Chapter 7.9, Problem 16P

Interpretation Introduction

Interpretation:

The reason that supports why ${\text{pK}}_{\text{a}}$ of para-nitrophenol is $7.14$ but ${\text{pK}}_{\text{a}}$ of meta- nitrophenol is $8.39$ should be identified using resonance contributors.

Concept Introduction:

Electronic effect:

Electron donating groups decreases acidity by inductive effect (withdrawal of electrons through a sigma bond).  Electron withdrawal increases acidity.  Electron-donating substituents destabilize a base, and decrease the strength of its conjugate acid; electron-withdrawing substituents stabilize a base, which increase the strength of its conjugate base.

Effect of delocalized electrons on ${\text{pK}}_{\text{a}}$Values:

Due to Inductive effect, an anion is stabilized by a nearby electronegative atom.  In the same way, electron delocalization also effective on stabilization of anion.

Rule: The strength of an acid depends on the stability of its conjugate base.