Acids and Bases - Organic Chemistry Concepts: An EFL Approach - page 87

So far, we have given simple guidelines to explain the relative acidity of carbox-
ylic acids as a functional class. We can use the same general ideas of X-H bond
strength and the stability of the conjugate base anion when we study other func-
tional classes.
6.4 GENERAL FUNCTIONAL GROUP ACIDITY
Carboxylic acids are generally the most acidic of the organic compounds. How-
ever, because nearly all organic compounds have hydrogen, most other classes
of compounds can show some acidity. It is possible to draw a scale of acidity
which covers all classes of organic compounds. However, only some of these
classes have acidities which are in the useful range.
There is an inverse relationship between substrate acidity and the basicity of the
deprotonated conjugate. Therefore, it is also possible to make a scale of organic
bases. However, it is more useful to have all organic compounds on a common
pK a scale. This scale shows the ability of compounds to donate or accept pro-
tons, either in their protonated acidic or deprotonated conjugate base forms.
The weaker the acid, the stronger the conjugate base.
We must carefully study functional groups when we try to determine acid-
ity. For example in Figure 6.6 , the overall functional group in a carboxylic
acid is shown as a combination of the carbonyl and the alcohol functional
groups. Both of these functional groups affect the acidity of a carboxylic acid.
In other compound types, each functional group must be evaluated for its
effect on acidity.
FIGURE 6.6
The origins of the carboxyl group.
The examples in Table 6.2 show the wide range of pK a values. At one end of the
scale, carboxylic acids are the most acidic. They have the lowest pK a values and
have the weakest conjugate bases. This shows the efficiency of the combination
of inductive effect and charge delocalization.
At the other end of the scale, the hydrocarbons alkanes, alkenes, and alkynes
have high pK a values. This shows that without either inductive effect or charge
delocalization, there is extremely low acidity. Within the range of hydrocarbons,
the higher the bond order of the carbon at the acidic C-H, the greater the acidity.
This causes alkynes, with pK a ≈ 25, to be in the useful range.