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.
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.