Benedict’s solution is an alkaline copper sulfate solution which is used to detect the presence of aldehyde groups. In the presence of Benedict’s solution, the aldehyde group is oxidized and the aqueous blue Cu2+ ion is reduced to a red Cu2O precipitate. Sugars such as glucose, which produce the red precipitate when Benedict’s solution is added, are called reducing sugars because they can reduce Cu2+ to Cu+. Which of the following carbohydrates would give a positive reaction with Benedict’s reagent? a. galactose; b. sorbitol (see below); c. β-ethylglucoside; d. gluconate .

Galactose is a sugar with six carbons and one aldehyde group present at the end. The presence of a free aldehyde group in galactose makes it a reducing sugar. Galactose can reduce the Cu2+ of Benedict solution into Cu+ and produce a red precipitate. Sorbitol, β-ethylglucoside, and gluconate do not have aldehyde group to reduce Cu2+ ions and therefore, do not give the positive test with Benedict solution.