Which is More Acidic: Ethene or Acetylene?

Introduction

Both ethene and acetylene are unsaturated hydrocarbons with C-H bonds. Neither is a strong acid by normal standards, but acetylene is dramatically more acidic. The secret lies in how the carbon atoms are hybridized.

Acidic Protons

The acidic protons are the C-H hydrogens. In ethene, the carbons are sp2-hybridized (double bond). In acetylene, the carbons are sp-hybridized (triple bond). The hybridization determines how tightly the carbon holds its electrons.

Governing Factor: Hybridization

The key factor is hybridization and s-character. sp orbitals have 50% s-character, while sp2 orbitals have only 33%. s orbitals are spherical and closer to the nucleus, so more s-character means the orbital holds electrons more tightly.

Conjugate Base Stability

When ethene loses H+, the carbanion sits in an sp2 orbital (33% s). When acetylene loses H+, the carbanion sits in an sp orbital (50% s). The acetylide carbanion is far more stable because its electrons are held much closer to the carbon nucleus.

pKa Comparison

Ethene has a pKa of ~44 and acetylene has a pKa of ~25. That 19-unit difference is enormous - acetylene is roughly 10^19 times more acidic. This makes acetylene acidic enough to be deprotonated by strong bases like NaNH2.

Related Topics