Modern nomenclature for sugars owes a great deal to 19th-century chemist Emil Fischer, a famous pioneer in the field of sugar chemistry. Among his numerous contributions is a system for categorizing sugars that remains in use today. One feature of this system is the distinction between two broad "families" of sugars called aldoses and ketoses. Glucose is an aldose, while the closely related sugar fructose is a ketose.
Aldoses and Ketoses
Simple five- and six-carbon sugars such as glucose, fructose and sorbose interconvert between linear and cyclic forms, a little like snakes that can either roll up into a coil or lie flat in a straight line. Although the cyclic form predominates in solution, the difference between an aldose and a ketose is more obvious in the linear form. Basically, an aldose has an oxygen double-bonded to a carbon at the end of a carbon chain, while a ketose has an oxygen double-bonded to a carbon with two carbons on either side of it.
Hemiacetals
With the exception of the aldose carbon, all the other five carbons in glucose each have an alcohol or oxygen-hydrogen group attached to them. When they convert from the linear form to the cyclic form, one of these alcohol groups will attack the aldose carbon to form a ring-shaped product called a hemiacetal. The resulting ring may be either five-membered or six-membered, depending on which alcohol group is involved. Glucose does not form three- or four-membered rings, because these would be too unstable.
Conversion
Although it's much easier to tell ketoses and aldoses apart if you look at the linear structure, you can still see the difference in the cyclic or ring-shaped configuration. When the sugar forms a ring, the oxygen atom in that ring is always bonded to the aldose or ketose carbon. If this aldose/ketose carbon has two carbon atoms attached to it, the molecule is a ketose. If it has only one carbon atom attached to it, the molecule is an aldose.
Practice
Learning to see the hemiacetal in the cyclic glucose molecule can be a little difficult at first, but once you've figured it out it will help you to remember that glucose is an aldose and to tell it apart from other sugars. These distinctions are especially important; sugars like glucose and fructose share the same molecular formula, but their structures are different. You can find drawings of the linear and cyclic structures of glucose under the link in the Resources section below.
References
- International Union of Pure and Applied Chemistry: Nomenclature of Carbohydrates
- "Organic Chemistry, Structure and Function"; Peter Vollhardt et al.; 2011
