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Alkyne Reactions. Reduction of Alkynes Relative Reactivity of 2 p -bonds Alkynes react like alkenes, but twice Hydrogenation of alkynes goes to alkanes Electrophilic Attack Most of the Alkene reaction we learned will work for alkynes x 2. cis -Alkenes from Alkynes

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alkyne reactions
Alkyne Reactions
  • Reduction of Alkynes
    • Relative Reactivity of 2 p-bonds

Alkynes react like alkenes, but twice

        • Hydrogenation of alkynes goes to alkanes
        • Electrophilic Attack
        • Most of the Alkene reaction we learned will work for alkynes x 2
slide2
cis-Alkenes from Alkynes
    • Alkynes can be Hydrogenated under alkene conditions
      • Pd/C or PtO2, 1 atm H2
      • Gives saturated alkane product
    • Lindlar Catalyst gives single H2 addition
      • Lindlar Catalyst = 5% Pd-CaCO3, Pb(OAc)2, quinoline =
      • Less active catalyst surface, so only one p-bond is added to
      • H2 addition is syn (like for alkenes) and gives cis-alkene product
  • trans-Alkenes
    • Na is a strong reducing agent: Na Na+ + e-
    • Na dissolves in NH3(l) to give Na+ + e- (solvated electron)
    • Alkynes exposed to Na/NH3(l) are selectively reduced to trans-alkenes
slide3
Mechanism: (see book for orbital picture)
  • Electrophilic Addition to Alkynes
    • Alkynes are much like alkenes
      • p-bonds readily attacked by electrophiles
      • Terminal alkynes follow Markovnikov rule
        • Electrophile ends up on less substituted Carbon
        • Nucleophile ends up on more substituted Carbon
slide4
HX Additions to Alkynes
    • Single addition usually gives anti product
    • The second addition gives the Geminal Dihaloalkane (Markovnikov Rule)
    • Terminal Alkynes give Marknovnikov Addition Products
    • Stopping the reaction after only one addition is difficult for terminal alkynes
slide5
Halogenation of Alkynes
    • Anti addition of a single X2 molecule can be done to get vicinal dihaloalkane anti addition product
    • The second addition gives a tetrasubstituted product
  • Ketones from Alkynes: Mercuric Ion Catalyzed Hydration
    • Like alkenes, Alkynes can be hydrated
    • The enol product undergoes tautomerization (interconversion of isomers by C=C, H shift) to give a ketone product
    • The Enol and Ketone are called tautomers
    • The Hg2+ cation catalyzes the reaction; mechanism not understood yet

Enol

Ketone

slide6
The hydration step follows the Markovnikov Rule
  • Symmetric Alkynes give only 1 product
  • Unsymmetric Alkynes give product mixtures
slide7
Anti-Markovnikov Additions to Alkynes
    • Radical HBr additions
      • HBr adds to Alkynes by radical mechanism with radical initiator ROOR
      • Anti-Markovnikov Products due to need for most stable radical intermediate
    • Aldehydes from Hydroboration-Oxidation of Alkynes
      • Like with alkenes, BHR2 adds to less substituted side of an alkyne
      • R groups prevent boration of both p-bonds
      • Oxidation results in the enolaldehyde tautomerization
slide8
Alkenyl Halide Chemistry
    • Alkenyl Halides don’t do SN1 or SN2 reactions
      • Alkenyl Halides preferentially eliminate to alkynes instead of substituting
      • Need strong base to get elimination, other nucleophiles give N.R.
      • Simple nucleophiles don’t give the substitution products
      • Alkenyl cation that would form is very unstable
    • Alkenyl Organometallics can form and act as nucleophiles
slide9
Organocuprate Chemistry
    • Most organometallic reagents don’t react with alkyl halides
      • Alkyl and Alkenyl Metal reagents don’t attack haloalkanes fast enough
      • Alkynyl Metal reagents can react with haloalkanes
    • Organocuprates are more reactive
      • Formation of Organocuprates
      • Organocuprates will couple their R groups with Haloalkanes
slide10
Either alkyl or alkenyl organocuprates work for the coupling reaction
  • Along with the Alkynyl Anion reactions, we now can form many different C—C bonds