281.

Explain: In aqueous solution order is Et₂NH,>Et₃N > EtNH₂.

In aqueous solution, the K_b order is Et₂NH > Et₃N > EtNH₂
This may be explained by the following factors:
(a)    Steric factor: The size of the alkyl group is more than that of hydrogen and, therefore, it hinders the attack of acid on the amine and therefore, basic strength decreases. Now crossing of alkyl group increases from primary to tertiary amines. As a result, their basic strength decreases. This is called steric hindrance. According to this effect, alkyl amines should be least basic.

(b)    Solvation of ions: When amines are dissolved in water, they undergo hydration through hydrogen bonding. The protonated amines form hydrogen bonds with water molecules and release energy called hydration energy. Now, greater the extent of hydrogen bonding in protonated amine, more will be its stabilization and consequently, greater will be the tendency of amine to change into a cation and greater will be the strength of the corresponding amine. As can be seen below, the hydration of protonated amine due to hydrogen bonding decreases as:
Et NH₃⁺ > Et₂ NH₂⁺ > Et₃NH⁺

The tertiary ammonium ion is less hydrated than secondary ammonium ion, which is less hydrated than primary ammonium ion. Thus, tertiary amines have less tendency to form ammonium ion and consequently are less basic.
As a consequence of combining effects of inductive effect and solvation, the secondary amines are the strongest bases among amines and basic strength varies as:
Et₂NH > Et₃ N > Et NH₂