Fructose
Home Up Glucose Optical Activity Ring Closure Reducing Sugars Disaccharides and Polysaccharides Fructose Sucrose

 

Fructose

Another monosaccharide that is of particular importance is fructose. It can be derived from glucose by rearranging the top portion of the molecule to be a ketose instead of an aldose. It's at least a two-step process in which hydrogens are moved up the C#1 and electrons are rearranged to form a new double bond. Note that the OH groups in fructose are in the same positions that they are in glucose, the difference being that the second carbon is a carbonyl group instead of having a hydroxyl group.

Structures of D-glucose and D-fructose. [67064.jpg]

Ring Closure

When fructose forms a ring, the process is in some ways similar to that for glucose and in some ways different. Again, it's the OH group on the next to the last carbon atom, which is the last asymmetric carbon atom, that loops up and attaches to the carbonyl group. But in this case, the carbonyl group is on the second carbon atom instead of on the first.

Structure and model of fructose with annotation indicating where ring closure occurs. [67066.jpg]
In this picture the model is being arranged so that the #5-OH is brought close to the double bond in the #2 carbonyl group. The #1C is to the lower right side and the #6C is to the upper left side, in preparation for the conventional orientation for ring sugars. 

67068.jpg (19775 bytes)
When ring closure occurs, the hydrogen from the #5-OH shifts over to create a new #2-OH group. In this form, the new OH goes down (it is almost hidden behind the #1C that sticks up) making this a-D-fructose. Remember that the conventional orientation is with the #6C in the up position.

Fructose has a ring that contains only five atoms instead of six. A sugar molecule with a 5-membered ring is sometimes referred to as a furanose.

Structure of fructose and model in alpha ring form. [67067.jpg]
The ring can also close into a beta form in which the new #2-OH is in the up position (same side of the ring as the #6C). This model shows b-D-fructose.

Structure of fructose and model in the eta ring form. [67069.jpg]

Lab Work

When you are in the lab, use a model kit to put together a model of fructose in the linear (open) form and have that checked by an instructor to make sure that you have the OH's in the right positions. Then use that to make both the alpha and the beta ring forms. Have an instructor check to make sure that you've used the right connections and have the new OH in the proper position.

 

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E-mail instructor: Sue Eggling

Clackamas Community College
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