Problem 93 - A Complete Resource Book in Chemistry for JEE Main

Question

Cc1ccc(N)cc1 Cc1ccc(N)cc1 I II III IV (a) IV \(>\… # Arrange the following in decreasing order of basicity: Nc1cccc(N)c1 N#Cc1ccc(N)cc1 Cc1ccc(N)cc1 I II III IV (a) IV \)>\mathrm{I}>\mathrm{III}>\mathrm{II}\( (b) \)\mathrm{IV}>\mathrm{I}>\mathrm{II}>\mathrm{III}\( (c) IV \)>\mathrm{III}>\mathrm{I}>\mathrm{II}\( (d) \)\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}$

Step-by-Step Solution

Answer

Structure III (Cc1ccc(N)cc1) is more basic than Structure II (N#Cc1ccc(N)cc1). Arrange as IV > I > II > III.

Step 1: Understand Basicity in Organic Chemistry

In organic chemistry, basicity refers to the ability of a molecule to accept protons (H+). Amines and their derivatives are often basic, with basicity influenced by electronic factors like the presence of electron-withdrawing or electron-donating groups, and resonance in aromatic compounds.

Step 2: Identify Chemical Structures

Look at the chemical structures given as SMILES strings: 1. Nc1cccc(N)c1 - a benzene ring with two amino groups at different positions. 2. N#Cc1ccc(N)cc1 - a benzene ring with a cyano (-CN) group and an amino group. 3. Cc1ccc(N)cc1 (or compound I) - a benzene ring with a methyl group (-CH3) and an amino group.

Step 3: Analyze Electron-Donating/Withdrawing Effects

Identify the electron-withdrawing or donating groups attached to each benzene ring: - In Structure 1, the amino group is an electron-donating group, increasing basicity. - In Structure 2, the cyano group is electron-withdrawing, decreasing basicity. - In Structure 3, the methyl group is slightly electron-donating, increasing basicity slightly.

Step 4: Evaluate Resonance Effects

Consider resonance stabilization in each compound: - In Structure 1, having amino groups allows for resonance stabilization with nitrogen's lone pairs contributing to increased basicity. - Structure 2 has a cyano group, reducing resonance stabilization with the amine nitrogen, decreasing basicity. - Structure 3 has slight resonance contribution due to the methyl group, which stabilizes through hyperconjugation.

Step 5: Arrange Basicity from Most to Least

Based on analysis, Structure 1 with a primary amino group and Structure 3 with a methyl group attached are both more basic than Structure 2 with a cyano group. This corresponds to option (b): IV > I > II > III.

Key Concepts

Proton Acceptors

Proton acceptors in organic chemistry are molecules or ions that have the ability to accept a hydrogen ion, denoted as \( H^+ \). These typically possess lone pairs of electrons that can form a bond with the proton. The strength of a proton acceptor is closely linked to its basicity.

In a basic molecule, the presence of nitrogen, such as in amines, greatly affects its ability to accept protons. Amines are commonly found among proton acceptors, thanks to the lone pair of electrons on the nitrogen atom. When a molecule accepts a proton, it can neutralize a positive charge, resulting in a more stable structure.

This capacity to accept protons makes such molecules valuable in many chemical reactions, including neutralization reactions between acids and bases. Understanding how different atoms and groups in a molecule influence its basicity is crucial for predicting chemical behavior.

Electron-Donating Groups

Electron-donating groups (EDGs) are parts of a molecule that donate electron density towards other parts of the molecule. This donation occurs through inductive or resonance effects. Common examples of EDGs include alkyl groups like methyl, and functional groups like amino groups.

When an electron-donating group is present in a molecule, it enhances the molecule's ability to accept protons, thereby increasing its basicity.

• The methyl group, often considered an EDG, provides extra electron density to stabilize positive charges through what is known as hyperconjugation, although its effect is modest compared to stronger EDGs.
• The amino group, with its lone pair of electrons, is a strong electron donor. It allows for resonance that increases the molecule's electron density, thereby enhancing basicity.

The impact of EDGs on a molecule's basicity depends on their position and nature, playing a crucial role in how the molecule interacts within chemical reactions.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) remove electron density from a molecule, typically making it less basic. They achieve this effect through inductive or resonance withdrawal.

Common EWGs include groups like cyano (-CN) and carbonyl groups, which withdraw electron density towards themselves. This can lower the electron density available on other parts of the molecule, such as a nitrogen atom in an amine group, thereby reducing its ability to accept protons.

• The presence of an EWG can significantly decrease basicity, as seen in the molecules from the exercise. A cyano group is known for strong electron withdrawal, drastically reducing the basicity of the attached amine.

Understanding how EWGs influence molecular basicity is integral for predicting reactions and behaviors in organic chemistry.

Resonance Effects

Resonance involves the delocalization of electrons across a molecule, allowing it to stabilize through multiple contributing structures. When considering basicity, resonance effects can significantly alter a molecule's ability to accept a proton.

Resonance can either increase or decrease basicity:

• When electrons can be delocalized towards a basic center like an amine, this can enhance basicity by stabilizing the added positive charge upon protonation.
• In contrast, if resonance allows electron density to be withdrawn from the base center, it reduces basicity. This occurs when electron-withdrawing resonance structures dominate.

For example, in an aromatic compound with an amino group, resonance can stabilize the lone pair on nitrogen, making it more available to accept protons, hence increasing basicity.

Aromatic Compounds

Aromatic compounds are cyclic molecules with a unique electronic configuration that results in an enhanced stability. They are governed by Huckel's rule, which states that a molecule must have \(4n + 2\) pi electrons to be considered aromatic.

The presence of substituents on an aromatic ring can affect its chemical properties significantly:

• Substituents like amino groups that donate electron density can increase the aromatic compound's basicity by making the nitrogen's lone pair more available for protonation.
• In contrast, substituents that withdraw electron density, like nitriles, can decrease the basicity of the aromatics.

Aromaticity often adds an additional layer of complexity to the study of organic molecules but provides a remarkable stability that influences how these compounds behave both chemically and in terms of their chemical reactivity.