. Further
evidence of the relationship between resonance stabilization of anions and acidity can be
seen when comparing the pK
a
values of carboxylic acids to the pK
a
values of alcohols.
