Acids and Bases - Organic Chemistry Concepts: An EFL Approach - page 91

6.5 GENERAL FUNCTIONAL GROUP BASICITY
In the Brønsted-Lowry definition, we compare organic substrates based on
their ability to act as bases and accept a proton. In the examples of acid-
ity mentioned earlier, we saw different conjugate bases which are caused by
deprotonation of the corresponding acids. Because acidity and basicity have
an inverse relationship, the weaker the acid (higher pK a ), the stronger the
conjugate base.
Therefore, alkanes give extremely strong conjugate bases because of their
very low acidity ( pK a ≈ 50). The alkoxides, from alcohols ( pK a ≈ 16), are in
fact commonly used as relatively strong bases in a wide range of organic
reactions.
Organic compounds in their protonated form can also act as bases. To do this,
they need to have a functional group which can provide the pair of electrons
which is needed to bind an additional proton. The classification of organic bases
is almost the same as in the Lewis definition. In other words, they must be able
to donate a pair of electrons. In this topic we only show the examples in which
this electron pair is shared with a proton.
This means that all organic compounds which have heteroatoms such as O, N, S,
or halogen can be basic. This is because they all have lone pairs of non-bonded
electrons. Amines, alcohols, and ethers are the most common examples.
In most cases, the reaction of a lone pair as a base simply means protonation
to give the charged species such as the ammonium or oxonium. As a measure
of base strength, we define the constant K b in the same way as for K a . This
is shown in Figure 6.12 . Again, it is more convenient to express this as pK b
in terms of -log K b . As with pK a , the lower the value of pK b , the stronger the
base.
FIGURE 6.12
Expression of basicity.
To allow direct comparison on a single scale, it is common to express pK b values
in terms of the pK a of the conjugate acids. Because of the simple inverse rela-
tionship, we get values from the relationship pK a + pK b = 14. Selected values are
listed in Table 6.3 . This gives a single scale of the abilities of organic compounds
to donate or accept protons.