Q64E-a - Organic Chemistry (Mcmurry)

Question

Use the pK_a  table in Appendix B to determine in which direction the

Equilibrium is favored.

(a)

Step-by-Step Solution

Answer

A reactant is favoured in this reaction

Step 1: Definition of pK a

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.

Step 2: pK a values of Phenol and Benzoic acid

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.

Step 3: Identify in which direction equilibrium will be favoured

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.