CARBOHYDRATES
Di, poly-Carbohydrates Carbo MiniTopics
Review
 Elmhurst College
Classification Glucose Fructose  Chemistry Department
Carbo - Isomers Galactose Ribose  Virtual ChemBook

 Fructose

Fructose is more commonly found together with glucose and sucrose in honey and fruit juices. Fructose, along with glucose are the monosaccharides found in disaccharide, sucrose.

Fructose is classified as a monosaccharide, the most important ketose sugar, a hexose, and is a reducing sugar.

An older common name for fructose is levulose, after its levorotatory property of rotating plane polarized light to the left (in contrast to glucose which is dextrorotatory).

Bees gather nectar from flowers which contains sucrose. They then use an enzyme to hydrolyze or break apart the sucrose into its component parts of glucose and fructose.

High Fructose Corn Syrup


Click for larger image 


Ring Structure for Fructose:

The chair form of fructose follows a similar pattern as that for glucose with a few exceptions. Since fructose has a ketone functional group, the ring closure occurs at carbon # 2. See the graphic on the left.

In the case of fructose a five membered ring is formed. The -OH on carbon #5 is converted into the ether linkage to close the ring with carbon #2. This makes a 5 member ring - four carbons and one oxygen.

Steps in the ring closure (hemiketal synthesis):
1. The electrons on the alcohol oxygen are used to bond the carbon #2 to make an ether (red oxygen atom).
2. The hydrogen (green) is transferred to the carbonyl oxygen (green) to make a new alcohol group (green).

The ring structure is written with the orientation depicted on the left for the monosaccharide and is consistent with the way the glucose is depicted.

Hemiketal Functional Group:

The anomeric carbon is the center of a hemiketal functional group. A carbon that has both an ether oxygen and an alcohol group (and is attached to two other carbons is a hemiketal.

Open graphic of hemiacetal/hemiketal in a new window

Which carbon in the structure on the left is the anomeric carbon?
Then check the answer from the drop down menu.

Compare Alpha and Beta Fructose:

The Beta position is defined as the -OH being on the same side of the ring as the C # 6. In the ring structure this results in a upwards projection for the -OH on carbon # 2.

The Alpha position is defined as the -OH being on the opposite side of the ring as the C # 6. In the ring structure this results in a downward projection for the -OH on carbon # 2.

The alpha and beta label is not applied to any other carbon - only the anomeric carbon, in this case # 2.

Open graphic of Alpha/Beta Fructose in a new window

Compare Alpha and Beta Fructose - Chime in new window




Click for larger image 

 

Compare Glucose and Fructose in the Chair Structures:

The six member ring and the position of the -OH group on the carbon (#4) identifies glucose from the -OH on C # 4 in a down projection in the Haworth structure).

Fructose is recognized by having a five member ring and having six carbons, a hexose.

Both glucose and fructose may be either alpha or beta on the anomeric carbon, so this is not distinctive between them

Compare Glucose and Fructose - Chime in new window

Quiz on Glucose and Fructose:

Write down your answers.
Then check the answers from the drop down menu.

Which structure is shown in the graphic - glucose or fructose?
Is the structure alpha or beta ?  
 Open graphic of Chair Structures in a new window

Which structure is shown in the graphic - glucose or fructose?
Is the structure alpha or beta ?  
 Open graphic of Chair Structures in a new window
 Open graphic of Haworth Structures in a new window