Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following, the strongest base is (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (b) \(\mathrm{p}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) (c) \(\mathrm{m}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C} \mathrm{H}_{2} \mathrm{NH}_{2}\)

Step-by-Step Solution

Answer

The strongest base is (d) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C} \mathrm{H}_{2} \mathrm{NH}_{2} \).

Step 1: Understanding Basicity

The basicity of an amine is determined by its ability to donate an electron pair. The inductive effect, resonance, and the presence of electron-withdrawing or electron-donating groups influence this ability.

Step 2: Analyze Each Compound

Review each option to determine the presence of electron-withdrawing or electron-donating groups and how they affect electron pair availability on the nitrogen atom: - **(a) \\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \):** Aniline, where nitrogen is attached to a benzene ring, and resonance reduces basicity. - **(b) \\( \mathrm{p}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2} \):** The para-nitro group is a strong electron-withdrawing group, reducing electron density and basicity. - **(c) \\( \mathrm{m}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2} \):** The meta-nitro group still withdraws electrons, reducing basicity, but less than the para position. - **(d) \\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C} \mathrm{H}_{2} \mathrm{NH}_{2} \):** A benzylamine, where the nitrogen is not directly attached to the benzene ring, reducing resonance effects and allowing greater availability of the electron pair.

Step 3: Determining the Strongest Base

Identify which compound has the highest availability of nitrogen's lone pair for donation: - Compound (d) \\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C} \mathrm{H}_{2} \mathrm{NH}_{2} \) has no electron-withdrawing groups attached directly to the nitrogen, and the lone pair is not delocalized into the benzene ring, making it more basic compared to other options.

Key Concepts

Inductive Effect

The inductive effect in amines refers to the shifting of electron density in a molecule due to the electronegativity of atoms or groups attached to it. When we talk about the inductive effect in the context of amines, we're looking at how attached groups can either donate or withdraw electrons through sigma bonds, significantly affecting the basicity.

• **Electron-Donating Groups**: These groups, like alkyl chains, push electrons towards the nitrogen atom, increasing the electronic density on the nitrogen and enhancing its ability to donate an electron pair. With more electron density, the amine's basicity usually increases.
• **Electron-Withdrawing Groups**: Such as halogens or nitro groups, pull electron density away from the nitrogen atom. This reduction in electron density makes the nitrogen less willing to donate its lone pair, thereby decreasing basicity.

Understanding the inductive effect is crucial when analyzing the basicity of different amines because it influences how readily the nitrogen atom can donate its lone pair of electrons.

Resonance Effects in Amines

Resonance effects in amines are another significant factor that influences basicity. When we talk about resonance, we refer to the delocalization of electrons across a molecule. In amines, resonance can either diminish or increase the basicity based on how electrons move.

• **Aromatic Structures**: When an amine is part of an aromatic system like benzene, the lone pair on the nitrogen can participate in resonance with the pi system of the benzene ring. This electron delocalization reduces the availability of the nitrogen's lone pair for protonation, usually decreasing basicity compared to non-aromatic amines.
• **Non-Aromatic Systems**: For amines where the nitrogen is not part of an aromatic system, resonance effects are less pronounced, meaning the lone pair remains localized and more available to donate, thus increasing the amine's basicity.

By considering resonance, we can better predict the basicity of amines based on their structural context, as seen with aromatic versus non-aromatic amines.

Electron-withdrawing and Electron-donating Groups

Electron-withdrawing and electron-donating groups play a critical role in determining the basicity of amines through their impact on electron density at the nitrogen atom.

• **Electron-Donating Groups (EDGs)**: These groups increase electron density at the nitrogen atom, making it easier for the nitrogen atom to share its electron pair. Common examples of EDGs include alkyl groups that push electron density towards the nitrogen, enhancing the amine's basicity.
• **Electron-Withdrawing Groups (EWGs)**: Conversely, EWGs like nitro groups pull electrons away from the nitrogen atom. This makes the nitrogen less capable of donating its lone pair, thus reducing basicity. The effect of EWGs can also depend on their position relative to the nitrogen atom. For example, a para-positioned nitro group in aromatic amines can lead to a stronger electron-withdrawing effect than a meta-positioned one.

Understanding the influence of these groups is essential for predicting basicity. It’s the balance between these opposing forces that determines how easily the nitrogen in an amine can donate its lone pair, thus acting as a base.