Formulas and Terminology

Let's take a look at a few simple sugars to see what kinds of formulas these compounds actually have.

By looking at their names, you can see that quite often, simple sugars will have "-ose" endings on their names. In these formulas (also shown in Example 2 in your workbook) you can see that each contains quite a few hydroxyl groups. Also notice that there is an aldehyde group in the first and second formulas, and a ketone group in the third one. So, these compounds are aldehydes or in this case, a ketone, with several OH's attached.

Structural formulas of D-galactose, D-glucose and D-fructose. [67001.jpg]

This leads to a different definition of carbohydrates. That is to say that carbohydrates are polyhydroxy aldehydes and ketones. We also have to throw in this kicker, "or their derivatives," and you'll see why before too long.

When referring to a carbohydrate that has an aldehyde group, we call it an aldose. The first two compounds above are aldoses. The third compound is referred to as a ketose, because it is a polyhydroxy ketone.

In addition, we can count the number of carbons and incorporate that into a name. Each of the compounds above have six carbons, so all of these compounds can be referred to as being hexoses. The first is also an aldose because it has an aldehyde group. We can combine both of those terms and call it an aldohexose. Glucose is also an aldohexose and the fructose is a ketohexose.

It's not necessary that carbohydrates have six carbon atoms, it's also quite possible for them to have different numbers. For example, here (and in Example 2 in your workbook) we have two very important simple sugars, ribose and 2-deoxyribose. These are found in RNA and DNA which we'll be talking about in a succeeding lesson. Both of these only have five carbons, so both of these would be considered pentoses. Because they are also aldoses, they can also be called aldopentoses.

Structures of ribose and 2-deoxyribose. [67002.jpg]

Another important point to notice in looking at these two formulas is that it's not necessary to have an oxygen attached to each carbon atom. It is necessary that you have more than one hydroxy group in order to be a carbohydrate, but not necessarily an oxygen for every carbon atom.

Keep in mind that all of these terms are categories rather than the names of specific compounds.

# of Cs		aldehyde	ketone
3	triose	aldotriose	ketotriose
4	tetrose	aldotetrose	ketotetrose
5	pentose	aldopentose	ketopentose
6	hexose	aldohexose	ketohexose
7	heptose	aldoheptose	ketoheptose

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E-mail instructor: Sue Eggling

Clackamas Community College
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