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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. cis -Alkenes from Alkynes

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Alkyne Reactions

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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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

  6. The hydration step follows the Markovnikov Rule • Symmetric Alkynes give only 1 product • Unsymmetric Alkynes give product mixtures

  7. 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

  8. 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

  9. 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

  10. 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

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