1 / 10

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

israel
Télécharger la présentation

Alkyne Reactions

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related