Lecture on Alkynes

Overview

• Topic: Alkynes and their reactivity
• Key Points: Molecular structure, orbital hybridization, bond characteristics, acidity, reactions

Structural Aspects of Alkynes

Molecular Bonds

• Sigma Bond (σ): Strong, head-to-head overlap, represented with green marker
• Pi Bonds (π): Weaker, side-to-side overlap, represented with blue and pink markers
  • Pi bonds are perpendicular to the molecular plane

Orbital Hybridization

• SP Hybridization: Carbon with 2 electron groups (1 sigma bond + 2 pi bonds)
  • Bond Angle: 180 degrees, making it difficult to incorporate into ring structures
• Bond Lengths:
  • Ethane (sp3): Largest bond length
  • Ethylene (sp2): Intermediate bond length
  • Acetylene (sp): Shortest bond length due to 50% s-character

Acidity and Reactivity

Terminal Hydrogens

• Acidity Order: Alkane < Alkene < Alkyne
  • Alkane pKa ≈ 50, Alkene pKa ≈ 44, Alkyne pKa ≈ 25
• Formation of Acetylide Ion: Strong bases (e.g., sodium amide) deprotonate terminal alkynes

Effective Electronegativity

• SP-hybridized carbon more electronegative than sp2 or sp3, stabilizing the conjugate base better

Reactions Involving Alkynes

Acid-Base Reactions

• Strong Bases Needed: Sodium amide, sodium hydride, carboanions
  • Equilibrium Consideration: Stronger bases are required to shift equilibrium toward alkyne formation

Elimination Reactions

• Di-alkyl Halides: Can undergo elimination to form alkynes
  • Geminal Di-alkyl Halides: Halides on the same carbon
  • Vicinal Di-alkyl Halides: Halides on adjacent carbons
  • Both follow E2 mechanism

Reducing Alkynes

• Hydrogenation: Alkyne to alkane with H2 and metal catalysts (Ni, Pt, Pd)
  • Poisoned Catalyst: Lindlar's catalyst (Pd poisoned with CaCO3 and PbO)
    • Produces cis-alkene
• Dissolving Metal Reduction: Sodium metal in liquid ammonia produces trans-alkene

Summary

• Alkynes have unique bonding and reactivity due to their structure and hybridization
• Their acidity allows for specific reactions with strong bases
• Alkynes can be converted into specific products based on the choice of reagents and conditions