CBSE Class 12 Chemistry: Amines (Marking Scheme Solution)

(i) Decreasing order of basic strength in aqueous solution:

• Step 1 (Governing Principle): [0.25 Mark] Basic strength in aqueous solution depends on the combined effect of inductive effect ($+I$), solvation effect (H-bonding), and steric hindrance. For ethyl amines, the order is determined by the balance of these factors.
• Step 2 (Working): [0.25 Mark] The order is $(\text{C}_2\text{H}_5)_2\text{NH} > \text{C}_2\text{H}_5\text{NH}_2 > (\text{C}_2\text{H}_5)_3\text{N}$. The secondary amine is most basic due to maximum stability of the conjugate acid via solvation and $+I$ effect, while the tertiary amine is less basic due to steric hindrance to solvation.
• Step 3 (Final Answer): [0.0 Mark] $(\text{C}_2\text{H}_5)_2\text{NH} > \text{C}_2\text{H}_5\text{NH}_2 > (\text{C}_2\text{H}_5)_3\text{N}$

(ii) Increasing order of solubility in water:

• Step 1 (Governing Principle): [0.25 Mark] Solubility depends on the extent of intermolecular hydrogen bonding with water molecules. Higher molar mass and hydrophobic hydrocarbon groups decrease solubility.
• Step 2 (Working): [0.25 Mark] $\text{C}_6\text{H}_5\text{NH}_2$ (Aniline) has a large hydrophobic phenyl group, making it least soluble. $\text{C}_2\text{H}_5\text{NH}_2$ (Primary) has more H-bonding sites than $(\text{C}_2\text{H}_5)_2\text{NH}$ (Secondary) relative to its size.
• Step 3 (Final Answer): [0.0 Mark] $\text{C}_6\text{H}_5\text{NH}_2 < (\text{C}_2\text{H}_5)_2\text{NH} < \text{C}_2\text{H}_5\text{NH}_2$

(iii) Decreasing order of their $pK_b$ values:

• Step 1 (Governing Principle): [0.25 Mark] $pK_b$ is inversely proportional to basic strength ($pK_b = -\log K_b$). A stronger base has a lower $pK_b$ value.
• Step 2 (Working): [0.25 Mark] Basic strength order: $\text{C}_2\text{H}_5\text{NH}_2 > \text{NH}_3 > \text{C}_6\text{H}_5\text{NH}_2$. Since $pK_b$ is the inverse, we reverse the order.
• Step 3 (Final Answer): [0.0 Mark] $\text{C}_6\text{H}_5\text{NH}_2 > \text{NH}_3 > \text{C}_2\text{H}_5\text{NH}_2$

💡 EL10 Concept Insight: Think of basic strength like a "magnet" for protons; the more the electrons are pushed toward the nitrogen (by $+I$ effect) and the easier it is for water to surround it, the stronger the base. $pK_b$ is just a "scorecard" where a lower number means a stronger base, so the weakest base gets the highest $pK_