Ring Closure

All of the sugar molecules that we've worked with so far are referred to as monosaccharides. One very important reaction of monosaccharides is ring closure. Ring closure, essentially, involves an internal or intramolecular addition reaction.

Structures for five monosaccharides (galactose, glucose, fructose, ribose, deoxyribose). [67039.jpg]

The OH group from the next to last carbon atom adds to the carbon-oxygen double bond in the carbonyl group. The hydrogen from the OH bonds to the oxygen from the carbonyl group and the oxygen from the OH bonds to the carbon from the carbonyl group.

Structure of D-glucose showing location of intramolecular addition. [67040.jpg]

The process is illustrated for you in your workbook (in Example 9) and I will try to show you how that happens using a model. When you are in the lab, be sure to practice manipulating a glucose model to close it into its ring form.

Diagram showing the first steps in the ring closure of glucose (Example 16) [67041.jpg]

First, let's get oriented. The carbonyl group including carbon #1 is near my hand. Carbon #6 is at the top of this picture.

Ring closure will involve the OH group on carbon #5 and the carbonyl group (carbon #1). The molecule bends and shifts until these two groups are in proximity to one another so that the OH group can attack the double bond.

Model of glucose oriented for ring closure. [67042.jpg]

When the OH group attacks the double bond, the pi bond (in the C=O) breaks and the hydrogen from the OH group attaches to the oxygen that was in the carbonyl to make a new OH group. The oxygen from the OH attaches to the carbon in the carbonyl group to make a new bond that closes the ring. Notice that when the double bond breaks, the carbon from the carbonyl group is now bonded to four atoms instead of just three. Therefore, it is tetrahedral and asymmetric.

žOžH

žC = O
˝
H

žO H
˝ ˝
žCžO
˝
H

In this picture, you're looking at the top of the ring. Notice that the newly formed OH group on the newly asymmetric carbon #1 is, essentially, up. Also, carbon #6 is up. This is called the beta (b) arrangement.

Model of glucose in beta ring form. [67043.jpg]

If, when the ring closing had occurred, the newly formed OH had gone down instead of up, then this would be referred to as the alpha (a) arrangement for this newly formed OH group.

Model of glucose in alpha ring form. [67044.jpg]

That orientation can be represented in diagrams like these (from Example 9 in your workbook) in which we show the beta (b) form as having the newly formed OH group from the first carbon atom going up (lower left). The #2 and #3 carbon atoms are presumed to be closer to you and the #5 carbon is on the back side of the ring. Up is defined, essentially, as being the direction in which the sixth carbon is pointing.

In the diagram to the lower right we have the alpha (a) form, in which the newly formed OH points down.

Structures of alpha and beta D-glucose (from Example 16). [67045.jpg]

Quite often, to simplify the diagrams, the carbons that are shown in the ring are left out and the rings are simplified to look like this. Each corner represents a carbon, and then the sixth carbon is the one that's shown pointing up. On the left is a-D-glucose in which the OH on C#1 is down (or opposite the direction from the ring as is C#6). On the right is b-D-glucose in which the newly formed OH on C#1 is up (or on the same side of the ring as is C#6).

Structures of alpha and beta D-glucose. [67046.jpg]

I would like to point out something about diagrams such as these for the ring forms of glucose. These kinds of diagrams are written in a way that makes it convenient to represent where the OH's are in the ring form. These are not the actual shapes of the molecules. A diagram such as this one does a better job of representing the shape of the molecule, but it's much less convenient to write.

Sometimes the structures are drawn with heavier lines to depict that they are bonding atoms that are closer to you than the rest of the molecule. This is an attempt to indicate a third dimension in the molecular structures.

Structure of glucose with wedged bonds. [ring.h1.jpg]

Sometimes the structures are drawn like this to more accurately represent the three-dimensional arrangement of all the parts of the molecule. Which structural representation is used depends on what features of the molecule are being emphasized. Keep in mind that all of these are legitimate representations, but each has its disadvantages as well as its advantages.

Chair structure of glucose. [ring.h2.jpg]

Having worked with models before, you are very much aware that none of these representations actually do full justice to the actual shape of the molecule and the flexibility that the molecule has.

When you are in the lab, be sure to use the diagram in your workbook (Example 9) to construct a model of the open-chain form of D-glucose and then convert that to the ring form and identify it as being either the alpha (a) or the beta (b) form. After you've done that, have an instructor check your work.

Ring Conversion

Glucose molecules spend most of their time in the ring forms rather than in the open chain form with which we started our discussion. Actually, I believe that crystalline glucose is the a-D-glucose form. In solution, both of these forms are present with about two-thirds of the molecules in the beta (b) form and about one-third in the alpha (a) form. In solution, the alpha (a) and beta (b) forms are constantly turning back and forth into one another by moving through the open-chain form.

The alpha (a) chain can open with the hydrogen of the C#1-OH going back to the ring-closing oxygen. That opens the ring and reforms the double bond between C#1 and its oxygen. The ring can then close again into the beta (b) configuration. These three forms are in equilibrium in solution and the molecules will make the change many, many times per second.

Equation showing the interconversions between the alpha, open and beta forms. [67050.jpg]

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E-mail instructor: Sue Eggling

Clackamas Community College
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