Why is 2-chloropropene achiral?
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.