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.

(a) Arrange the following in increasing order of their basic character. Give reason:
\[ \includegraphics[width=0.5\linewidth]{ch29i.png} \]

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 Affecting Basicity The basicity of amines is primarily determined by the electron density on the nitrogen atom. The more electron-donating the substituents, the more basic the amine will be, as they increase the availability of the lone pair on nitrogen for protonation. Conversely, electron-withdrawing groups decrease the electron density on nitrogen and thus reduce its basicity. Electron-donating groups (such as \( \text{-NH}_2 \) and \( \text{-CH}_3 \)) increase the basicity because they increase the electron density on nitrogen. Electron-withdrawing groups (such as \( \text{-NO}_2 \) and \( \text{-CN} \)) decrease the basicity because they pull electron density away from nitrogen, making the lone pair less available to accept a proton. 

Step 2: Analyzing the Substituents Let's break down the substituents attached to the nitrogen atoms in the given compounds: \( \text{-NH}_2 \) is an amine group, which is an electron-donating group. Thus, it will increase the electron density on nitrogen, making the amine more basic. \( \text{-NO}_2 \) is a strong electron-withdrawing group. It pulls electron density away from the nitrogen, thus decreasing the availability of the nitrogen's lone pair and reducing the basicity. \( \text{-CH}_3 \) is an electron-donating group, but not as strongly as \( \text{-NH}_2 \). Therefore, it increases basicity, though to a lesser extent compared to \( \text{-NH}_2 \). 

Thus, the increasing order of basicity is: \[ \text{-NO}_2 < \text{-NO}_2 (\text{para}) < \text{-NH}_2 < \text{-NH}_2 < \text{-CH}_3 \]