Question

Point out the correct decreasing order of $p K_{b}$ values of following amines $C _{2} H _{5} NH _{2}, C _{6} H _{5} NHCH _{3},\left( C _{2} H _{5}\right)_{2} NH$ and $C _{6} H _{5} NH _{2}$

• A. $\left( C _{2} H _{5}\right)_{2} NH > C _{2} H _{5} NH _{2}> C _{6} H _{5} NHCH _{3}> C _{6} H _{5} NH _{2}$
• B. $\left( C _{2} H _{5}\right)_{2} NH > C _{6} H _{5} NHCH _{3}> C _{6} H _{5} NH _{2}> C _{2} H _{5} NH _{2}$
• C. $C _{6} H _{5} NH _{2}> C _{6} H _{5} NHCH _{3}>\left( C _{2} H _{5}\right)_{2} NH > C _{2} H _{5} NH _{2}$
• D. $C _{2} H _{5} NH _{2}>\left( C _{2} H _{5}\right)_{2} NH > C _{6} H _{5} NHCH _{3}> C _{6} H _{5} NH _{2}$

Answer 1

Answer: (C)

Order of $p K_{b}$ values
$C _{6} H _{5} NH _{2}> C _{6} H _{5} NHCH _{3}>\left( C _{2} H _{5}\right)_{2} NH > C _{2} H _{5} NH _{2}$
This is simply because $+I$-effect of $C _{2} H _{5}$ group increases the basicity of amine while $-I$-effect of $C _{6} H _{5}$ group decreases the basicity of amine.