Which is More Acidic: Acetic Acid or Chloroacetic Acid?

Introduction

Both molecules are carboxylic acids with similar structures. The only difference is that one hydrogen on the methyl group has been replaced with a chlorine atom. How does this small change affect acidity?

Acidic Protons

Both acids have their acidic proton on the carboxylic acid O-H group. The structural difference is on the adjacent carbon: acetic acid has three C-H bonds, while chloroacetic acid has a C-Cl bond replacing one C-H.

Governing Factor: Induction

The key factor is the inductive effect. Chlorine is electronegative (EN = 3.0) and pulls electron density away from the carboxylate group through the sigma bond framework. This electron withdrawal helps stabilize the negative charge on the conjugate base.

Conjugate Base Stability

Both conjugate bases are carboxylate anions with resonance stabilization. But in chloroacetate, the Cl atom pulls electron density away from the negatively charged oxygens through sigma bonds. This additional stabilization makes chloroacetate a weaker base.

pKa Comparison

Acetic acid has a pKa of 4.8 and chloroacetic acid has a pKa of 2.9. The roughly 2-unit difference means chloroacetic acid is about 80 times more acidic, entirely due to the inductive effect of one chlorine atom.

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