Question

Why can alkynes have cis and trans isomers?

Answer 1

Alkynes cannot exhibit cis / trans isomerism.

Explanation:

An alkyne linkage is linear: `R-C-=C-R'`; geometric isomerism across the triple bond cannot be supported. On the other hand, a suitably substituted olefin, `R_1R_2C=CR_3R_4`, can support such isomerism, as can suitably substituted cyclic structures. In each of the cases, the connectivity of each isomer is the same. `C1` is connected to `C2`, is connected to `C3` etc. but the spatial orientation, the geometry, is different.