Detection
Benedict's reagent and Fehling's solution are used to test for the presence of a reducing sugar. The reducing sugar reduces copper(II) ions in these test solutions to copper(I), which then forms a brick red copper(I) oxide precipitate. 3,5-Dinitrosalicylic acid is another test reagent that allows quantitative spectrophotometric measurement of the amount of reducing sugar present.
Reducing sugars can also be detected with the addition of Tollens reagent. When Tollens reagent is added to an aldehyde, it precipitates silver metal, often forming a silver mirror on clean glassware.
Sugars having acetal or ketal linkages are not reducing sugars, as they do not have free aldehyde chains. They therefore do not react with any of the reducing-sugar test solutions. However, a non-reducing sugar can be hydrolysed using dilute hydrochloric acid. After hydrolysis and neutralization of the acid, the product may be a reducing sugar that gives normal reactions with the test solutions.
All carbohydrates respond positively to Molisch's reagent but the test has a faster rate when it comes to monosaccharides.
Read more about this topic: Reducing Sugar