Question

Why do alkanes and alkynes, unlike alkenes, have no geometric isomers?

Answer 1

Because the carbon skeleton of alkanes and alkynes do not offer the possibility of geometric isomerism.

Explanation:

Structural isomerism is determined by the `C-C` connectivity of the molecule. Geometric isomers have the same `C-C` connectivity, but differing geometries. Thus, a simple olefin such as `"2-butylene"` clearly has cis and trans isomers, however, for each isomer, `C-C` connectivity is the same: `C1` connects to `C2`......connects to `C4`.

On the other hand, for alkanes and alkynes, there is no possibility for geometric isomerism, as differing conformations will equilibrate to give an average structure.

So when we represent alkanes and alkynes as formulae on paper, it is useful to draw the molecule with maximum symmetry, as this represents the likely geometry in solution.