Introduction to Alkynes

General Overview

• Alkynes are unsaturated hydrocarbons containing a carbon-carbon triple bond.
• Formula: CnH2n-2 (fewer hydrogens than alkenes).
• Represent two degrees of unsaturation.

Nomenclature

• Ends with yne (e.g., from ane to yne).

Classification of Alkynes

• Internal Alkynes: Triple bond is located somewhere in the chain.
• Terminal Alkynes: Triple bond at the end, also called monosubstituted acetylenes.

Structure and Hybridization

• Carbon atoms in triple bonds are sp-hybridized.
  • Mixes s orbital with one 2p orbital, creating two sp hybrid orbitals.
  • Arranged linearly (180°) with two unhybridized 2p orbitals at 90°.
• In acetylene (C2H2)
  • Carbon atoms form a sigma bond via sp orbital overlap.
  • Each carbon's p orbitals overlap to form pi bonds.
• Triple bond contains 1 sigma and 2 pi bonds.

Cyclic Alkynes

• Smallest cycloalkyne is cyclononyne (9-membered ring), decomposes at room temperature due to ring strain.

Acidity of Terminal Alkynes

• More acidic than alkanes and alkenes due to hybridization state:
  • Alkanes: sp3
  • Alkenes: sp2
  • Alkynes: sp
• s orbitals are more electronegative than p orbitals.
• Greater s-character in hybrid orbitals stabilizes negative charge better.
• Terminal alkynes can be deprotonated with bases like sodium amide or sodium hydride, forming acetylide ion.

Reactions Involving Alkynes

• Acetylide ions as strong bases and nucleophiles.
• React with unhindered 1° substrates via SN2 mechanism.

Additional Resources

• Practice quizzes and problems available on various topics related to alkynes.
• External links provided for further exploration of topics like alkene reactions, alkynes in organic synthesis, etc.