Lecture on Reactions Associated with Alkynes

Introduction to Alkyne Reactions

• Example: Reaction of 2-butyne with hydrogen gas and palladium over carbon.
  • Initial reaction forms an alkene via syn addition.
  • Continues to form an alkane, adding four hydrogen atoms across the triple bond.

Use of Lindlar's Catalyst

• Example: Reaction of 2-butyne with hydrogen gas and Lindlar's catalyst.
  • Stops at the alkene level, forming a cis-alkene.
  • Lindlar’s catalyst consists of palladium mixed with barium sulfate and quinoline.

Metal-Ammonia Reduction

• Reagents: Sodium metal (or lithium) and liquid ammonia.
  • Stops at the alkene level, yielding a trans-alkene.
  • Mechanism involves formation of a radical anion and then a trans-alkene.

Mercury-Catalyzed Hydration

• Reagents: Mercury sulfate with water and sulfuric acid.
  • Produces an enol that tautomerizes into a ketone.
  • Similar to oxymercuration-demercuration.

Hydroboration-Oxidation

• Reagents: R2BH or SIA2BH in THF, followed by hydrogen peroxide and hydroxide.
  • Converts terminal alkynes into aldehydes and internal alkynes into ketones.
  • Proceeds with anti-Markovnikov addition.

Reactions with Hydrobromic Acid (HBr)

• Example: Reaction of alkynes with HBr.
  • First addition turns alkyne to alkene, second to alkane.
  • Follows Markovnikov addition unless peroxides are present, leading to anti-Markovnikov.

Halogenation

• Example: Reaction with bromine (Br2) in dichloromethane.
  • Alkynes react to form alkenes and then alkanes with anti-addition of halogens.

Formation of Vicinal and Geminal Dihalides

• Example: Use of sodium amide to convert dihalides to alkynes.
  • Sodium amide favors terminal alkynes; potassium hydroxide at high temperatures favors internal alkynes.

Mechanisms of Conversion

• Mechanisms show how to convert internal alkynes to terminal alkynes and vice versa using specific conditions and reagents.

Synthesis of Carbon-Carbon Bonds

• Example: Use of acetylide ions to form carbon-carbon bonds.
  • Acetylide ion reacts with alkyl halides to form new carbon-carbon bonds.

Conclusion

• Overview of alkyne reactions provides various routes for transforming alkynes into different products.
• Important to understand the conditions and reagents that influence product formation.