Question

What functional groups are present in carbohydrates?

Answer 1

Carbohydrates can contain hydroxyl (alcohol) groups, ethers, aldehydes and/or ketones.

Carbohydrates are chains (or polymers) of basic sugar molecules such as glucose, fructose and galactose. In order to see which functional groups are present in carbohydrates, we must look at the functional groups present in the more basic building blocks. Saccharides - and by extension carbohydrates - are composed of only three atoms: carbon, hydrogen and oxygen. The structure for one of the most common saccharides, glucose, is shown here.

Here we can identify multiple hydroxyl (alcohol) functional groups and one aldehyde functional group. Alcohols are characterized by `-OH` and aldehydes by `CH=O`. This basic structure accounts for two of the four functional groups. Turning to another basic saccharide, fructose, we can identify a ketone functional group, as shown in the figure below.

Here, because the `C=O` bond is bridged by two carbons instead of one carbon and one hydrogen, it is a ketone functional group.

Lastly, we must consider functional groups that arise through the linking of saccharides. Below is the structure of a disaccharide carbohydrate consisting of glucose and fructose. Notice that here both glucose and fructose are drawn in their cyclic ring form.

The two saccharides are linked through an oxygen atom. This link is called a glycosidic bond. Since the glycosidic bond has the form, `R-O-R`, it is an ether.