Cumulenes containing an EVEN number of consecutive double bonds (such as 1,3-dichloroallene) are ste — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Even-numbered cumulenes (allenes) utilize mutually perpendicular unhybridized p-orbitals to form consecutive $\pi$ -bonds. This strictly forces the terminal substituents into orthogonal (perpendicular) planes. Because the groups are $90^\circ$ apart rather than flat, the distances between substituents cannot vary to form cis/trans shapes, meaning they do NOT show Geometrical Isomerism. However, if appropriately substituted, this perpendicularity allows them to show Optical Isomerism.