isomer

Benzene-1,2-dicarboxylic acid has the lowest melting point.

Since the melting point is relative to the intermolecular forces.
Benzene-1,2-dicarboxylic acid forms intramolecular hydrogen bond so that reducing of the number of intermolecular hydrogen bonds of the molecules.

2008-08-29 18:44:52 補充：
第一位回答者,你忘記了carboxyl group的特性...

First of all, the m.p. of a substance is closely related to dipole moment and
symmetry. If you say don't consider dipole moment and symmetry, I can't
have the ability to answer your question.

If consider dipole moment and symmetry, the arrangement should be as
follows.

1,4- benzenedicarboxylic acid has greastest m.p.
While 1,3- benzenedicarboxylic acid has moderate m.p and
1.2- benzenedicarboxylic acid has lowest m.p.

The greatest chance for the dipole moment to cancel such as
1,4- benzenedicarboxylic acid will have the highest
m.p., while the one has largest dipole moment such as
1,2- benzenedicarboxylic acid should have lowest m.p.

m.p. = melting point