Geometric Isomers in Alkenes & Alkynes: Why?

[gymstar]

Hi everyone... can anyone help me out? I am not sure if i am on the rigth track.

Why do geometric isomers exist in alkenes, but not in alkynes?

Does it have something to do with the that alkenes have a double bond and alkynes have a triple bond? Also the pi bond restricts the rotational movement of the molecule and locks the groups in place. But how does that change in alkynes? I understand why geometric isomers cannot exist in alkanes beacuse of the sima (single bond) but why can't they exist in the alkynes?

Any help is greatly appriciated... thanks

 

[movies]

There is restricted rotation with alkynes too, but rotation around the triple bond wouldn't form a different compound as it could with a double bond. Think about the shape of the molecules; alkynes are linear with two substituents, one on each end. Alkenes are not linear: there are four distinct positions where different groups can be. Switching the position of two of the groups at one end of an alkene can form a geometric isomer, but in an alkyne there aren't two distinct groups to switch!

 

[campa]

alkynes have a triple bond which ristricts these of having 4 extra bonds (2 per C making the triple bond)which you should have when considering a geometric isotomer

 

[gymstar]

Thanks a lot for clearing that up. It makes perfect sense now.

thanks