Question: Among the following compounds, the most acidic is: A) p-nitrophenol B) p-hydroxybenzoic acid C...

Question

Among the following compounds, the most acidic is:
A) p-nitrophenol
B) p-hydroxybenzoic acid
C) o-hydroxybenzoic acid
D) p-toulic acid

Answer 1

Solution

To solve this question, we must first have knowledge about Benzoic Acid and its substituted forms. Then we need to assess the properties and then compare the given option among them on the basis of properties of conjugation and then only we can conclude the correct answer.

Complete answer:
Before we move forward with the solution of this given question, let us first understand some basic concepts:
Benzoic Acid: is an organic compound which is described and represented by the chemical formula ${C_6}{H_5}COOH$ . It consists of a carboxyl group attached to a benzene ring. Therefore, benzoic acid is said to be an aromatic carboxylic acid.
This compound exists as a crystalline, colorless solid under normal conditions. The term ‘benzoate’ refers to the esters and salts of ${C_6}{H_5}COOH$ .
Step 1: Since, all the options except p-nitrophenol, are acids. So there is no chance that p-nitrophenol, which is not even an acid, is the most acidic among the given options. So this is definitely not our answer.
Step 2: And now, among the given substituted benzoic acid, o-hydroxy acid is the strongest acid although $- OH$ causes electron donation by resonance effect which tends to decrease acid strength. And this is due to a very high stabilisation of conjugate base by intramolecular H-bond which outweigh the electron-donating resonance effect of $- OH$ .
The overall order of acid-strength of given four acids is:
o-hydroxybenzoic acid $(p{K_\alpha } = 2.98) >$ p-toluic acid $(p{K_\alpha } = 4.37) >$ p-hydroxybenzoic acid $(p{K_\alpha } = 4.58)$
So, clearly we can conclude that the correct answer is Option C.

Note: Benzoic acid is used in the industrial production of the aromatic compound phenol. This is done via a process known as oxidative decarboxylation. It can be noted that the ideal temperature under which this process can be carried out is in the range of $300$ to $400$ degrees celsius.