The key difference between polyhydroxy aldehydes and polyhydroxy ketone is that polyhydroxy aldehydes contain an aldehyde group with many –OH groups whereas polyhydroxy ketones contain a ketone group with many –OH groups.
The terms polyhydroxy aldehydes and polyhydroxy ketones describe the structures of carbohydrates. Both these compounds have a number of hydroxyl groups (-OH groups) and carbonyl groups (either aldehydes or ketone groups).
CONTENTS
1. Overview and Key Difference
2. What are Polyhydroxy Aldehydes
3. What are Polyhydroxy Ketones
4. Side by Side Comparison – Polyhydroxy Aldehydes vs Polyhydroxy Ketone in Tabular Form
5. Summary
What are Polyhydroxy Aldehydes?
Polyhydroxy aldehydes are organic compounds containing many hydroxyl groups (-OH) and an aldehyde group (-C(=O)H). Here, the carbonyl group occurs at the terminal carbon atom. We call them “aldoses”. Furthermore, this carbon atom of the aldehyde group can combine with one of the hydroxyl groups (when the compound is in an aqueous solution) to form a cyclic compound that we call it “hemiacetal”.
Figure 01: Different Aldoses
Monosaccharides are good examples of polyhydroxy aldehydes. These are simple sugars and they act as the building blocks of carbohydrates such as disaccharides and polysaccharides.
What are Polyhydroxy Ketones?
Polyhydroxy ketones are organic compounds containing many hydroxyl groups and a ketone group (-C(=O)-). Here, the carbonyl group occurs at the second carbon atom from the terminal of the molecule. We call these compounds “ketoses”. Same as aldoses, these compounds also can combine with a hydroxyl group of the same molecule (removing a water molecule) to form a cyclic compound, which is hemiketal.
Some monosaccharides occur as ketoses. The simplest ketose is dihydroxyacetone. Moreover, it has three carbon atoms, and the carbonyl group is at the second/middle carbon atom. All the ketose monosaccharides are reducing sugars.
Figure 02: Different Ketoses
We can differentiate between aldoses and ketoses by Seliwanoff’s test in which we heat the sample of monosaccharides in the presence of an acid and resorcinol. The basis of this test is dehydration of the molecules. The dehydration occurs quickly in ketoses and in aldoses it occurs very slowly. Here, ketoses give a dark red colour while aldoses give a pink colour of the reaction mixture.
What is the Difference Between Polyhydroxy Aldehydes and Polyhydroxy Ketone?
Polyhydroxy aldehydes are organic compounds containing many hydroxyl groups (-OH) and an aldehyde group (-C(=O)H) while Polyhydroxy ketones are organic compounds containing many hydroxyl groups and a ketone group (-C(=O)-). So, the key difference between polyhydroxy aldehydes and polyhydroxy ketone is their functional group: aldehyde group or ketone group.
However, both these compounds have carbonyl groups. But, in polyhydroxy aldehydes, the carbonyl group occurs at the terminal of the molecule while, in polyhydroxy ketones, it occurs at the second carbon atom from one terminal. So, this is also a difference between polyhydroxy aldehydes and polyhydroxy ketone. Further, both these compounds can form cyclic compounds via combining the functional group with one of the hydroxyl groups, removing a water molecule. Here, the cyclic compound that forms from polyhydroxy aldehyde is as “hemiacetal” while for polyhydroxy ketones it is “hemiketal”.
Below is a summary of the difference between polyhydroxy aldehydes and polyhydroxy ketone.
Summary – Polyhydroxy Aldehydes vs Polyhydroxy Ketone
In summary, polyhydroxy aldehydes are organic compounds containing many hydroxyl groups (-OH) and an aldehyde group (-C(=O)H) while Polyhydroxy ketones are organic compounds containing many hydroxyl groups and a ketone group (-C(=O)-). Therefore, the key difference between polyhydroxy aldehydes and polyhydroxy ketone is the functional group: aldehyde group or ketone group. However, both these compounds have carbonyl groups. In polyhydroxy aldehydes, the carbonyl group occurs at the terminal of the molecule whereas in polyhydroxy ketones it occurs at the second carbon atom from one terminal.
Reference:
1. “25.10: Carbohydrates.” Chemistry LibreTexts, Libretexts, 23 Feb. 2019, Available here.
Image Courtesy:
1. “Family tree aldoses” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia
2. “Family tree of d-ketoses” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia