Organic Chemistry Chapter 11 Summary

Alkyne Structure

  • Alkynes contain a carbon-carbon triple bond.
  • General formula: CnH(2n-2) (four fewer hydrogens than alkanes).
  • Terminal alkynes: triple bond at chain end with one hydrogen; Internal alkynes: carbons bonded to each carbon of the triple bond.

Alkyne Bonding

  • Consists of one sigma bond and two pi bonds.
  • Each carbon is sp-hybridized with linear geometry (180° bond angle).

Naming Alkynes

  • Use suffix -yne instead of -ane for IUPAC naming.
  • Diynes and triynes for compounds with two or three triple bonds; enynes for mixed double and triple bonds.

Physical Properties of Alkynes

  • Similar to hydrocarbons of comparable shape and weight.
  • Low melting and boiling points; increases with carbon number.
  • Soluble in organic solvents, insoluble in water.

Acetylene (Ethyne)

  • Simplest alkyne, used frequently in welding due to high heat during combustion.

Preparation of Alkynes

  • Formed through elimination reactions of dihalides or conversion from alkenes.
    • E2 eliminations to yield a triple bond.

Addition Reactions of Alkynes

  • Undergo similar addition reactions as alkenes; hydrogen halides, halogenation, hydration.
  • Terminal alkynes can be deprotonated to form acetylide ions (strong nucleophiles).

Hydrohalogenation of Alkynes

  • Alkynes react with HX to form vinyl halides (Markovnikov addition).
  • Hydrohalogenation is slower for alkynes than alkenes.

Tautomerization

  • Keto-enol tautomerization occurs; enols are less stable than ketones.

Reactions of Acetylide Ions

  • Acetylide ions formed from terminal alkynes react with electrophiles (e.g., alkyl halides).
  • Strong nucleophiles; can open epoxide rings.

Retrosynthetic Analysis

  • Reverse engineering from target compounds to starting materials.
  • Focus on new C–C bond formation and functional group conversions in synthesis.