Lecture Notes on Reactions Involving Alkynes

Hydrogenation of Alkynes

• Catalysts: Hydrogen gas with palladium over carbon
• Initial Reaction: Turns alkyne into an alkene with syn addition of hydrogen
• Final Reaction: Further hydrogenation turns alkene into alkane, adding four hydrogen atoms across the triple bond

Lindlar's Catalyst

• Purpose: Stops hydrogenation at the alkene level, forming a cis alkene
• Catalyst Composition: Palladium mixed with barium sulfate in quinoline, sometimes with methanol
• Result: Converts an alkyne into a cis alkene

Metal Ammonia Reduction

• Reagents: Sodium metal and liquid ammonia, or lithium metal
• Product: Trans alkenes
• Mechanism:
  • Sodium gives one electron, forming a radical anion
  • Reaction with ammonia leads to vinyl radical and vinyl anion
  • Final product is a trans alkene

Reactions with Mercury Sulfate

• Reagents: Alkyne with mercury sulfate, water, and sulfuric acid
• Product: Converts alkynes into ketones through the formation of an enol intermediate
  • Enol tautomerizes to a ketone

Hydroboration-Oxidation

• Reagent: R2BH followed by hydrogen peroxide
• Product: Produces an enol intermediate that tautomerizes to aldehydes
  • Terminal alkynes require R2BH to avoid double addition

Addition of Halogens and Hydrogen Halides

• HBr Addition: Forms an alkane with Markovnikov addition
  • Terminal alkynes lead to geminal dihalides
• Br2 Addition:
  • One equivalent forms a dibromoalkene
  • Two equivalents form a tetrabromoalkane

Mechanisms and Specific Reactions

• Hydroboration-Oxidation of Alkynes
  • Enol intermediate tautomerizes to an aldehyde for primary carbons, ketone for secondary carbons
• Role of Peroxides: In HBr reactions with alkynes, leads to anti-Markovnikov addition

Conversion and Isomerization

• Dihalides to Alkynes: Using sodium amide for terminal alkynes and potassium hydroxide for internal alkynes
• Mechanism of Deprotonation:
  • Sodium amide removes terminal alkyne hydrogen irreversibly
  • Potassium hydroxide requires high temperatures

Terminal Alkyne Synthesis

• Using Sodium Amide: Converts vicinal dihalides to terminal alkynes
• Mechanism: Involves deprotonation followed by nucleophilic substitution

Summary

• Alkyne reactions are versatile, producing alkanes, alkenes, ketones, and aldehydes
• Importance of catalysts and conditions (temperature, solvents) in determining the final product
• Mechanisms often involve intermediates like radical anions, enols, and vinyl radicals

Practical Applications

• Development of Carbon-Carbon Bonds: Via nucleophilic attack by acetylide ions

These notes cover the key reactions and mechanisms associated with alkynes, including hydrogenation, halogenation, and methods to control the stereochemistry and regiochemistry of products.