Question

Sketch a representation of the disaccharide sucrose (table sugar). Label clearly the portion of the disaccharide that originates from glucose, the portion that originates from fructose, and the glycoside linkage between the rings.

Short answer

A representation of the disaccharide sucrose can be sketched by drawing the α-glucose chair conformation and β-fructose furanose conformation connected through an α-1,2 glycosidic bond between the C1 carbon atom of glucose and the C2 carbon atom of fructose. Label the six-membered ring as originating from glucose and the five-membered ring as originating from fructose. Additionally, label the glycoside linkage between the two sugar rings.

Step-by-step solution

Recognize glucose and fructose structures

Glucose and fructose are monosaccharides, the basic units of carbohydrates. Glucose has a six-membered ring structure, while fructose has a five-membered ring structure. Glucose is an aldohexose, and its structure can be represented as a chair conformation with hydroxyl (-OH) groups attached. Fructose is a ketohexose, and its structure can be represented as a furanose ring with hydroxyl groups attached.

Identify the glycoside linkage between glucose and fructose

In sucrose, glucose and fructose units are linked by an α-1,2 glycosidic bond. This means that the α conformation of glucose is linked to the β conformation of fructose at the first (C1) carbon atom of glucose and the second (C2) carbon atom of fructose. The glycoside linkage is formed through a condensation reaction, in which a water molecule is released as a result of the bond formation.

Draw a representation of sucrose

Now that we know the structures of glucose and fructose, as well as the glycoside linkage between them, we can draw a representation of sucrose. First, draw the glucose molecule in its α chair conformation and the fructose molecule in its β furanose conformation. Then, connect the two sugar rings through the glycoside linkage between C1 of glucose and C2 of fructose. Make sure to show the hydroxyl groups and other functional groups present in both glucose and fructose.

Label the portions of the sucrose molecule

Finally, label the different portions of the sucrose molecule. Indicate which part of the molecule is derived from glucose (the six-membered ring) and which part is derived from fructose (the five-membered ring). Also, label the glycoside linkage that connects the two sugar rings together (the bond between C1 of glucose and C2 of fructose).

Key concepts

Glucose and Fructose Structure

Understanding the structures of glucose and fructose is key to grasping how these two monosaccharides combine to form the disaccharide sucrose. Glucose is often depicted in its stable six-membered ring, also known as the Haworth projection. In this cyclic form, glucose is known as an aldohexose, indicating that it contains an aldehyde group and has six carbon atoms. Fructose, on the other hand, is a ketohexose - containing a ketone group, and also has six carbon atoms but predominantly exists in a five-membered ring structure. Both of these sugars have multiple hydroxyl (-OH) groups attached to their carbon atoms, creating multiple sites for potential chemical reactions.

The arrangement of these hydroxyl groups differs between glucose and fructose and is significant in determining their chemical properties and reactions with other molecules, like forming glycosidic bonds. The structural details are crucial when it comes to identifying the origins of the components in larger sugar molecules such as sucrose.

Glycoside Linkage in Sucrose

Central to the formation of sucrose is a special type of covalent bond known as the glycosidic linkage. This bond is what holds the two monosaccharides – glucose and fructose – together. Specifically, in sucrose, an α-1,2 glycosidic bond is formed when the anomeric carbon of glucose (C1 - the carbon attached to the aldehyde group) bonds to the second carbon atom of fructose (C2).

This glycosidic linkage is formed through a dehydration synthesis, or condensation reaction, where a molecule of water is released. It is the removal of the hydroxyl group from the anomeric carbon of glucose and the hydrogen from the OH group of fructose that allows the bond to form, creating the sweet molecule known as table sugar. In chemical diagrams, this bond is typically represented as a line connecting the two sugar rings, indicating a bridge between the distinct parts of the sucrose molecule.

Monosaccharides: The Building Blocks

Monosaccharides are the most basic form of carbohydrates and include simple sugars like glucose, fructose, and galactose. These molecules are single units that can be linked together to form more complex carbohydrates. As the name suggests, 'mono-' meaning one, and '-saccharide' meaning sugar, monosaccharides are single sugar units with three to seven carbon atoms.

Monosaccharides have a backbone of carbon atoms and are classified based on the number of carbon atoms they contain. For example, trioses have three, while hexoses like glucose and fructose have six. These sugars have different functional groups that make them react uniquely, particularly with regard to how they form glycosidic linkages. In the context of sucrose, it is the linking of two hexose monosaccharides, one being an aldohexose and the other a ketohexose, that forms a disaccharide.