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.

enter image description here


2,3-butadiene is not a IUPAC name. It should be 1,2-butadiene. I think you are talking about 1,3-butadiene (natural rubber). In any case, neither of the molecules have a chiral center; therefore, no optical activity.

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    $\begingroup$ Buta-1,3-dien is not an Allene, so I think the OP is actually talking about Buta-1,2-dien. $\endgroup$ Apr 28 '14 at 7:47
  • $\begingroup$ Yes, exactly martin $\endgroup$
    – user34304
    Apr 28 '14 at 10:01
  • $\begingroup$ OK, my mistake. But there is no chiral center since C1 has 2 hydrogens. 2,3-hexadiene would be chiral. $\endgroup$
    – LDC3
    Apr 28 '14 at 13:37
  • $\begingroup$ As is penta-2,3-diene (1,3-dimethylallene). $\endgroup$
    – ron
    Apr 28 '14 at 14:40
  • $\begingroup$ Also, natural rubber is comprised of isoprenes (i.e. isopentadienes), not butadienes. $\endgroup$
    – Curt F.
    Apr 21 '15 at 15:21

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