91Ó°ÊÓ

We define pK_a in terms of K_a , i.e., acid dissociation constant. If we have a stronger acid, then its acid dissociation constant will be more. So, we can say that the acid dissociation constant is directly proportional to the acidic strength of a molecule. Later for more convenience, pK_a was introduced. pK_a is defined as negative of the logarithm of K_a . The lesser the pK_a value, the more the acidic strength will be.

${\mathbf{pK}}_{\mathbf{a}}\mathbf{=}\mathbf{-}\mathbf{log}{\mathbf{K}}_{\mathbf{a}}$

For example, methanol (CH₃OH ) has a 15.5 pK_a value.

Phenol is a compound in which a hydroxyl group (OH) is attached to a phenyl ring (C₆H₅). It is an organic compound, and also it follows (4n + 2). Therefore, it is aromatic in nature.

Benzoic acid is also an aromatic compound. It is acidic in nature due to the presence of one carboxylic group on the benzene ring.

Equilibrium is always favored in the direction in which acidic strength is less. In this reaction, phenol has more pK_a value, and benzoic acid has a lesser pK_a value; therefore, the acidic strength of benzoic acid is more. Hence, equilibrium will be favored on the reactant side of the reaction.