Is anyone familar with the equilibrium reactions that convert the ketone group of fructose into an aldehyde group so fructose give a positive Fehling's Test? I know it will as we received so in class but not sure how the reaction takes place.
It's explained at this site. Scroll down to the first paragraph under Figure 2 (about 60% down the page). Fructose is converted to glucose and mannonse under th alkaline conditions of the Fehling's test solution.
I didn't post the site. Here it is.
http://www.chemie.uni-regensburg.de/Organische_Chemie/Didaktik/Keusch/D-Fehling-e.htm
DrBob,
Does Figure 2 demonstrate that Fructose is giving a positive result (as an aldose)even though it has been converted to glucose and manose? Thanks for the clarification.
Yes, I am familiar with the equilibrium reactions that convert the ketone group of fructose into an aldehyde group, allowing fructose to give a positive Fehling's Test. I can explain how this reaction takes place.
The conversion of the ketone group of fructose (also known as a fructofuranose ring) into an aldehyde group (fructopyranose ring) occurs through a process called mutarotation. Mutarotation refers to the spontaneous interconversion between different anomers (isomers differing in the configuration at the anomeric carbon) of a sugar molecule in solution.
Fructose in its crystalline form predominantly exists as a fructofuranose ring, which has a ketone functional group. However, when fructose is dissolved in an aqueous solution, it undergoes mutarotation. This process involves the opening of the fructofuranose ring and the formation of a fructopyranose ring, which has an aldehyde functional group.
The mutarotation happens due to the equilibrium between the open-chain form (aldose form) and the cyclic forms (hemiacetal forms) of the monosaccharide. In the case of fructose, the keto form (ketose form) is converted into an aldehyde form through tautomeric shifts and ring opening and closing reactions.
When fructose is subjected to Fehling's Test, the presence of aldehyde groups in the fructopyranose form leads to a positive result. Fehling's Test is a chemical test that detects the presence of reducing sugars, such as aldehydes, by their ability to reduce the cupric ions (Cu2+) present in Fehling's solution. Positive Fehling's Test is observed as a formation of a reddish-brown precipitate of copper(I) oxide.
In summary, the equilibrium reactions of mutarotation allow the conversion of the ketone group of fructose into an aldehyde group, enabling fructose to give a positive Fehling's Test.