AL CHEM

圖片參考：http://i1096.photobucket.com/albums/g340/pingshek/2011-4-16.png?t=1302889272


http://i1096.photobucket.com/albums/g340/pingshek/2011-4-16.png?t=1302889272

trans-butenedioic acid tends to form a diester. Since esters are not very soluble in water, this will lower the solubility of trans-butenedioic acid.
http://en.wikipedia.org/wiki/Fumaric_acid

- C-COOH bonds are polar.
- In actual experiments, COOH in trans-butenedioic acid will form intermolecular H- bond with H2O molecules, while COOH in cis-butenedioic acid will form intramolecular H-bond within COOH in cis-butenedioic acid.
- Thus, the solubility in water of trans-butenedioic acid is greater than that of cis-butenedioic acid in actual experiment.

i feel doubtful on miraco's explanation.
in fact, i'm thinking about completely opposite situations.

for the dicarboxylic acid to be dissolved - i.e. dispersed - in water, what would be the major force involved?
polarity of molecule is surely present, but the major issue should be hydrogen-bonding.

when the acid molecules are dispersed, attraction between molecules in solid is interrupted, which is endothermic in nature; after that, there's attraction between acid molecules and water molecules, i.e. disolvation. disolvation is exothermic.

enthalpy of dissolution = lattice enthalpy (+ve) + enthalpy of disolvation (-ve)

which acid, maleic (cis-) or fumaric (trans-), has higher lattice enthalpy?
surely it's fumaric, as it has much more extensive inter-molecular H-bondings. therefore, much more energy is needed to overcome the attraction between the acid molecules.

on the other hand, intermolecular H-bond is not so extensive in maleic acid ('coz they go intra-molecular). lattice is weaker, breaking of lattice is relatively more feasible.


yet, it's still difficult to tell..