Why is glucaric acid soluble in water but not mucic acid? The chemical formulae seem to be the same so why is it that one is more soluble than the other?
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$\begingroup$ One seems to be symmetric(mucic acid) whereas the other is not.. Also what have you attempted to find the solution so far? $\endgroup$– Safdar FaisalAug 21, 2020 at 9:59
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$\begingroup$ Symmetry might actually answer it, I looked at the structures again and mucic acid has a plane of symmetry so that might explain why it isn't soluble. Thanks! I wanted to know about their solubilities because in my biochemistry textbook they said that you can identify galactose in samples by oxidation as it forms insoluble crystals. So I wondered why glucaric acid is soluble. $\endgroup$– SidAug 21, 2020 at 10:37
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1 Answer
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Wikipedia lists the melting points of the sugar acids as:
Glucaric (or saccharic) acid: $\pu{125-126 ^\circ C}$
mucic (galactaric) acid: $\pu{230 ^\circ C}$.
The very great solubility of all the sugary materials (glucaric acid: $\pu{63 mg/mL}$1, or perhaps $\pu{912 mg/mL}$2) is due to their hydroxyl groups, which are very compatible with $\ce{H2O}$. Mucic acid, even though, it has as many hydroxyls as glucaric, is soluble only to the extent of $\pu{3.3 mg/mL}$, although it was predicted from its structure to dissolve reasonably well ($\pu{63.1 g/L}$3). (Note the differences in units; I used the same numbers and units as in the original reference.)
The likely reason for the great difference in solubility and melting point is the way the mucic acid crystal packs. This has shown up in density and x-ray crystallography4.
Looking at the structures, or playing with molecular models (typically, one molecule at a time - you never have enough atoms to make a half a dozen molecules, nor the time to rotate them all and pack them, nor the straight-line vision to see how well they pack) doesn't give you the same sense of difference that the solubility and melting point give.
Maybe this is not terribly unusual: tartaric acid(s), a simpler diacid, shows similar differences in solubility and melting point just because of the chirality of the halves of the molecule. Melting points: D or L: $\pu{171-174 ^\circ C}$; DL $\pu{206 ^\circ C}$; meso: $\pu{147/ 165 ^{\circ} C}$. Solubilities (T=?): D or L: $\pu{1.33 kg/L}$; DL: $\pu{0.21 kg/L}$; meso: $\pu{1.25 kg/L}$ (Wikipedia).
References
- https://www.drugbank.ca/drugs/DB03603
- https://pubchem.ncbi.nlm.nih.gov/compound/Glucaric-acid#section=Experimental-Properties
- https://hmdb.ca/metabolites/HMDB0000639
- The crystal structure of galactaric acid (mucic acid) at −147 °C: An unusually dense, hydrogen-bonded structure by George A.Jeffrey, Richard A.Wood, Carbohydrate Research, Volume 108, Issue 2, 16 October 1982, Pages 205-211, DOI: 10.1016/S0008-6215(00)81790-5
answered Aug 21, 2020 at 13:57
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$\begingroup$ Thank you so much for providing the links! I understood from ref. 4 that hydrogen bonding in the crystal state of mucic acid is the reason for its decreased solubility, but that begs the question as to why the crystal structure of glucaric acid doesn't have hydrogen bonding to the same extent as mucic acid? $\endgroup$– SidAug 21, 2020 at 15:48
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1$\begingroup$ @Sid: You're welcome! But I gave you the answer: Make a dozen molecular models...or maybe do a crystal x-ray analysis...or maybe do it in your mind. Exercise those brain muscles. Or just look in awe at Nature's ability to make things interesting, and go on to a simpler problem. $\endgroup$ Aug 22, 2020 at 14:11
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