# Chirality of buta-1,2-diene

My teacher says buta-1,2-diene is symmetric and hence optically inactive but I think it's the opposite, because allenes with an even number of double bonds are optically active because they have no plane of symmetry. Which answer is correct?

Let's see if a picture can help. Newman projections can be handy in analyzing the stereochemistry of allenes. On the left is a Newman projection of methyl allene (buta-1,2-diene). There is a sigma plane of symmetry that is perpendicular to the screen and contains the 3 allenic carbons plus the methyl group. Any molecule with a plane of symmetry is achiral. On the other hand, look at the Newman projection to the right, 1,3-dimethyallene (penta-2,3-diene). There is no plane symmetry in this case, only a $\ce{C_2}$ axis. Draw the mirror image of the pictured allene and you will find that it is not superimposable on its mirror image. 1,3-dimethylallene is a chiral molecule.