[Julie Smith]

Hi, I'm going to be doing an experiment with these two acids, but I needed to get some things cleared up.

I know that fumaric acid has a much higher melting point than maleic acid (300*C vs. 138*C) but I'm wondering why? Does it have to do with the molecular polarities of the two isomers or maybe the different intermolecular/intramolecular hydrogen bonding in the two isomers? or maybe both things? Also, if these are some of the reasons why,which I think they are, which one would be the predominant factor?

thanks

[sjb]

Hi, I'm going to be doing an experiment with these two acids, but I needed to get some things cleared up.

I know that fumaric acid has a much higher melting point than maleic acid (300*C vs. 138*C) but I'm wondering why? Does it have to do with the molecular polarities of the two isomers or maybe the different intermolecular/intramolecular hydrogen bonding in the two isomers? or maybe both things? Also, if these are some of the reasons why,which I think they are, which one would be the predominant factor?

thanks

Typically, I'd suggest that the fact you can only form 1 intermolecular hydrogen bond with each maleic acid compared to the two available to fumaric may cause a difference in melting point. Perhaps see http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/chembox_diprotic_tit-e.htm ?

[Julie Smith]

well fumaric acid is non-polar and maleic acid is polar so does that mean that becuase fumaric acid is non-polar it has a higher melting point?

I know in terms of intermolecular hydrogen bonding, since fumaric acid has strong h-bonding that it leads to a higher melting point, but in terms of polarity of the two isomers why does it have the higher melting point?

thanks

[AWK]

well fumaric acid is non-polar and maleic acid is polar so does that mean that becuase fumaric acid is non-polar it has a higher melting point?

Both acids are isomers, they contain the same polar groups.

Moreover, nonpolar compounds usually show lower melting points

[Julie Smith]

Both acids are isomers, they contain the same polar groups.

Moreover, nonpolar compounds usually show lower melting points

so does that mean that both of them are either polar or non-polar? Does having a dipole moment mean that something is polar because maybe that's what I'm getting confused with? Maleic acis has a dipole moment of 7.22D and fumaric acid has it of 0D.

[Borek]

Both are polar, although only one has a dipole moment.

[Julie Smith]

Both are polar, although only one has a dipole moment.

oh ok, so I guess the melting point is influenced by the dipole moment. But then why does fumaric acid have a much higher melting point than maleic acid, when according to this it shouldn't?

[Julie Smith]

Also, when reacting maleic acid and fumaric acid with a metal, for eg. Mg, and solid NaHCO3, would the two react at completely different rates?

I'm asking because maleic acid 's solubility in water is 78g/100mL and fumaric acid's is something like 0.70g/100mL. does it have something to do with their different Ka values?

thanks.

[AWK]

Mainly different intermolecular/intramolecular hydrogen bonding in the two isomers cause different physical and chemical properties.