Alkyne Acidity and Acetylide Anions

The acidity of hydrocarbons, their ability to donate a proton, is significantly influenced by the hybridization of the carbon atom bonded to the hydrogen. Alkynes, with their sp-hybridized carbons (50% s-character), exhibit greater acidity compared to alkenes (sp² hybridization, 33% s-character) and alkanes (sp³ hybridization, 25% s-character). While it's sometimes stated that increased s-character makes the carbon more electronegative and thus the C-H bond more polarized, the primary reason for the increased acidity of alkynes lies in the stability of the resulting acetylide ion. Acetylene has a pKa of approximately 25, making it considerably more acidic than ethylene (pKa ~44) and ethane (pKa ~50). This acidity allows alkynes like acetylene to react with strong bases, such as sodium amide (NaNH₂), to form acetylide ions. These acetylide ions, being strong nucleophiles and bases, are crucial intermediates in organic synthesis. They readily react with alkyl halides, facilitating the formation of new carbon-carbon bonds and enabling the synthesis of more complex molecules, including pharmaceuticals, natural products, and polymers. While incredibly useful, acetylide ions are also highly reactive and require careful handling due to the potential for explosive metal acetylide formation.

Examples of Acetylide Formation

Here are a few examples illustrating the formation of acetylide ions:

• From Acetylene:
  HC≡CH + NaNH₂ → HC≡C⁻Na⁺ + NH₃
• From a Terminal Alkyne:
  R-C≡CH + BuLi → R-C≡C⁻Li⁺ + BuH (Bu = Butyl group)

Key Concepts and Further Points

• Stability of Acetylide Ions: The increased acidity of alkynes is primarily due to the stability of the resulting acetylide ion. This stability arises from two key factors:
  
  • s-Character and Charge Density: The negative charge resides in an sp orbital with high s-character. s orbitals are closer to the nucleus, meaning the electron density of the negative charge is held closer to the carbon nucleus, compacting the charge and stabilizing the anion. This is the *primary* reason for the increased acidity.
• Basicity of Acetylide Ions: Acetylide ions are not only strong nucleophiles but also strong bases. This basicity is important to consider when choosing reaction conditions.
• Reactions with Alkyl Halides (SN2): The reaction of an acetylide ion with a primary alkyl halide is a classic SN2 reaction, leading to the formation of a substituted alkyne. Steric hindrance at the alkyl halide is a crucial factor.
• Limitations: Acetylide ions will not effectively react with sterically hindered alkyl halides (e.g., tertiary alkyl halides) or aryl halides. In such cases, other synthetic strategies are required.
• Metal Acetylides: Reactions with certain metal salts can produce metal acetylides, some of which are shock-sensitive explosives. This is a significant safety concern, especially with heavy metals like silver and copper.