Question

The number of localised lone pair in purine is (A) 4 (B) 3 (C) 2 (D) 1

Short answer

In the purine molecule, there are 2 localized lone pairs, which are located on N7 and N9 nitrogen atoms. These lone pairs are not involved in resonance or bond formation within the molecule. Therefore, the correct answer is (C) 2.

Step-by-step solution

Draw the structure of purine

To find the number of localized lone pairs in the purine molecule, we must first draw its structure as follows: Purine is a bicyclic heterocyclic organic compound composed of a pyrimidine ring fused to an imidazole ring. The structure of purine can be represented like this: ``` N N \ / C = C - N || || / \ / \ N C H C \ / \ / \ H N H \ || | | C = C - C | | || H N N || H ```

Identify the localized lone pairs

Now, we will identify the atoms in the purine structure that have lone pairs. In purine, there are four nitrogen (N) atoms: N1, N3, N7 and N9. Each of these nitrogen atoms has a lone pair of electrons. N1: One lone pair N3: One lone pair N7: One lone pair N9: One lone pair Localized lone pairs are those that are not involved in resonance or bond formation within the molecule. In purine, the lone pairs on N1 and N3 are involved in resonance with the adjacent double bonds and thus are not localized. The lone pairs on N7 and N9 are not involved in resonance and are considered localized.

Count the localized lone pairs

Now that we have identified the localized lone pairs in the purine structure, we can count the number of these lone pairs: Localized lone pairs = N7 (1) + N9 (1) = 2

Match the answer to the given options

We have found that there are 2 localized lone pairs in the purine molecule, so the correct answer is: (C) 2

Key concepts

Localized Lone Pairs

Localized lone pairs are electrons that do not participate in bonding or resonance. In molecules like purine, which has a unique bicyclic structure, determining which lone pairs are localized requires understanding their interaction with the molecule.

In the case of purine, we examine the nitrogen atoms since they possess lone pairs. Purine consists of four nitrogen atoms:

• N1
• N3
• N7
• N9

Only the lone pairs on N7 and N9 are localized because they do not engage in resonance with nearby bonds. They're simply available electrons on these nitrogens. In contrast, the lone pairs on N1 and N3 are delocalized due to their involvement in resonance with the adjacent double bonds. This distinction is crucial for understanding electron behavior in large, complex molecules.

Nitrogen Atoms in Heterocycles

Nitrogen atoms in heterocyclic compounds like purine play a critical role in both the structure and reactivity of the molecule. Heterocycles refer to rings containing atoms of at least two different elements as members of its ring(s), and nitrogen is a common heteroatom in these systems.

In purine, the nitrogen atoms contribute to the aromatic character of the rings, influencing the stability and chemical properties of the structure. Specifically:

• N1 and N3 participate in the pyrimidine ring.
• N7 and N9 are part of the imidazole ring.

Each nitrogen has its lone pair of electrons, which can affect the molecule's electron distribution, especially when considering involvement in resonance. Understanding these positions and roles is essential in predicting chemical interactions and behavior in biological and chemical contexts.

Resonance in Organic Molecules

Resonance is a fundamental concept in organic chemistry that describes the delocalization of electrons within a molecule. This phenomenon provides additional stability to the molecule because the charge distribution is spread out over a larger area.

In purine, the resonance primarily involves certain lone pairs of the nitrogen atoms. For N1 and N3, their lone pairs participate in resonance with adjacent carbon atoms, contributing to multiple resonance structures. This delocalization allows purine to maintain its aromaticity, a stabilizing feature that is crucial for many biological processes, especially in the context of DNA and RNA.

By understanding resonance, chemists can predict how molecules like purine will react in different chemical environments, as well as anticipate reactivity and interaction with other molecular structures. It's a key tool in the toolkit of both synthetic and biological chemistry.