Does it have something to do with its ketone group or a different property? Please explain.
Dear Alexxandra,
very good thinking. In aqueous solution, fructose is an open-chain to a small extent and is present predominantly as α- or β-fructopyranose, which partially merge by mutarotation. In crystalline form, fructose exists as a closed pyran ring (fructopyranose). The keto group which is only present in fructose's open chain form has now turned into an OH- group by nucleophilic substitution during ring formation. Therefore, the keto group can't play any role in incorporating water into fructose's crystalline structure. Crystals of fructose exist as either anhydrous β-d-fructopyranose or as the dihydrate or hemihydrate (one molecule of water of crystallization per two molecules) of β-d-fructopyranose. Dihydrate fructose crystals can dissolve in their own water of hydration and must therefore be handled very carefully. The water of crystallisation is not chemically bound, but only weakly bound by hydrogen bonds (which are electrostatic forces) and may normally be removed by heating.
Wishing you best of success,
Johannes
Dear Alexxandra Jane Ty,
differences in degrees of hygroscopicity between d-glucose and d-fructose are observed because of diffences in crystal growth morphology. When sugars crystallize, intermolecular hydrogen bonds form between the sugar molecules and, therefore, the hydroxyl groups are unavailable to hydrogen bond with water. This is the case to a larger extent in glucose being an aldo sugar than in fructose being a keto sugar. Fructose is more soluble than glucose and hard to crystallize because it is more hygroscopic and holds onto water stronger.
The difference between alpha and beta glucose is in the position of one of the four -OH groups. The carbon next to the oxygen atom carrying an -OH group in the hexagonal ring is called the anomeric carbon. If the -OH group attached to it is below the ring, the molecule is alpha glucose. If the -OH group is above the ring, the molecule is beta glucose. Since the linear and cyclic forms of glucose inter-convert with each other, alpha glucose can turn into beta glucose and vice versa. If you take a sample of pure alpha glucose and put it into water, you'll end up with a sample that is part alpha and part beta glucose. In beta-D-glucose all of the OH-substituents occupy the sterically preferred equatorial position, therefore it is more stable and the most widely distributed sugar in nature. Again, differences in water uptake are due to differences in the cristalline structure of both forms.
Best regards, Johannes
Thank you Johannes Haedrich for the explanation. Just to clarify further, what is it about the fructose having that ketone group that makes it hold on to water stronger in comparison to glucose? In other words, what is it about being a ketone that makes it solubize so easily? I thought ketones are less reactive given the stability of having more steric hindrance which inhibits attack on the carbonyl group?
Dear Alexxandra,
very good thinking. In aqueous solution, fructose is an open-chain to a small extent and is present predominantly as α- or β-fructopyranose, which partially merge by mutarotation. In crystalline form, fructose exists as a closed pyran ring (fructopyranose). The keto group which is only present in fructose's open chain form has now turned into an OH- group by nucleophilic substitution during ring formation. Therefore, the keto group can't play any role in incorporating water into fructose's crystalline structure. Crystals of fructose exist as either anhydrous β-d-fructopyranose or as the dihydrate or hemihydrate (one molecule of water of crystallization per two molecules) of β-d-fructopyranose. Dihydrate fructose crystals can dissolve in their own water of hydration and must therefore be handled very carefully. The water of crystallisation is not chemically bound, but only weakly bound by hydrogen bonds (which are electrostatic forces) and may normally be removed by heating.
Wishing you best of success,
Johannes
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