Textbook Question
Use the Fischer projection for D-gulose in problem 13.69 to answer each of the following:
b. Draw the Fischer projection and name the product formed by the oxidation of D-gulose.
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20. Carbohydrates
Oxidation of Monosaccharides
3:10 minutesProblem 75b
Textbook Question
Will the following carbohydrates produce a positive Benedict’s test?
b. lactose
Verified step by step guidance1
Understand the Benedict's test: The Benedict's test is used to identify reducing sugars. Reducing sugars are carbohydrates that have a free aldehyde group (-CHO) or a free ketone group (-C=O) that can reduce copper(II) ions (Cu²⁺) in Benedict's reagent to copper(I) oxide (Cu₂O), forming a red precipitate.
Determine the structure of lactose: Lactose is a disaccharide composed of glucose and galactose linked by a β-1,4-glycosidic bond. It is important to note that one of the monosaccharide units in lactose has a free aldehyde group when the glycosidic bond is hydrolyzed.
Analyze lactose as a reducing sugar: Lactose is classified as a reducing sugar because it contains a hemiacetal group in its structure. This hemiacetal group can open up to form a free aldehyde group under the conditions of the Benedict's test.
Relate the chemical behavior of lactose to the Benedict's test: Since lactose can produce a free aldehyde group, it will react with Benedict's reagent, reducing Cu²⁺ to Cu₂O and producing a positive result (red precipitate).
Conclude the result: Based on the chemical properties of lactose and its ability to act as a reducing sugar, lactose will produce a positive Benedict's test.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Benedict's Test
Benedict's test is a qualitative assay used to detect reducing sugars in a solution. When a reducing sugar is present, the copper(II) ions in the Benedict's reagent are reduced to copper(I) oxide, resulting in a color change that can range from green to brick red, depending on the concentration of the sugar. This test is commonly used in biochemistry to identify sugars like glucose and fructose.
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Reducing Sugars
Reducing sugars are carbohydrates that can donate electrons to other molecules, thus reducing them. This property is due to the presence of a free aldehyde or ketone group in their structure. Common examples include monosaccharides like glucose and disaccharides like lactose, which can act as reducing agents under certain conditions.
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Lactose
Lactose is a disaccharide sugar composed of glucose and galactose, commonly found in milk. It is classified as a reducing sugar because it has a free aldehyde group that can participate in redox reactions. Therefore, lactose can produce a positive result in a Benedict's test, indicating its reducing properties.
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