The correct order of basic strength in aqueous solution for the bases NH3, H2N-NH2, CH3CH2NH2, (CH3CH2)2NH and…

Step 1 — Hydrazine vs ammonia

$\ce{H2N-NH2}$: the $\ce{-NH2}$ group exerts $-I$ effect on the other N → weaker base than $\ce{NH3}$.

Step 2 — Ethylamines in aqueous solution

In aqueous solution, basic strength depends on inductive effect AND solvation:

• $(\ce{Et})_2\ce{NH}$: 2 ethyl groups (induction) + good solvation → highest
• $\ce{EtNH2}$: 1 ethyl group + good solvation → second
• $(\ce{Et})_3\ce{N}$: 3 ethyl groups (induction) but poor solvation (steric) → lower than expected
• $\ce{NH3}$: no alkyl groups → lowest among amines

Step 3 — Complete order:

$\ce{H2N-NH2} < \ce{NH3} < (\ce{Et})_3\ce{N} < \ce{EtNH2} < (\ce{Et})_2\ce{NH}$ → Option (c)

Key Points to Remember:

• Hydrazine < $\ce{NH3}$ (electron-withdrawing $\ce{-NH2}$ group)
• In aqueous solution: $(\ce{Et})_2\ce{NH}$ > $\ce{EtNH2}$ > $(\ce{Et})_3\ce{N}$ > $\ce{NH3}$
• Triethylamine: steric hindrance reduces solvation → lower than diethylamine