Question

Amines have a lone pair of electrons on nitrogen atom due to which they behave as Lewis base. 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. But 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 at the p₊than at m₋ position.
(a) Arrange the following in the increasing order of their basic character. Give reason:
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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

Step 1: Understanding the Factors Influencing Basicity
The basicity of amines is mainly determined by the electron density on the nitrogen atom. The more electron-donating the substituents, the more basic the amine will be, as they enhance the availability of the nitrogen's lone pair for protonation. On the other hand, electron-withdrawing groups decrease the electron density on nitrogen, reducing its basicity. Electron-donating groups (like \(–NH_2\) and \(–CH_3\)) increase the basicity by raising the electron density on nitrogen.
Electron-withdrawing groups (such as \(–NO_2\) and \(–CN\)) decrease the basicity as they pull electron density away from nitrogen, making the lone pair less available to accept a proton.
\bigskip Step 2: Analyzing the Substituents
Now, let’s analyze the substituents attached to the nitrogen atoms in the given compounds:
\(–NH_2\) is an amine group, an electron-donating group. It will increase the electron density on nitrogen, making the amine more basic.
\(–NO_2\) is a strong electron-withdrawing group. It pulls electron density away from nitrogen, reducing the availability of the lone pair and lowering the basicity.
\(–CH_3\) is also an electron-donating group, but weaker than \(–NH_2\). Therefore, it increases basicity, but not as strongly as \(–NH_2\).
\bigskip Step 3: Arranging in Order of Basicity
Now, let’s rank the compounds based on the strength of their electron-donating or electron-withdrawing effects:
\(–NO_2\) (electron-withdrawing) will have the lowest basicity.
\(–NO_2\) (when positioned para) will be less basic than \(–NH_2\) due to the electron-withdrawing nature of \(–NO_2\).
\(–NH_2\) will be the most basic because it is a strong electron-donating group.
\(–CH_3\) is a mild electron-donating group, so it will be more basic than \(–NO_2\), but less basic than \(–NH_2\).
Therefore, the increasing order of basicity is: \[ \text{NO}_2<\text{NO}_2<\text{CH}_3<\text{NH}_2 \]