Which of the following has more $\mathrm{pK}_{\mathrm{a}}$ value?
Which of the following has more $\mathrm{pK}_{\mathrm{a}}$ value?
2-Nitrophenol
3-Nitrophenol
4-Nitrophenol
2, 4-Dinitrophenol
Solution
Greater $\mathrm{pK}_{\mathrm{a}}$ value means smaller $\mathrm{K}_{\mathrm{a}}$ or weaker acid. Due to the presence of an electron-withdrawing $-\mathrm{NO}_2$ group at the meta position, 3- nitrophenol is the weakest acid among the given options.
The effect of $-\mathrm{NO}_2$ is nullified at the meta position.