Reactions Associated with Alkynes

Reduction with Hydrogen Gas

• Reagents: Hydrogen gas (H₂) and Palladium/C catalyst
• Process:
  • Converts alkynes to alkanes through syn addition
  • First, the alkyne is reduced to an alkene, then further reduced to an alkane
  • Example: 2-butyne with hydrogen gas gives butane

Lindlar's Catalyst

• Reagents: Lindlar's catalyst (Palladium, Barium Sulfate, Quinoline) with H₂
• Process:
  • Converts alkynes to cis-alkenes
  • Stops at alkene level, does not proceed to alkane

Sodium Metal and Ammonia

• Reagents: Sodium (or Lithium) metal, Liquid Ammonia
• Process:
  • Converts alkynes to trans-alkenes
  • Mechanism involves formation of radical anions and vinilic radicals
  • Ends with the formation of trans-alkenes

Oxymercuration-Demercuration

• Reagents: Mercury(II) sulfate, Water, Sulfuric acid
• Process:
  • Converts alkynes to ketones via enol intermediates
  • Enol tautomerizes to more stable ketone

Hydroboration-Oxidation

• Reagents: R₂BH (Sia₂BH often used), H₂O₂, OH⁻, H₂O
• Process:
  • Converts alkynes to aldehydes (for terminal alkynes) or ketones (for internal alkynes)
  • Proceeds with anti-Markovnikov addition

Halogenation

• Reagents: Br₂, Cl₂
• Process:
  • Converts alkynes to tetrahalides or dihalides
  • Anti-addition of halogens

Hydrogen Halide Addition

• Reagents: HBr, HCl
• Process:
  • Markovnikov addition of halogens
  • Can produce geminal dihalides
  • Anti-Markovnikov addition with peroxides

Alkylation of Terminal Alkynes

• Reagents: Sodium Amide, Alkyl Halides (e.g., Ethyl Bromide)
• Process:
  • Terminal alkynes can form acetylide ions
  • Acetylide ions react with alkyl halides to form new carbon-carbon bonds

Decoupling with Sodium Amide vs. KOH

• Reactions:
  • Sodium Amide (NaNH₂): Forms terminal alkynes
  • Potassium Hydroxide (KOH): Forms internal alkynes at high temps

Mechanisms Involved

• Radical anions, vinilic radicals, and mercuric enols
• Examples of E2 elimination reactions and nucleophilic attacks forming C-C bonds

These key reactions and mechanisms demonstrate the versatility of alkynes as intermediates in organic synthesis, particularly in forming alkenes, alkanes, ketones, aldehydes, and various halogenated compounds.