Organic Chemistry MCAT Practice Exam

Which test indicates the presence of reducing sugars?

1. Benedict's test

2. Fehling's test

3. Tollen's test

4. All of the above

The correct answer is: All of the above

The correct answer is that all of the tests mentioned can indicate the presence of reducing sugars. Reducing sugars are carbohydrates that have free aldehyde or ketone groups, which can donate electrons to other molecules, hence "reducing" them. Benedict's test specifically detects aldehyde groups in sugars, allowing for the identification of reducing sugars such as glucose and fructose. When a reducing sugar is present, the copper(II) ions in the Benedict's reagent are reduced to copper(I) oxide, which precipitates out of the solution, forming a color change that can range from green to red, depending on the sugar concentration. Fehling's test works similarly to Benedict's test; it also relies on the ability of reducing sugars to reduce copper(II) ions, leading to a color change indicating their presence. This test also differentiates between various types of carbohydrates based on their reducing ability. Tollen's test is primarily used for aldehydes but can also be used for reducing sugars. In this test, silver ions are reduced to metallic silver, indicating the presence of a reducing agent, which includes reducing sugars as well. Thus, since all three tests can identify reducing sugars through different chemical reactions involving the reduction of metal ions, it's