Cumulenes containing an EVEN number of consecutive double bonds (such as allenes) are stereochemical — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Even-numbered cumulenes utilize mutually perpendicular unhybridized p-orbitals to form consecutive $\pi$-bonds. This forces the terminal substituents into orthogonal planes. Because the groups are $90^\circ$ apart rather than flat, they do NOT show Geometrical Isomerism (though they may show Optical Isomerism).