Difference Between Ketose and Aldose

Ketose vs Aldose
 

Carbohydrates are a group of compounds that are defined as “polyhydroxy aldehydes and ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones.” Carbohydrates are the most abundant type of organic molecules on the earth. They are the source of chemical energy for living organisms. Not only this, they serve as important constituents of tissues. Carbohydrate can be again categorized into three as monosaccharide, disaccharides and polysaccharides. Monosaccharides are the simplest carbohydrate type. Monosaccharide has the formula of C_x(H₂O)_x. These cannot be hydrolyzed into simpler carbohydrates. They are sweet in taste. All monosaccharides are reducing sugars. Therefore, they give positive results with benedicts’ or Fehling’s reagents.  Monosaccharides are classified according to,

• The number of carbon atoms present in the molecule
• Whether they contain an aldehyde or keto group

Therefore, a monosaccharide with six carbon atoms is called a hexose. If there are five carbon atoms, then it is a pentose. These are further divided based on whether they have an aldehyde group or a ketone group.

Ketose

As described above, one way of classifying monosaccharides is using functional groups present in the molecule. Therefore, if the monosaccharide has a ketone group, it is called a ketose. For example, fructose is a ketose. It has the following structure.

Carbon with the ketone group always gets the number two.  When forming the ring, fructose forms a five member ring, which is a hemiketal. These monosaccharides are further divided based on the number of carbon atoms present. If there are five carbon atoms, it is known as a ketopentose and, if there are six carbon atoms, it is known as a ketohexose. Fructose, sorbose, tagtose and psicose are some ketohexoses. They have three chiral centers and, therefore, eight stereoisomers. Ribulose and xylulose are ketopentoses, and they only have two chiral centers.

Aldose

A monosaccharide with an aldehyde group is called as aldose. For example, glucose has an aldehyde group and it has the following structure.

The carbon atom with the aldehyde group is always assigned number one. And there are five other hydroxyl groups in the molecules. For monosaccharides, we can draw a linear structure like above or a cyclic structure. In solution, majority of the molecules are in cyclic structure. For example, when a cyclic structure is forming in glucose, the -OH on carbon 5 is converted into the ether linkage, to close the ring with the aldehyde group carbon 1. This forms a six member ring structure. The ring is also called a hemiacetal ring, due to the presence of carbon that has both an ether oxygen and an alcohol group. Other than glucose there are other molecules with six carbon atoms and an aldehyde group. Allose, altrose, glucose, mannose, gulose, idose, and talose are other types of aldohexoses. All these have four chiral centers, and thus have 16 stereoisomers.  Ribose, xylose, arabinose, and lyxose, are aldopentoses with five carbon atoms and an aldehyde group.

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