Question

Show how aldoses and ketoses can form five- and six-membered rings.

Short answer

Aldoses are monosaccharides with carbonyl group as aldehyde in their structure. Example: glucose, ribose, and galactose.

Ketoses are monosaccharides, where the carbonyl group is a ketone. Example: fructose and ribulose.

Step-by-step solution

Introduction of aldose and ketose.

Aldoses are monosaccharides with carbonyl group as aldehyde in their structure. Example: glucose, ribose, and galactose.

Ketoses are monosaccharides, where the carbonyl group is a ketone. Example: fructose and ribulose.

Explanation for the formation of five- and six-membered rings.

In aldose and ketose, the hydroxyl group reacts with either aldehyde or ketone functional monosaccharide group to form cyclic hemiacetal and hemiketal.The Haworth projection conveniently represents the structure of these cyclic rings. In this, the heavier ring bond projects in front of the plain of paper while the lighter ring bond is projected behind it.

Explanation for the formation of five membered rings.

Referring to the figure, ketone forms five-membered ring. The hydroxyl group on fifth carbon is converted into ether bond to close the ring with carbon number 2. This makes five-membered ring with four carbon and one oxygen atom (Fructofuronose).

Explanation for the formation of six-membered rings.

Referring to the figure, aldose forms six-membered ring. The carboxyl group on the sixth carbon is joined to first carbon to close the ring. This makes six-membered ring with five carbon and one oxygen atom (Glucopyranose).