Question

Ribose is an example of: -
(a) Aldopentose
(b) Ketopentose
(c) Aldohexose
(d) Ketohexose

Answer 1

Hint: Ribose is a five-carbon membered ring. It contains aldehyde groups. It is a product obtained by the hydrolysis of DNA or RNA. It is found in the furanose form.

Complete step by step answer:
Let us first study monosaccharides and the structure of pentose and hexose.
Monosaccharides are the simplest carbohydrates that cannot be hydrolyzed into smaller molecules. They contain 3 to 7 carbon atoms. There are twenty monosaccharides which occur in nature. These are of two types:
Aldose: monosaccharides containing an aldehyde (-CHO) group are called aldose. Since the aldehyde group is monovalent, therefore, it is always present at one end of the carbon chain, i.e., at$$C_1$$.
Ketose: monosaccharides containing a keto ( >C=O) group are called ketose. Since the keto group is divalent, it can be present anywhere along the carbon chain. However, in all the naturally occurring ketose, the keto group is always present at a carbon atom next to the terminal carbon, i.e., at$$C_2$$.
Aldoses and ketose are further classified as trioses, tetroses, pentoses, hexoses, heptoses, etc. according to as they contain three, four, five, six, seven, etc.
Ribose is a sugar that is obtained by the hydrolysis of the RNA molecule.
It has five carbon atoms in the ring.
It contains aldehyde groups. Hence, it is aldopentose.
The structure of ribose is:

Cyclic hemiacetal of an aldopentose or a cyclic hemiketal of a ketohexose is called the Furanose ring.
There are four carbon and one oxygen atom with the anomeric carbon to the right of the oxygen in the furanose ring of ribose. The substituent on the highest numbered chiral carbon atom will point downwards out of the plane in L-configuration and will point upward in the D-configuration. The furanose structure of ribose is given below:

 Note: Two sugars are obtained by hydrolysis of DNA and RNA, these are deoxyribose and ribose, both these sugars are found in the furanose form so don’t get confused between them. They just differ by only one alcohol group. Ribose has one extra alcohol group.