Chapter 6, Problem 6.77P

Interpretation Introduction

(a)

Interpretation:

It is to be explained why the pKa values of all three isomers in which a methyl group is attached to the ring are higher than the pKa of phenol itself.

Concept introduction:

When comparing uncharged acids, the stronger acid is the one whose negatively charged conjugate base is more stable. Electron donating substituents attached to the ring decrease the acidity as compared to non-substituted rings. Alkyl group is an electron donating group, thus, it will destabilize the negatively charged conjugate base to a greater extent than non-substituted rings.

Interpretation Introduction

(b)

Interpretation:

Why the meta isomer has the lowest $\text{pKa}$ value of the three isomers is to be explained.

Concept introduction:

When comparing uncharged acids, the stronger acid is the one whose negatively charged conjugate base is more stable. Electron donating substituents attached to the ring decrease the acidity as compared to non-substituted rings. Methyl groups is an electron donating group, thus, it will destabilize the negatively charged conjugate base to a greater extent than non-substituted rings. Greater the resonance stabilization of the conjugate base, stronger is the acid. In the resonance contributors, the structure in which an atom bearing a negative charge becomes destabilized as the number of electron-donating groups attached to it increases. This structure will not contribute significantly to the resonance hybrid.