[cdsnose]

Why is the boilling point of p-dichlorobenzene low while the melting point is high as opposed to the meta and ortho dichlorobenzene? Usually when the boiling point is high, the melting point is high too. I've looked and read my book and still do not know the answer. I've also asked a tutor in my school but to now avail. I would appreciate your help on this. thank you!

[Donaldson Tan]

the reason lies in packing. the chloro-position influences the arrangement of molecules in its solid state. different arrangement results different extent of inter-molecular bonding, and hence different melting point. i hope this brief explaination will guide you to the right track.

[movies]

Yes, as Geodome said, the packing of the crystals is very important to the melting point because you must overcome the crystal packing energy.  This can vary greatly even for relatively similar molecules.

The boiling point, however, is most affected by polarity and molecular mass.  Since the dichlorobenzene isomers all have the same mass, you must consider the differences in polarity.  Think about the net dipole moment of the three isomers and I think that you will find the answer.