Question

What is the decreasing order of basicity of primary, secondary and tertiary ethyl substituted amines and $N{H}_3$ in aqueous phase?

• A. $N{H}_3>C_2{H}_{5}N{H}_2>(C_2{H}_5{)}_{2}N{H}_2>(C_2{H}_5{)}_{3}N$
  
• B. $(C_2{H}_5{)}_{3}N>(C_2{H}_5{)}_{2}NH>C_2{H}_{5}N{H}_2>N{H}_3$
  
• C. $(C_2{H}_5{)}_{2}NH>C_2{H}_{5}N{H}_2>(C_2{H}_5{)}_{3}N>N{H}_3$
  
• D. $(C_2{H}_5{)}_{2}NH>(C_2{H}_5{)}_{3}N>C_2{H}_{5}N{H}_2>N{H}_3$
  

Answer 1

The correct option is D $(C_2{H}_5{)}_{2}NH>(C_2{H}_5{)}_{3}N>C_2{H}_{5}N{H}_2>N{H}_3$


When $R=C{H}_3-C{H}_2-$ group in aliphatic amine, the basic strength in aqueous phase would be $2^{\circ }>3^{\circ }>1^{\circ }>N{H}_3$
The basicity of an amine in aqueous solution depends upon the stability of the ammonium cation, which in turn depends upon a combination of the following three factors:
(1) +I effect of the alkyl group
(2) Extent of hydrogen bonding with water molecules.
(3) Steric effect of the alkyl groups

In the aqueous phase, the substituted ammonium cations gets stabilized not only by (+I) effect of the alkyl group but also by solvation with water molecules.
Hence, (d) is the correct option.