Chapter 17.SE, Problem 64AP

Interpretation Introduction

Interpretation:

p-Nitrophenol and 2,6-dimethyl-4-nitrophenol both have a pKa of 7.15 but 3,5-dimethyl-4-nitrophenol has pKa of 8.25. Why 3,5-dimethyl-4-nitrophenol is much less acidic is to be explained.

Concept introduction:

Resonance delocalization occurs only when the molecule is planar facilitating the overlapping of the orbitals. If the substituent groups present prevent co-planarity of the molecule then resonance is not possible.

pKa = -log Ka, where Ka is the dissociation constant of the acid. Thus a stronger acid has a low pKa value.

To explain:

Why 3,5-dimethyl-4-nitrophenol is much less acidic with pKa of 8.25 when compared to p-nitrophenol and 2,6-dimethyl-4-nitrophenol both of which have a pKa of 7.15.