CHM030 4.2: Reactions of Alkynes

• alkynes


  hydrocarbons that contain a carbon-carbon triple bond
• general formula of alkynes

  CnH2n-2
• Addition of H2 to alkynes

  most often used catalyst: Lindlar catalyst (a specially prepared form of palladium metal)
• Lindlar catalyst can be prepared by precipitating palladium on calcium carbonate and treating it with lead acetate and quinoline
• Addition of HX to alkynes

  the H atom adds to the terminal carbon of the triple bond

  the X atom adds to the internal, more highly substituted carbon
• TRUE OR FALSE

  TRUE: In the addition of HX to alkynes, the regioselectivity of addition to a monosubstituted alkyne usually follows Markovnikov’s rule.
• Addition of HX to Alkynes

  The reaction can usually be stopped after the addition of 1 molar equivalent of HX to yield a vinylic halide

  vinylic means “on the C=C double bond”
• Addition of HX to Alkynes

  An excess of HX leads to formation of a dihalide product.
• Addition of X2 to alkynes

  • Bromine and chlorine are added to alkynes to give dihalide addition products with anti-stereochemistry.
  • Either 1 or 2 molar equivalents can be added
• Addition of H20 to alkynes

  Addition of water takes place when an alkyne is treated with

  aqueous sulfuric acid in the presence of mercuric sulfate

  catalyst
• Addition of H20 to alkynes

  A mixture of both possible ketones results when an internal

  alkyne (R―C ≡ C―R') is hydrated
• Addition of H2O to Alkynes

  Only a single product is formed when a terminal alkyne (R―C ≡ C―H) is hydrated
• Formation of Acetylide Anions

  When a terminal alkyne (R―C ≡ C―H) which is weakly acidic is treated with a strong base such as sodium amide, NaNH2 , the terminal hydrogen is removed and an acetylide anion is formed.
• Formation of Acetylide Anions

  The presence of an unshared electron pair on the negatively charged alkyne carbon makes acetylide anions both basic and nucleophilic.
• Terminal alkynes 


  can be prepared by reaction of acetylene with sodium amide, NaNH2
• Internal alkynes


  can be prepared by reaction of a terminal alkyne with sodium amide, NaNH2
• TRUE OR FALSE

  TRUE: The one limitation to the reaction of an acetylide anion with an alkyl halide is that only primary alkyl halides, RCH2X, can be used.
• Addition of H2 to Alkynes

  Because hydrogenation occurs with syn stereochemistry,

  alkynes give cis alkenes when reduced.