The molecular formula for Fructose is C6H12O6. The molecular formula for glucose is also C6H12O6. How will you describe each one of them chemically? (25 points) i. Fructose is a 6-carbon containing hexose. Glucose is also a 6-carbon containing hexose. ii. Are Fructose and glucose Furanose/pyranose? Which is which? iii. Why is Sucrose (a disaccharide containing glucose and fructose) a non-reducing sugar
The Correct Answer and Explanation is:
i. Fructose and Glucose as Hexoses:
Both fructose and glucose are 6-carbon sugars, making them hexoses. They both share the same molecular formula C6H12O6, meaning they contain the same number of carbon, hydrogen, and oxygen atoms. However, despite this similarity, they differ in their structural arrangement and chemical properties.
- Glucose is an aldose because it contains an aldehyde functional group (–CHO) at the end of its carbon chain.
- Fructose, on the other hand, is a ketose because it contains a ketone functional group (–C=O) at the second carbon position.
This structural difference gives rise to different chemical and biological behaviors between the two sugars.
ii. Furanose/Pyranose Forms:
Fructose and glucose both form cyclic structures when they dissolve in water. The difference lies in which type of ring structure they form:
- Glucose primarily forms a pyranose ring, a six-membered ring structure. This occurs when the hydroxyl group on the fifth carbon atom reacts with the aldehyde group on the first carbon atom, creating a stable six-membered ring (like a pyranose).
- Fructose, however, forms a furanose ring, which is a five-membered structure. This happens when the hydroxyl group on the fifth carbon reacts with the ketone group on the second carbon, forming a five-membered ring structure, characteristic of furanose sugars.
iii. Sucrose as a Non-Reducing Sugar:
Sucrose is composed of glucose and fructose linked by a glycosidic bond. The glycosidic linkage between the two monosaccharides occurs between the anomeric carbon of glucose (carbon 1) and the anomeric carbon of fructose (carbon 2). When this bond is formed, the anomeric carbons of both sugars are involved in the linkage, leaving no free aldehyde or ketone group that could undergo oxidation.
Because reducing sugars require a free aldehyde or ketone group to act as a reducing agent (donating electrons), the lack of any free carbonyl group in sucrose means it cannot act as a reducing sugar. Therefore, sucrose is classified as a non-reducing sugar, unlike other disaccharides such as maltose, which contains a free aldehyde group in one of its monosaccharides, allowing it to be a reducing sugar.
