Problem 72 - 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) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2}-\mathrm{NH}_{2}\) (c) \(\mathrm{p}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) (d) \(\mathrm{m}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\)

Step-by-Step Solution

Answer

The strongest base is (b) \(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2-\mathrm{NH}_2\).

Step 1: Assess basic structure of each compound

For compound (a) \(\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2\), it is an aniline derivative where the amine group is directly attached to the benzene ring. For compound (b) \(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2-\mathrm{NH}_2\), the amine group is attached to a benzyl group, which is not directly connected to the benzene ring. Compounds (c) and (d) both have an \(\mathrm{NO}_2\) group, which is an electron-withdrawing group, at the para and meta positions relative to the amine group, respectively.

Step 2: Determine effect of electron withdrawing groups

Compounds (c) \(\mathrm{p}-\mathrm{NO}_2-\mathrm{C}_6\mathrm{H}_4\mathrm{NH}_2\) and (d) \(\mathrm{m}-\mathrm{NO}_2-\mathrm{C}_6\mathrm{H}_4\mathrm{NH}_2\) both have a nitro group that withdraws electrons from the amine group, reducing its basicity. The para position in (c) withdraws more than the meta position in (d), due to resonance effects.

Step 3: Compare basicity of amine groups

The amine group in compound (b) is located on a benzyl group, which is an electron-releasing group from the benzene due to hyperconjugation in the \(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\) group. This enhances the electron density on the nitrogen, increasing its basicity compared to direct attachment as in compound (a) and less affected by deactivating groups as in (c), and (d).

Step 4: Identify the strongest base

Since the electron releasing properties make the benzylamine group in \(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2-\mathrm{NH}_2\) (compound b) more basic than the other structures, compound (b) is the strongest base.

Key Concepts

Amines

Amines are a fundamental category of organic compounds characterized by the presence of a nitrogen atom bonded to one or more alkyl or aromatic groups. The general formula for an amine is RNH₂, R₂NH, or R₃N, depending on the number of carbon-containing substituents attached to the nitrogen.

• Primary amines: Characterized by one alkyl or aromatic group attached to the nitrogen (RNH₂).
• Secondary amines: Have two groups attached to nitrogen (R₂NH).
• Tertiary amines: Feature three carbon-containing groups bonded to the nitrogen (R₃N).

Amines are structurally similar to ammonia and behave as bases. The presence of the lone pair of electrons on the nitrogen atom makes amines capable of accepting protons, a key aspect of their basicity.
This capability contributes to their function in reactions and their importance in organic chemistry.

Electron-withdrawing groups

Electron-withdrawing groups (EWGs) are substituents attached to a molecule that pull electron density away from other parts of the molecule. They achieve this through inductive or resonance effects, making nearby atoms or groups less electron-rich. Common EWGs include groups such as -NO₂ (nitro), -CN (cyano), and carbonyl-containing groups like -COOH (carboxyl).
The presence of EWGs in a molecule can greatly decrease the basicity of amines. This occurs because the electron-withdrawing group diminishes the availability of the lone pair electrons on the nitrogen, which are necessary for accepting protons.

• Nitro groups are particularly strong EWGs, affecting molecular properties by resonance and inductive effects.
• These groups play a critical role in regulating chemical reactivity and stability, especially in aromatic compounds.

Resonance effects

In organic chemistry, resonance refers to the delocalization of electrons across multiple atoms or bonds, which can stabilize structures and modify chemical properties. Aromatic systems, such as those found in benzene and its derivatives, are often subject to significant resonance effects.
For example, in para-nitroaniline, the nitro group can participate in resonance, drawing electron density from the ring and the amine, decreasing the basicity of the nitrogen:

• Resonance can result in the electron pair on nitrogen being spread out into the aromatic system, reducing its availability to accept a proton.
• This diminishes the molecule’s basicity due to less localized control over its electron cloud.

Thus, resonance effects can significantly influence the chemical behavior and acid-base properties of aromatic amines.

Basicity in organic compounds

Basicity in organic compounds, particularly amines, is primarily influenced by the electron density around the nitrogen atom. The ability of the nitrogen to donate its lone pair of electrons determines the strength of a base. Several factors affect basicity, including:

• Substituent Effects: Electron-donating groups enhance basicity by increasing electron density on the nitrogen, while electron-withdrawing groups decrease it.
• Resonance and Inductive Effects: These can either stabilize extra electron density or remove it, altering the nitrogen’s proton affinity.
• Hybridization: Compounds with sp³ hybridized nitrogen are typically more basic than those with sp², due to increased availability of electrons.

In the given compounds, benzylamine (C_6H_5CH_2-NH_2) is more basic because the benzyl group enhances electron density on nitrogen. Meanwhile, amines attached directly to an aromatic ring or those with EWGs like nitro groups, exhibit reduced basicity.
Basicity plays a crucial role in many organic reactions, influencing molecular properties and reactivity.