The decreasing order of basicity of the following amines in aqueous solution is

(A) Ethylamine
(B) N-Ethylethanamine
(C) N,N-Diethylethanamine
(D) Benzamine

Choose the correct answer from the options given below:

(B), (C), (A), (D)

The correct answer is Option (2) → (B), (C), (A), (D)

Compounds with proper names

• (A) Ethylamine: CH₃CH₂NH₂ (1° aliphatic amine)
• (B) N-Ethylethanamine: (CH₃CH₂)₂NH (2° aliphatic amine)
• (C) N,N-Diethylethanamine: (CH₃CH₂)₃N (3° aliphatic amine)
• (D) Benzamine (Aniline): C₆H₅NH₂ (aromatic amine)

Determining the basicity of amines in aqueous solution is a bit more complex than in the gas phase because we have to balance three competing factors: inductive effect, solvation (hydration) effect, and steric hindrance.

Factors Influencing Basicity

1. Inductive Effect ($+I$): Alkyl groups are electron-donating. More alkyl groups should make the nitrogen more electron-rich and thus more basic.
2. Solvation Effect: In water, the protonated amine (cation) is stabilized by hydrogen bonding with water molecules. Smaller cations with more hydrogens (primary amines) are better solvated and stabilized than tertiary amines.
3. Steric Hindrance: Bulky alkyl groups can block the approach of water molecules or the proton itself.

Ranking the Amines

For ethyl-substituted amines in aqueous solution, the experimental order is a unique "middle-ground" result of these factors:

• (B) N-Ethylethanamine ($2^\circ$): Secondary amines strike the perfect balance between the $+I$ effect of two ethyl groups and sufficient solvation. They are the most basic.
• (C) N,N-Diethylethanamine ($3^\circ$): Despite having three electron-donating groups, the steric hindrance is high and solvation of the resulting cation is poor, making it less basic than the $2^\circ$ amine.
• (A) Ethylamine ($1^\circ$): This has the best solvation but only one $+I$ group, placing it below the $3^\circ$ amine (specifically for the ethyl series).
• (D) Benzamine (Aniline): The lone pair on nitrogen is delocalized into the benzene ring via resonance, making it significantly less available for donation. Aromatic amines are almost always the least basic in this comparison.

The Resulting Order

The decreasing order of basicity for ethyl-substituted amines in aqueous solution is:

Secondary ($2^\circ$) > Tertiary ($3^\circ$) > Primary ($1^\circ$) > Aromatic

Note: If these were methyl groups instead of ethyl, the order would change to $2^\circ > 1^\circ > 3^\circ$. The larger size of the ethyl group pushes the tertiary amine ahead of the primary one.