# Does 1,3-dichloroallene possess a C2 symmetry axis?

Is there an $$C_2$$ axis of symmetry in $$\ce{Cl-CH=C=CH-Cl}$$?

A $$C_2$$ axis of symmetry means that I should be able to rotate the molecule about an axis by $$180^\circ$$. But I would need a composition of the following two rotations

1. $$90^\circ$$ rotation about $$\ce{C=C=C}$$ axis
2. $$180^\circ$$ about the perpendicular bisector of $$\ce{C=C=C}$$.

Would that count as a $$C_2$$ axis of rotation?

• The rule is that the molecule must be indistinguishable before and after the symmetry operation. Therefore is not the C$_2$ axis a line from the midpoint between the two Cl atoms (or H atoms) to the central C atom ? The only other operations are E and S$_4$ Jun 10, 2020 at 8:11
• Sometimes, the perfectly OK and preferred formula does not help. In this case it's useful to draw it with 4 stereochem. wedges insted of 2. Jun 10, 2020 at 9:17

A $$C_2$$ axis is indeed here—although not the one that you are thinking of: your suggestion of a $$C_4$$ then $$C_2$$ rotation does not qualify as a $$C_2$$ symmetry operation. There is a true $$C_2$$ axis which bisects the allene and is tilted by $$45^\circ$$ from both the $$\ce{Cl-C-Cl}$$ and $$\ce{H-C-H}$$ planes.
2. Type in an identifier, e.g. the name, 1,3-dichloropropa-1,2-diene, or CAS number, 83682-32-0.
3. Open console (FileConsole) and enter show pointgroup to list symmetry elements and draw pointgroup to add them to the picture: