Question

The order of basic strength of methyl substituted amines and $NH _{3}$ in aqueous solution is

• A. $\left( CH _{3}\right)_{2} NH > CH _{3} NH _{2}>\left( CH _{3}\right)_{3} N > NH _{3}$
• B. $\left( CH _{3}\right)_{3} N >\left( CH _{3}\right)_{2} NH > CH _{3} NH _{2}> NH _{3}$
• C. $\left( CH _{3}\right)_{2} NH > CH _{3} NH _{2}> NH _{3}>\left( CH _{3}\right)_{3} N$
• D. $NH _{3}> CH _{3} NH _{2}>\left( CH _{3}\right)_{2} NH >\left( CH _{3}\right)_{3} N$

Answer 1

Answer: (A)

Three effects are most useful to decide basic character of simple aliphatic amines in aqueous solution.

(i) Inductive effect, which favour tertiary amine to be more basic due to greater $+I$ effect of three alkyl groups directly attached to nitrogen.

(ii) Solution effect, which favour primary amine to be more basic due to greater solvation of its cation.

(iii) Steric factor Which causes tertiary amine to be less basic due to steric hindrance to approach of proton towards nitrogen. On the basis of combining effect of the above said parameters the correct order is

$ \left( CH _{3}\right)_{2}> CH _{3} NH _{2}>\left( CH _{3}\right)_{3} N > NH _{3}$