[mandy]

When chlorobenzene is nitrated, the para and ortho product are obtained.  Why is one of the products more polar than the other one.  I did a TLC analysis and the para form seems to be more polar, but I don't understand how one can be more polar since they have the same subtituents.  I think it must have something to do with how close the substituents are to ecah other but I don't understand what kind of effect this has.

[macman104]

I believe it does have to do with the position of the substituents.  However, I will say that the polarity difference between the two is fairly small.  Anyway, I believe that the difference in polarity comes from the ability for a group of these molecules to align with each other.  If you think about how initially you learned that a highly branched alkane will have a lower boiling point than a straight chain alkane of the same size, I believe this is similar in situation.  The para compound can achieve a more "linear" shape than the ortho compound.  At least that is the reason I've come up with, good question though!