Question

Why is 1,4-dichloroanthracene polar?

Answer 1

Because it is not symmetric so the bond dipoles do not cancel out.

Explanation:

You only get a zero dipole moment for an organic molecule when the bond dipoles cancel each other out. For example, in 1,4-dicholorbenzene the two chlorines will pull electrons from the aromatic ring, but those two pulls exactly cancel each other:

However, take a look at 1,4-dicholoranthracene:

Yes--across the molecule the two chlorines will cancel each other out, but the electrons in the rest of the aromatic rings will still be pulled toward the ring with the two chlorines on it.

You could fix this by substituting the three ring:

Now there is a zero dipole moment because all the forces on the electrons of the aromatic bond are the same!