Acids and Bases - Organic Chemistry Concepts: An EFL Approach - page 84

FIGURE 6.2
Lewis acid-base definition.

Figure 6.3 shows how in Brønsted-Lowry terms, the acidity constant K a describes
the strength of any acid in water solution. This can also be written as a pK a value,
in which pK a = -log K a . Therefore, a stronger acid has a larger K a and a lower pK a .
A weaker acid has a smaller K a and a higher pK a .

FIGURE 6.3
Expression of acidity.

There is an inverse relationship between the acid strength and the base strength
of its conjugate base. If an acid is strong, its conjugate base does not bind the
proton strongly and so it is a weak base. Especially in organic compounds, it is
the structure and stability of the conjugate species which has the greatest effect
on acid or base strength.

6.3 ACIDITY OF CARBOXYLIC ACIDS
Carboxylic acids are a good example for a general discussion of acidity. They
show clearly the role of the electronic factors which you need to study when
looking at the acidity of other functional classes.

The two aspects of carboxylic acids to think about are:

their higher acidity relative to other functional classes;

n

the differences of acidity over many different carboxylic acids.

n

FIGURE 6.4
Acidity of carboxylic acids.