Chapter 14, Problem 14.57P

Interpretation Introduction

Interpretation:

The given compounds L-P are to be ranked in order of decreasing acid strength, from lowest pK_a to highest pK_a.

Concept introduction:

The strength of an acid depends on the stability of conjugate anion (base) formed by the removal of the acidic proton. The stronger acid has higher the stability of the conjugate base. The stronger the acid, the higher is its ${\text{K}}_{\text{a}}$ value, and consequently, the lower its ${\text{pK}}_{\text{a}}$ value.

In case of organic compounds, the stability of the conjugate base depends on several factors - the electronegativity of the atom(s) that bears the negative charge, resonance delocalization of the negative charge, and hybridization of the atom that bears the negative charge.

The stability of the conjugate base increases with increasing electronegativity of the atom that bears the charge, increasing number of atoms over which the charge is delocalized, and finally, increasing s orbital character of the atom that bears the charge. In cyclic compounds with a conjugated $\text{π}$ system, the stability increases as - $\text{antiaromatic < nonaromatic < aromatic}$.