[wicked-wanderer]

Hi,

I have a question regarding the differences between two similar organic compounds with differing solubility. I'd very much appreciate your help

Both compounds contain two heteroaromatic moieties. One is rigid, highly conjugated and very insoluble in organic solvent. The other is very soluble; it contains the same two functional groups, however they are separated by a linker. As a result, the degree of conjugation is high and it is also flexible. My question is, does both the rigidity and conjugation of the former compound contribute to its insolubility?

Many thanks for your help

[OC pro]

Structures would help a lot...

[Babcock_Hall]

Without structures the question is impossible to answer.  However, I would hazard a guess that if the linker were derived from polyethylene glycol, that the oxygen atoms would contribute to greater water solubility.

[fledarmus]

I'm not sure exactly what question you are asking. Are you comparing water solubility with organic solubility and wondering why the more rigid structure might be more water soluble than the less rigid structure? Or are you comparing crystallinity and wondering why the more rigid structure might crystallize out of an organic solvent while the floppier structure stays in solution?

Assuming that you are asking the second question - In general, aromatic structures can get some boost to their intermolecular forces through pi-stacking interactions, where the ring systems stack flat against each other in the crystal structure. The flatter the molecule, the more likely this is to happen. When you have a flat group linking two aromatic systems into a plane you can get stronger intermolecular interations; if the two groups are anchored at an angle to each other, or if they are attached by a floppy group so they are free to move around (and especially if the gauche-butane interactions of the floppy group tend to hold them at an angle), they will not stack as well and the resulting material will be less likely to crystallize.