Which is More Acidic: Methane or Ammonia?

Introduction

Methane and ammonia are both simple hydrides from the second period. Neither is a strong acid, but one gives up its proton much more readily. Can you predict which?

Acidic Protons

Methane has four equivalent C-H bonds, while ammonia has three equivalent N-H bonds. The highlighted atom in each molecule is the central atom that holds the negative charge in the conjugate base.

Governing Factor: Electronegativity

The key factor is electronegativity. Nitrogen (EN = 3.0) is more electronegative than carbon (EN = 2.5). A more electronegative atom stabilizes a negative charge better because it attracts electrons more strongly.

Conjugate Base Stability

When methane loses H+, the resulting CH3- carbanion has a negative charge on carbon, which is not very electronegative. When ammonia loses H+, the NH2- amide ion has its charge on nitrogen, which handles negative charge better.

pKa Comparison

Methane has a pKa of ~48 and ammonia has a pKa of ~38. The 10-unit pKa difference means ammonia is about 10 billion times more acidic than methane - all because of nitrogen's higher electronegativity.

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