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Could someone explain to me why some alkenes containing at least one couple of different substituents at the two double-bonded C atoms (that is, some of those with at least two different substituents for both C atoms) are considered achiral? For instance, for 2-chloropropene or 1-bromo-2-chloro-2-fluoro-1-iodoethene, where (and what) in the structural formula of each of them is the plane of symmetry that makes it achiral or, in other words, what do their superposable mirror images look like? Thanks.

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    $\begingroup$ Welcome to Chemistrty.SE If you have any questions about how this site works visit our help center and take our tour. For your molecules, consider their three-dimensional geometry. For example, what is the geometry around the central carbon atom in 2-chloropropene? $\endgroup$ – Ben Norris Jul 12 '15 at 11:41
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    $\begingroup$ As your molecules are planar. The plane of the molecule is the element of symmetry that renders the molecule achiral. $\endgroup$ – Yomen Atassi Jul 12 '15 at 12:01
  • $\begingroup$ You can see the 3D geometry, e.g. 2-chloropropene and see that the molecule is flat. $\endgroup$ – Geoff Hutchison Jul 12 '15 at 16:34
  • $\begingroup$ Yes, I've checked it in the meantime and saw the plane of reflectional symmetry imposed by the C=C bond. It also explains why certain allenes are chiral (despite having no asymmetric C atom). Thanks! $\endgroup$ – Adrian Jul 12 '15 at 17:30
  • $\begingroup$ You are welcome to contribute an answer to your own question @Adrian - that way, future readers can benefit from this knowledge. $\endgroup$ – user15489 Jul 12 '15 at 20:33
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Any molecule that cannot be superimposed on its mirror image is described as being chiral. If the molecule is superimposable on its mirror image the object is called achiral.

For simple alkenes, the carbon atoms of the double bond and the four atoms bonded to them all lie in the same plane, which divides the molecule into two halves, which are mirror images of each other. This plane of symmetry ensures that the mirror image forms of the molecule are superimposable. Therefore, simple alkenes like 2-chloroprop-1-ene and 1-bromo-2-chloro-2-fluoro-1-iodoethene are achiral.

Nevertheless, owing to the barrier to rotation in alkenes, 1-bromo-2-chloro-2-fluoro-1-iodoethene exists as two (achiral) stereoisomers: (E)-1-bromo-2-chloro-2-fluoro-1-iodoethene and (Z)-1-bromo-2-chloro-2-fluoro-1-iodoethene.

(E)-1-bromo-2-chloro-2-fluoro-1-iodoethene  enter image description here

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