In aqueous solution, the basicity of ethyl-substituted aliphatic amines is determined by a combination of the inductive effect ($+I$), steric hindrance, and hydration energy (solvation effect) of the conjugate cation. The experimental basicity order is:

$$2^\circ \gt 3^\circ \gt 1^\circ \gt \text{NH}_3$$ $$(\text{C}_2\text{H}_5)_2\text{NH} \gt (\text{C}_2\text{H}_5)_3\text{N} \gt \text{C}_2\text{H}_5\text{NH}_2 \gt \text{NH}_3$$

Diethylamine ($(\text{C}_2\text{H}_5)_2\text{NH}$) achieves the optimal balance between $+I$ electronic stabilization and hydration of the cation, making it the most basic.

Relationship between basicity and $\text{p}K_b$:

$$\text{p}K_b = -\log K_b$$

The weaker the base, the lower is its $K_b$ and the higher is its $\text{p}K_b$ value.

• Aliphatic amines ($\text{C}_2\text{H}_5\text{NH}_2$, $(\text{C}_2\text{H}_5)_2\text{NH}$) are strong bases with $\text{p}K_b \approx 3.0 - 3.29$.
• Aniline ($\text{C}_6\text{H}_5\text{NH}_2$) is an aromatic primary amine where the lone pair of nitrogen is delocalized over the benzene ring through resonance, making it significantly less available for protonation. Thus, aniline is the weakest base among the given options and possesses the highest $\text{p}K_b$ value ($\text{p}K_b = 9.38$).

The higher the base dissociation constant ($K_b$), the stronger the base:

• Diethylamine ($(\text{C}_2\text{H}_5)_2\text{NH}$) is a secondary aliphatic amine. In aqueous solution, secondary amines exhibit the highest basic strength due to the optimum combination of inductive effect and hydration of the conjugate cation.
• Experimental $K_b$ values:
  • $(\text{C}_2\text{H}_5)_2\text{NH}$: $K_b = 5.6 \times 10^{-4}$ ($\text{p}K_b = 3.0$)

• $\text{C}_2\text{H}_5\text{NH}_2$: $K_b = 4.7 \times 10^{-4}$ ($\text{p}K_b = 3.29$)
• $\text{CH}_3\text{NH}_2$: $K_b = 4.4 \times 10^{-4}$ ($\text{p}K_b = 3.38$)
• $\text{C}_6\text{H}_5\text{N}(\text{CH}_3)_2$: $K_b = 1.15 \times 10^{-9}$ ($\text{p}K_b = 8.92$)