Question

There are four organic acids below. Compare the acidities of four compounds and explain.

1. benzoic acid

2. 4-nitrophenol

3. 3-nitrophenol

4. 4-Methoxyphenol

Answer 1

The most acidic amon these compounds is 1) Benzoic acid as it has free H⁺

the next most acidic compound will be 2) 4- nitrophenol followed by 3 nitrophenol and finally the 4- methoxyphenol

The reason for this is that in the phenolate anion we see the following resonance stabilization. The more the conjugate base is stabilized the stronger the acid.

So having a withdrawing group like nitro at the 4 position increases the stabilization of the phenolate and so it is a strong acid whereas in the case of a donating group like methoxy the stabilization reduces. Comparing the 4-nitrophenol with the 3-nitrophenol. Extra resonance stabilization is possible at the 4 position which does not happen at the 3 position.

So the final acidity comparision between the four is. benzoic acid is most acidic and 4-methoxyphenol is least acidic

benzoic acid> 4-nitrophenol> 3-nitrophenol>4-Methoxyphenol