Chapter 9: Alkynes - An Introduction to Organic Synthesis

Alkynes: An Introduction to Organic Synthesis

Overview of Alkynes

• Definition: Hydrocarbons containing carbon–carbon triple bonds.
• Simplest Alkyne: Acetylene (ethyne).
• Polyynes: Linear carbon chains of sp-hybridized carbon atoms.

Learning Objectives

• Naming Alkynes (9.1)
• Preparation of Alkynes (9.2): Elimination reactions of dihalides.
• Reactions of Alkynes (9.3): Addition of HX and X2.
• Hydration of Alkynes (9.4)
• Reduction of Alkynes (9.5)
• Oxidative Cleavage of Alkynes (9.6)
• Alkyne Acidity (9.7): Formation of acetylide anions.
• Alkylation of Acetylide Anions (9.8)
• Organic Synthesis Introduction (9.9)

Naming Alkynes

• General hydrocarbon naming rules apply.
• Suffix: -yne for alkynes.
• Indicate the position of the triple bond using the number of the first alkyne carbon in the chain.

Examples of Naming Alkynes

• 1-Butyne: CH₃CH₂C≡CH
• 3-Hexyne: CH₃C≡CCH₂CH₂CH₃
• Alkynyl Groups: CH₃CH₂C≡C and others.

Preparation of Alkynes

• Method: Elimination reactions of dihalides.
  • React a 1,2-dihaloalkane with KOH or NaOH for a two-fold elimination of HX.
  • Vicinal Dihalides: Formed byadding bromine or chlorine to an alkene; leads to a vinyl halide intermediate.

Reactions of Alkynes

• Addition of HX and X₂: Similar to alkenes but with distinct mechanisms due to triple bond.
  • π bonds broken in the process; for acetylene, it requires 202 kJ/mole to break a π bond.
  • Resulting addition products from bromine and chlorine lead to trans stereochemistry.

Hydration of Alkynes

• Catalyst: Mercury(II) sulfate enhances hydration.
• Produces an enol that rearranges to a ketone, following Markovnikov's rule.
• Keto-Enol Tautomerism: Rapid transfer of a proton yields isomeric compounds; important in the context of hydration.

Reduction of Alkynes

• Reductive conversion using H₂ over metal catalysts leads to alkenes and alkanes:
  • Using deactivated palladium yields cis alkenes.
  • Sodium or lithium in liquid ammonia yields trans alkenes through a different mechanism.

Oxidative Cleavage of Alkynes

• Strong oxidizing agents (O₃ or KMnO₄) cleave internal alkynes, yielding two carboxylic acids.
• Terminal alkynes form CO₂ as a product of cleavage.

Acetylide Anions

• Formed by the reaction of a strong base removing terminal hydrogen from an alkyne.
• Acetylide anions are highly nucleophilic.
• Can undergo alkylation reactions with alkyl halides to produce new alkynes.

Alkylation of Acetylide Anions

• Nucleophilic attack leads to the formation of new C-C bonds; further alkylation can yield internal alkynes from terminal alkynes.

Organic Synthesis

• Importance: Central to pharmaceuticals and chemical industries.
• Requires a strategic approach:
  • Organizing known reactions into synthetic routes.
  • Employing retrosynthetic analysis to plan syntheses.

Summary of Key Points

• Alkynes play a crucial role in organic synthesis, facilitating the production of various compounds.
• Understanding their reactions, derivatives (like acetylide anions), and stability is vital for effective synthesis planning.
• Major processes include hydration, reduction, and oxidative cleavage which are leveraged for synthetic applications.