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My teacher taught me that compounds having intramolecular H-Bonding are relatively less soluble in water as they will not involve in H-Bonding with water appreciably while those able to do intermolecular H-Bonding will appreciably form H-Bonding with water, and thus the latter ones will be relatively more soluble in water.

Applying this logic will make maleic acid less soluble than fumaric, but that's not the case. Wikipedia shows 478.8g/L for maleic, while just 4.9g/L for fumaric. Can someone help me with where I am going wrong?

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    $\begingroup$ You need to consider also the crystal lattice energy of each $\endgroup$
    – Andrew
    Commented May 8, 2021 at 21:13
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    $\begingroup$ chemistry.stackexchange.com/questions/66771/… $\endgroup$
    – Mithoron
    Commented May 8, 2021 at 22:04
  • $\begingroup$ chemistry.stackexchange.com/questions/51503/… $\endgroup$
    – Mithoron
    Commented May 8, 2021 at 22:07
  • $\begingroup$ So the teacher should have been more cautious? @Andrew $\endgroup$
    – Alchimista
    Commented May 9, 2021 at 8:33
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    $\begingroup$ Several factors could be important, but the first that comes to mind is polarity. The dipole moment of maleic acid is much larger than fro fumaric acid. $\endgroup$
    – matt_black
    Commented May 9, 2021 at 20:31

1 Answer 1

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Determining the crystal lattice energy would be helpful in comparing the starting points for a solubility investigation. Perhaps an approximation to the crystal stability is provided by the melting points: for fumaric acid, the mp is 287$^o$C; for maleic acid, 135$^o$C. 135$^o$C is a sort of normal mp for organic compounds. 287$^o$ seems quite high, suggesting a high lattice energy, and therefore, lower solubility for this isomer.

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Even if molecules have the possibility of intramolecular bonding, as indicated by molecular models or drawings on paper, the existence of intermolecular bonding is not precluded.

Sometimes explanations of an observation are used as predictions which then turn out to be incorrect. The explanations may still be useful, but as explanations of actual data, rather than as reliable predictions.

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