Problem 127 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

A nucleoside on hydrolysis gives: [KCET 2004] (a) a heterocyclic base and orthophosphoric acid (b) an aldopentose, a heterocyclic base and orthophosphoric acid (c) an aldopentose and a heterocyclic base (d) an aldopentose and orthophosphoric acid

Step-by-Step Solution

Answer

(c) an aldopentose and a heterocyclic base.

Step 1: Understand Nucleosides

A nucleoside is composed of a nitrogenous base (heterocyclic base) linked to a sugar molecule, typically a pentose sugar like ribose or deoxyribose. Nucleosides do not contain phosphate groups, which differentiates them from nucleotides.

Step 2: Definition of Hydrolysis

Hydrolysis is a chemical reaction involving the breaking of a bond in a molecule using water. When nucleosides are hydrolyzed, they break down into their individual components.

Step 3: Hydrolysis of Nucleosides

On hydrolysis, a nucleoside will yield its individual components: a nitrogenous base and a pentose sugar. There will be no phosphate released in the process, as nucleosides do not contain phosphate groups.

Step 4: Identify Components Released

The hydrolysis of nucleosides releases an aldopentose sugar and a nitrogenous base but not orthophosphoric acid, as there is no phosphate group present in nucleosides.

Key Concepts

Nitrogenous Base

Nucleosides are made up of two primary components, one of them being a nitrogenous base. These nitrogenous bases are organic molecules that contain nitrogen and have a ring structure. They are crucial as the building blocks for nucleic acids, which are fundamental to DNA and RNA structures. The most commonly known nitrogenous bases are adenine, guanine, cytosine, thymine (found only in DNA), and uracil (found only in RNA). Each base has a unique structure which allows it to pair specifically with another base through hydrogen bonding. For example, adenine typically pairs with thymine (or uracil in RNA), and guanine pairs with cytosine. This pairing is what facilitates the encoding of genetic information. So when a nucleoside is broken down during hydrolysis, it releases a nitrogenous base, allowing us to appreciate its structure and function as a separate entity.

Aldopentose Sugar

The other key component of nucleosides is the aldopentose sugar. Aldopentoses are a type of sugar that contain five carbon atoms, and an aldehyde group is located at one end. In nucleosides, the sugar can either be ribose or deoxyribose. These sugars form the central part of the nucleoside structure and link to the nitrogenous base via a glycosidic bond. - **Ribose** sugar is found in RNA and contains an OH group on its second carbon. - **Deoxyribose** sugar is found in DNA and lacks one oxygen atom compared to ribose, having only a hydrogen atom at this position. When nucleosides undergo hydrolysis, the aldopentose sugar is released, providing the necessary structure that supports various biochemical reactions and functional processes within cells.

Chemical Reaction

Hydrolysis is a critical type of chemical reaction that plays a significant role in breaking down complex molecules into simpler components. In the context of nucleoside hydrolysis, this involves the addition of a water molecule to rupture the bond between the nitrogenous base and the aldopentose sugar. During this reaction, the water molecule interacts with the glycosidic bond, facilitating its cleavage and ultimately resulting in the separation of the nucleoside into its two original components: the nitrogenous base and the aldopentose sugar. - **No Involvement of Phosphate Groups**: Since nucleosides do not contain phosphate groups, orthophosphoric acid is not released during this type of reaction. This characteristic distinguishes nucleosides from nucleotides, which include phosphate groups. Understanding hydrolysis as a chemical reaction gives insight into biochemical pathways and cellular processes that rely on the cleavage and formation of these complex molecules. This reaction is not only pivotal in biological systems but is also a foundational concept in chemistry education.