Acids - Arrow Pushing in Organic Chemistry - page 23

Scheme 2.7 Resonance forms of the malonate anion rationalized using arrow pushing.
As illustrated in Schemes 2.2 and 2.5, different organic anions form under different con-
ditions. Some, as illustrated in Scheme 2.2, form through spontaneous dissociation of an
acid while others, as illustrated in Scheme 2.5, require bases to extract protons and liberate
anions. Since, as illustrated in Scheme 2.1, acids are defined as substances that liberate
protons, and since the anions illustrated in Schemes 2.2 and 2.5 form upon liberation of
their respective protons, both carboxylic acids and substances such as dimethyl malonate
must be defined as acids.
Although carboxylic acids and substances such as dimethyl malonate can be classified as
acids, their relative acidities are clearly very different as illustrated by the different con-
ditions required to liberate protons and anions. In order to understand this phenomenon,
it is essential to first understand how acidities are measured.
2.3 HOW IS ACIDITY MEASURED?
Before discussing how acidity is measured, the definition of the equilibrium constant
should be reviewed. Referring to Scheme 2.1, illustrating that an acid is in equilibrium
with its dissociated ions, the degree of this dissociation as compared to the undissociated
acid is measured according to its equilibrium constant (K eq ). From general chemistry
coursework, we remember that K eq is the product of the ion concentrations divided by
the concentration of the undissociated acid (Fig. 2.3).
Since Figure 2.3 represents how equilibrium constants are calculated and since we are
specifically studying dissociation of acids, K eq can be redefined for acids as the acid
dissociation constant (K a ) illustrated in Figure 2.4.
Figure 2.3 Definition of the equilibrium constant (K eq ).