The key difference between aldohexose and ketohexose is that aldohexoses contain a formyl group, whereas ketohexoses contain a ketone group.

Both aldohexoses and ketohexoses are organic compounds which can be classified as monosaccharides containing six carbon atoms. These two types are subclasses of hexose, and they differ from each other according to the functional group they bear.

CONTENTS

1. Overview and Key Difference
2. What is Aldohexose 
3. What is Ketohexose
4. Side by Side Comparison – Aldohexose vs Ketohexose in Tabular Form
5. Summary

What is Aldohexose?

Aldohexoses are organic compounds containing six carbon atoms and a formyl functional group. These compounds contain carbonyl carbon atoms. The carbonyl carbon atom of an aldohexose is in position 1 of the six-membered linear structure of the monosaccharide. This carbonyl carbon forms a formyl group which is abbreviated by “-CHO“. This is a special case of aldoses. An aldose is any sugar molecule containing a formyl functional group.

An aldohexose is a derivative of an aldehyde having the structure H–C(=O)–(CHOH)5–H. A common example is glucose. In its linear form, an aldohexose contains four chiral centers; thus, there are 16 possible aldohexose stereoisomers, comprising 8 pairs of enantiomers.

Figure 01: D-Glucose

D-Glucose is a common example of an aldohexose. It contains six carbon atoms in a chain, and there is a formyl group at position 1 of the linear glucose structure.

What is Ketohexose?

Ketohexoses are organic compounds containing six carbon atoms and a ketone functional group. It is a derivative of ketose where a ketose is any sugar molecule containing a ketone functional group. The ketohexoses contain six carbon atoms and a carbonyl carbon at the middle of the carbon chain. Therefore, this carbon atom is essentially bound to two other carbon atoms (not bound to any hydrogen atoms) and an oxygen atom (via a double bond). In biological applications, the most common and important ketohexose is fructose.

In its linear structure, a ketohexose contains the carbonyl carbon center at the position 2 or 3. Therefore, in the linear structure, a ketohexose contains three chiral centers and eight possible stereoisomers. This means four pairs of enantiomers are possible for this compound.

Figure 02: D-Fructose

D-fructose is a common example of ketohexoses. This compound contains six carbon atoms in a chain and the carbonyl carbon is at the position 2 of the chain.

What is the Difference Between Aldohexose and Ketohexose?

Aldohexoses and ketohexoses are two subclasses of hexoses. A hexose is a monosaccharide compound containing six carbon atoms. The key difference between aldohexose and ketohexose is that aldohexoses contain a formyl group whereas ketohexoses contain a ketone group.

Moreover, when considering the position of the carbonyl carbon in these compounds, aldohexose contains a carbonyl carbon at position 1 while ketohexose contains a carbonyl carbon at position 2 or 3. However, ketohexoses containing the carbonyl carbon at position 3 is uncommon. A common example of an aldohexose is D-glucose while D-fructose is an example of a ketohexose that occurs in biological systems.

The following infographic summarizes the difference between aldohexose and ketohexose.

Summary – Aldohexose vs Ketohexose

A hexose is a monosaccharide compound containing six carbon atoms. There are two major forms of hexoses as aldohexoses and ketohexoses, depending on the functional group these compounds contain. The key difference between aldohexose and ketohexose is that aldohexoses contain a formyl group, whereas ketohexoses contain a ketone group.

Reference:

1. “14.3: Important Hexoses.” Chemistry LibreTexts, Libretexts, 23 Feb. 2019, Available here.
2. “Hexose.” Wikipedia, Wikimedia Foundation, 2 June 2020, Available here.

Image Courtesy:

1. “DGlucose Fischer” By Christopher King – Own work (CC BY-SA 3.0) via Commons Wikimedia

2. “D-Fructose” By Ayacop – Own work, Public Domain) via Commons Wikimedia

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