Why is maleic acid so soluble in water whereas fumaric acid is not? How can we explain this in terms of intra-molecular hydrogen bonding formation within maleic acid and inter-molecular hydrogen bonding formation with water. It seems that there is contradiction.
Also, how do we explain the difference in melting points between the two isomers?
A very good point! I'm sure there must be somewhere (mainly in the German, before WWII) literature an extensive treatment of this phenomena. Here just an opinion: very often it is the entropy contribution in the Gibbs function which favours certain solubility
"exceptions". Intitively, the more open fumaric should be more easily hydrated (and perhaps also is) than the closely packed maleic. However, a good, "close" packing of the rather stiff (only the COOH groups can rotate along the covalent bond!) fumaric molecule between the molecules of water (quasi ice structure!)is perhaps possible at all. On the other had, a packed structure of maleic makes it more fitting between the water molecules. Perhaps this could be supported by molecular dynamics calculations...a good PhD thema! Regards, P.
PS: why not to write to J.chem. educ.? they have a good historical column...