Chem 335 - Winter 1999 - Organic Chemistry II
Portland State University - Dr. Carl C. Wamser

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Chapter 10 - Alkynes

Structure

• carbon-carbon triple bond
• sp hybridization (linear)
• no cis-trans possibilities
• the two pi bonds are perpendicular
• high electron density
  (usually more reactive than alkenes)

Nomenclature

• -yne infix (with number)
• rules similar to alkenes
• with both -enes and -ynes, suffix is -enyne and numbering is from the end closer to a multiple bond
  
  (E)-4-hexen-1-yne
  
• common names: alkyl acetylene or dialkyl acetylene

Properties

• acidity
  sp C-H bonds are relatively acidic
  alkanes (sp3 C-H), pKa ~ 51
  alkenes (sp2 C-H), pKa ~ 44
  alkynes (sp C-H), pKa ~ 25
• sp C-H bonds can be deprotonated by bases with pKa > 25
  ( e.g., NaNH2 but not NaOH )
• alkyne anions are called acetylides and are good nucleophiles
• alkylation
  substitution of an alkyl group, using acetylide anion
  SN2, so alkyl group must be CH3 or 1°

Preparations - eliminations

• NaNH2 in liquid NH3 removes 2 HX from dihalides

Reactions - Hydrogenation

• catalytic hydrogenation to an alkane
  
• Lindlar catalyst + H2 - forms cis-alkene
  
• reduction by sodium in liquid NH3 - forms trans-alkene
  
• hydroboration followed by acid - forms cis-alkene
  

Addition reactions

• hydroboration followed by oxidation - forms ketone or aldehyde
  follows anti-Markovnikov addition (as with alkenes + BH3 )
  but enol is unstable - converts to a carbonyl compound

• keto-enol tautomers
  carbonyl compound is generally more stable than the enol isomer
  tautomers - isomers differing by the position of a hydrogen & double bond
  
• hydration of terminal alkynes to aldehydes using (sia)2BH
  problem - alkenylboranes from terminal alkynes can add more BH3 to the C=C bond
  solution - use a dialkylborane that can only add once
  
• halogenation
  addition of two equivalents of Cl2 or Br2
  
• hydrohalogenation
  addition of two equivalents of HCl or HBr
  forms geminal dihalide (see homework problem)
  
• hydration
  addition of water with acid catalysis or Hg+2 as catalyst
  follows Markovnikov addition (unlike hydroboration/oxidation)
  also goes through an enol intermediate

Organic synthesis

• retrosynthetic strategy
  • think backwards -
    What is a reaction that could be used to make this product?
  • visualize the construction of the carbon framework
    What reactions make new C-C bonds?
  • carry out functional group interconversions as necessary