Lecture on Reactions Associated with Alkynes

Hydrogenation of Alkynes

• Example: 2-butyne with hydrogen gas and palladium on carbon.
  • First hydrogen molecule turns alkyne to alkene (syn addition).
  • Second hydrogen molecule turns alkene to alkane.
  • Overall, four hydrogen atoms added across the triple bond.
• Example: 5-carbon alkyne with hydrogen gas and palladium on carbon results in an alkane.

Lindlar's Catalyst

• Reaction: 2-butyne with hydrogen gas using Lindlar's catalyst (palladium, barium sulfate, quinoline, methanol).
  • Stops at cis-alkene level (syn addition of hydrogen).

Metal-Ammonia Reduction

• Reagents: Alkyne, sodium metal (or lithium), and liquid ammonia.
  • Forms a trans-alkene.
• Mechanism:
  • Sodium donates electron, forming radical anion.
  • Radical anion reacts with ammonia, forming a vanillic radical.
  • Subsequent electron donation from sodium forms vanillic anion.
  • Final reaction with ammonia yields trans-alkene.

Hydration Reactions

• Mercury(II) Sulfate Reaction: Alkyne with HgSO4, H2O, H2SO4.
  • Forms enol, which tautomerizes to ketone.
• Hydroboration-Oxidation Reaction: Alkyne with R2BH or SIA2BH, followed by H2O2, OH-.
  • Forms enol, then converts to aldehyde for terminal alkynes, ketone for internal.

Halogenation and Hydrogen Halide Additions

• HBr Addition: Alkyne with HBr forms geminal dihalide.
  • Initial addition gives alkene with Markovnikov addition (bromine to more substituted carbon).
  • Second addition yields alkane with both bromines on same carbon.
• Br2 Addition: Two equivalents convert alkyne to alkane with four bromine atoms (anti-addition).

Synthesis Strategies

• Terminal vs. Internal Alkyne Formation:
  • Sodium amide (NaNH2) favors terminal alkyne.
  • Potassium hydroxide (KOH) at high temperature favors internal alkyne.
• Mechanism for Terminal Alkyne Formation:
  • Deprotonation sequences with NH2- lead to enolate formation, quenched with water.

Mechanisms and Examples

• Mercury(II) Sulfate Reaction Mechanism:
  • Alkyne attacks Hg2+, forming intermediate. Water attacks, forming enol, then tautomerizes to ketone.
• Formation of Carbon-Carbon Bonds:
  • Acetylide ion reacts with alkyl halides, forming new carbon-carbon bonds.

Key Concepts

• Catalysts:
  • Lindlar's for cis-alkene.
  • Metal-Ammonia for trans-alkene.
• Regiochemistry:
  • Markovnikov vs. anti-Markovnikov in hydrogen halide addition.
• Tautomerization:
  • Enols convert to more stable ketone or aldehyde forms.

These notes summarize the various reactions and mechanisms involving alkynes, focusing on hydrogenation, hydration, halogenations, and synthesis pathways including the use of different catalysts and reaction conditions.