Geometric Isomers in Alkenes & Alkynes: Why?

In summary, the existence of geometric isomers in alkenes is due to their double bond and the ability for the pi bond to restrict rotational movement. However, this is not the case for alkynes, which have a triple bond and a linear shape with only two substituents. This restricts the ability for geometric isomers to form.
  • #1
gymstar
3
0
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
 
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  • #2
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!
 
  • #3
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
 
  • #4
Thanks a lot for clearing that up. It makes perfect sense now.

thanks
 

FAQ: Geometric Isomers in Alkenes & Alkynes: Why?

What are geometric isomers?

Geometric isomers are molecules that have the same chemical formula and connectivity, but differ in the spatial arrangement of their atoms.

Why do alkenes and alkynes exhibit geometric isomerism?

Alkenes and alkynes have a double or triple bond, respectively, which restricts the rotation of the atoms around the bond. This leads to different spatial arrangements of the atoms and results in geometric isomerism.

How do geometric isomers differ from structural isomers?

Structural isomers have different chemical formulas and connectivity, while geometric isomers have the same chemical formula and connectivity but differ in spatial arrangement.

What is the difference between cis and trans isomers?

Cis isomers have similar groups on the same side of the molecule, while trans isomers have similar groups on opposite sides of the molecule. This results in different physical and chemical properties.

Why is it important to distinguish between geometric isomers?

Geometric isomers can have different physical and chemical properties, such as melting point, boiling point, and reactivity. This can have implications in many industries, including pharmaceuticals, where the different isomers may have different biological activities and effects.

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