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Reactions of Sugars

>> Parts of this equation/concept include:

A -	Ring Closure
B -	Polymerization

Two reactions of sugars are ring closure and polymerization. Both of these reactions are condensation reactions where water is lost.

In this reaction the aldehyde or ketone of the sugar reacts with an alcohol (hydroxyl) at the other end of the sugar. The alcohol group that reacts is the next-to-last one at the other end of the chain. By tradition, the end alcohol group and the carbon it is bonded to are directed up. The oxygen of the next-to-last hydroxyl group becomes the connecting oxygen of the ether. The hydrogen of this group bonds with the alcohol of the aldehyde or ketone, creating another hydroxyl group. This hydroxyl group can be oriented either up or down. The alcohol groups in the middle should remain on the same side of the carbon as in the linear version of the structure.

>> Example 1

Draw the hemiacetal form of the following sugars.

Solution:

The -isomer is shown, with the alcohol group on the same side as the CH₂OH group. The -isomer is also possible. It is

rotate and number the carbons

Either structure is correct. The upper one is the -isomer, the lower is the -isomer.

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The alcohol part of the hemiacetal (in the ring form of the sugar) reacts with an alcohol in another sugar (also in the ring form).

>> Example 2

When the following sugars react, what is the disaccharide formed?

Solution:

There are actually several possible correct answers to this question. Both sugars have a hemiacetal group and that group must participate in the polymerization.

The hemiacetal of the leftmost group is:

The alcohol of the hemiacetal will react with one of the alcohols from the other sugar, creating an ether link. For example,

On the other hand, the hemiacetal of the rightmost sugar is:

So if this group reacts with an alcohol from the sugar on the left, some possible disaccharides are:

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