Which is the correct order of basic strength of ethylamines in aqueous solution?

A $(C X_{2} H X_{5}) X_{3} N$ > $(C X_{2} H X_{5}) X_{2} NH$ > $C X_{2} H X_{5} NH X_{2}$
B $(C X_{2} H X_{5}) X_{2} NH$ > $(C X_{2} H X_{5}) X_{3} N$ > $C X_{2} H X_{5} NH X_{2}$
C $C X_{2} H X_{5} NH X_{2}$ > $(C X_{2} H X_{5}) X_{2} NH$ > $(C X_{2} H X_{5}) X_{3} N$
D $(C X_{2} H X_{5}) X_{2} NH$ > $C X_{2} H X_{5} NH X_{2}$ > $(C X_{2} H X_{5}) X_{3} N$

Answer: B

For ethylamines in aqueous solution the standard order is secondary > tertiary > primary. The ethyl groups'

+ I

effect is stronger than that of methyl, so

(C X_{2} H X_{5}) X_{3} N

overtakes

C X_{2} H X_{5} NH X_{2}

even though its cation is less solvated; steric hindrance and poor solvation keep it below the secondary amine.