Question

Amines have a lone pair of electrons on the nitrogen atom, due to which they behave as Lewis bases. Greater the value of \( K_b \) or smaller the value of \( pK_b \), stronger is the base. Amines are more basic than alcohols, ethers, esters, etc. The basic character of aliphatic amines should increase with the increase of alkyl substitution. However, it does not occur in a regular manner, as a secondary aliphatic amine is unexpectedly more basic than a tertiary amine in aqueous solutions. Aromatic amines are weaker bases than ammonia and aliphatic amines. Electron-releasing groups such as \( -CH_3 \), \( -NH_2 \), etc., increase the basicity, while electron-withdrawing substituents such as \( -NO_2 \), \( -CN \), halogens, etc., decrease the basicity of amines. The effect of these substituents is more pronounced at the para-position than at the meta-position.

Arrange the following in increasing order of their basic character. Give reason: 

Hint:

The basicity of amines is determined by the electron-donating or electron-withdrawing effect of the substituents attached to the nitrogen. Electron-donating groups increase basicity, while electron-withdrawing groups decrease it.

Answer 1

To arrange the given amines in increasing order of their basic character, we need to consider the electronic effects of the substituents on the aromatic ring and their positions relative to the amino group (\(-NH_2\)).

Given Compounds:
1. Aniline (\(C_6H_5NH_2\)) 2. p-Nitroaniline (\(NO_2-C_6H_4-NH_2\)) 3. p-Toluidine (\(CH_3-C_6H_4-NH_2\))

Key Concepts:

1. Basicity of Amines: The lone pair on nitrogen determines basicity. Electron-donating groups (\(-CH_3\)) increase electron density on nitrogen, enhancing basicity, while electron-withdrawing groups (\(-NO_2\)) decrease it.
2. Positional Effects: The effect of substituents is stronger at the para position than at the meta position due to resonance and inductive effects.

Analysis of Given Amines:

1. p-Nitroaniline (\(NO_2-C_6H_4-NH_2\)): The \(-NO_2\) group is strongly electron-withdrawing through both resonance and inductive effects, significantly reducing the electron density on nitrogen. This makes it the weakest base among the three.
2. Aniline (\(C_6H_5NH_2\)): The \(-NH_2\) group in aniline is less basic than aliphatic amines due to resonance delocalization of the lone pair into the benzene ring. It has no additional substituents to alter its basicity further.
3. p-Toluidine (\(CH_3-C_6H_4-NH_2\)): The \(-CH_3\) group is electron-donating (+I effect), increasing electron density on nitrogen. This makes it the strongest base among the three.

Increasing Order of Basicity:
\[ \text{p-Nitroaniline} < \text{Aniline} < \text{p-Toluidine} \]

Reason:
The \(-NO_2\) group destabilizes the protonated form (ammonium ion) by withdrawing electron density, while the \(-CH_3\) group stabilizes it by donating electron density. Aniline, with no substituents, has intermediate basicity.

Final Answer:
The increasing order of basic character is: \[ \boxed{NO_2-C_6H_4-NH_2 \ < \ C_6H_5NH_2 \ < \ CH_3-C_6H_4-NH_2} \]