Summary of Ch 7 Alkyne Reactions

Summary of Ch 7: Preparing Alkynes 1) Preparing terminal alkyne: a. From acetylene:

 b. From terminal alkene:

2) Preparing internal alkyne: a. From acetylene:

 b. From internal alkene:

Summary of Ch 7: Alkyne Reactions

T e trahaloalkan e Alkyne

 





  X

and  X added twice Onium ion intermediate 1,2-shift is not possible ! nd  Not a regioselective reaction (2  Br attacks the more substituted C of onium ion)  Not a stereoselective reaction

Dihaloalkan e  





Terminal Alkyne

  X

and  X added Onium ion intermediate 1,2-shift is not possible ! nd  Not a regioselective reaction (2  Br attacks the more substituted C of onium ion) Stereoselective   anti (-X and  X will be trans)

Dihaloalkan e-ge minal  



  H

and  X added twice Carbocation intermediate 1,2-shift is not possible ! Regioselective reaction !

            



most H. Second X from H-X adds to C ! st with 1  -X group  Not a stereoselective reaction

Vinyl halid e  



  H

and  X added Carbocation intermediate 1,2-shift is not possible ! Regioselective reaction !

            



most H.  Not a stereoselective reaction

Terminal Alkyne

K e to   



  H

and  OH added Keto-enol Tautomerization Carbocation intermediate 1,2-shift is not possible ! Regioselective reaction !

            



 Not a stereoselective reaction

K e to   



  H

and  OH added Keto-enol Tautomerization Onium ion intermediate ! 1,2-shift is not possible Regioselective reaction !

           



the most H. Stereoselective reaction: anti, but loss of stereochemistry during keto-enol tautomerization

K e to   



  H

and  OH added Keto-enol Tautomerization Regioselective reaction !  Non-   from H-       with the least H. Stereoselective reaction: syn, but loss of stereochemistry during keto-enol tautomerization

Alkan e Alkyne

  

  H

and  H added twice  Not a regioselective reaction Stereoselective reaction   syn addition,  but after the second equivalent of H2 added, stereochemistry is lost.

c i s 

- Alk e n e

  

  H

and  H added  Not a regioselective reaction Stereoselective reaction   syn addition = c i s   product.

t  ra n s - Alk e n e   

  H

and  H added  Not a regioselective reaction Stereoselective reaction   anti addition = t  ra n s   product.