Chapter 3, Problem 15PP

PRACTICE PROBLEM 3.15

Nitro groups have a large acid-strengthening effect on phenols. The $\text{p}{K}_{\text{a}}$ of 4-nitrophenol is 7.15, whereas that of phenol is 9.89, a factor of almost 1000. Account for the greater acidity of 4-nitrophenol relative to phenol on the basis of resonance and inductive effects. Your answer will help explain why 2,4,6-trinitrophenol (also called picric acid) is so exceptionally acidic $\left(\text{p}{K}_{\text{a}}=0.38\right)$ that it is more acidic than acetic acid $\left(\text{p}{K}_{\text{a}}=4.76\right)$.