Which is More Acidic: Chloroacetic Acid or Trichloroacetic Acid?

Introduction

Both molecules are chlorinated acetic acids. Chloroacetic acid has one Cl on the alpha carbon, while trichloroacetic acid has three. How does multiplying the number of electron-withdrawing groups affect acidity?

Acidic Protons

Both acids donate their proton from the carboxylic acid O-H group. The structural difference is entirely on the alpha carbon: one Cl versus three Cl atoms replacing the hydrogens.

Governing Factor: Induction

The key factor is cumulative induction. Each chlorine atom withdraws electron density through sigma bonds. With three chlorines pulling electron density away from the carboxylate, the inductive effect is approximately tripled.

Conjugate Base Stability

Both conjugate bases are carboxylate anions with resonance stabilization. Chloroacetate benefits from one C-Cl inductive pull. Trichloroacetate gets three times the inductive stabilization, making it a much weaker base and therefore a stronger acid.

pKa Comparison

Chloroacetic acid has a pKa of 2.9 and trichloroacetic acid has a pKa of 0.7. The 2.2-unit difference means trichloroacetic acid is about 160 times more acidic. Each additional Cl drops the pKa by roughly 1 unit.

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