why does sucrose not react with Benedicts solution until it has been treated with acid?

Answer 1

i think it might be because it is a complex sugar

Answer 2

Sugars will only react with Benedicts solution if they are reducing sugars. To be a reducing sugar, the sugar has to have an aldehyde group that is available to react with the copper in the Benedicts solution. In sucrose, the two simple sugars are bound together in a way the prevents the reaction from happening. When you react sucrose with acid, you are breaking the bond between the two monosaccharides, making glucose which does react nicely with Benedicts.

All said at another level, the two anomeric carbons of glucose and fructose are linked in a glycosidic bond so that neither anomeric carbon is available to react with the Benedicts. When you hydrolyze sucrose with acid, both glucose and fructose (for odd reasons...ask your organic chemistry prof) will be able to reduce the copper ions of Benedicts solution to Cu2O.