Q9P - Organic Chemistry (Mcmurry)

Question

Rank the following compounds in order of increasing acidity. Don’t look at a table of ${\mathbf{pK}}_{\mathbf{a}}$ data to help with your answer.

(a) Benzoic acid, p-methylbenzoic acid, p-chlorobenzoic acid

(b) p-Nitrobenzoic acid, acetic acid, benzoic acid

Step-by-Step Solution

Answer

(a) p-Methylbenzoic acid < Benzoic acid < p-Chlorobenzoic acid

 

(b) Acetic acid < Benzoic acid < p-Nitrobenzoic acid

Step1: Write the given carboxylic compounds in order of increasing acidity. (a)

We know that the substituents bonded to the benzene ring changes the carboxylic acid acidity.

 

• The electron-donating group present in the substituted benzoic acid decreases the acidity by destabilizing a negative charge on the $\left(-{\mathrm{COO}}^{-}\right)$ carboxylate anion

 

• The electron-withdrawing group present in the substituted benzoic acid increases the acidity by stabilizing a negative charge on the $\left(-{\mathrm{COO}}^{-}\right)$ carboxylate anion

 

Methyl $\left(-C{H}_3\right)$ substituent in p-Methylbenzoic acid reduces acidity by destabilizing a single negative charge on the carboxylate anion but chloro substituent in p-Chlorobenzoic acid increases acidity by stabilizing a single negative charge on the carboxylate anion. 

Hydrogen substituent in benzoic acid has no such effect because it is neither electron-withdrawing nor electron-donating.

Hence, p-Methylbenzoic acid is least acid while p-Chlorobenzoic acid is most acidic.

Therefore, the given compounds are arranged in order of increasing acidity as shown below:

p-Methylbenzoic acid < Benzoic acid < p-Chlorobenzoic acid

Step 2: Write the given carboxylic compounds in order of increasing acidity. (b)

Nitro $\left(-N{O}_2\right)$ substituent on the benzoic acid stabilizes the negative charge on the carboxylate $\left(-{C{O}_2}^{-}\right)$ anion but not hydrogen substituent in the benzoic acid because nitro substituent is an electron-withdrawing group. Therefore, p-Nitrobenzoic acid is the most acidic.

 

Acetic acid is least acidic than benzoic acid because methyl $\left(-C{H}_3\right)$substituent in acetic acid is an electron-donating group that destabilizes a negative charge. Since the benzene ring is a weakly electron-withdrawing group.

 

Therefore, the given compounds are arranged in order of increasing acidity as shown below:

Acetic acid < Benzoic acid < p-Nitrobenzoic acid