Why do appropriately substituted even-cumulenes (such as allenes of the type ) inherently fail to ex — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Even cumulenes (having 2, 4, 6... consecutive double bonds) contain a central $sp$ hybridized atom utilizing perpendicular p-orbitals to form adjacent $\pi$ bonds. This quantum mechanical requirement strictly forces the groups attached to the terminal $sp^2$ carbons to reside in orthogonal (perpendicular) planes. Since the groups are not in the same 2D plane, "cis" and "trans" relative spatial distances become identical, making GI impossible.