Read the following passage and answer questions.

Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base. Thus, the order of basicity of aliphatic amines should be: primary > secondary > tertiary, which is opposite to the inductive effect based order. Secondly, when the alkyl group is small, like $-CH_3$ group, there is no staric hinderance to H-bonding. In case the alkyl group is bigger than $-CH_3$ group, there will be steric hinderance to H-bonding. Therefore, the change of nature of the alkyl group, e.g., from $-CH_3$ to $-C_2H_5$ results in change of the order of basic strength. Thus, there is a subtle interplay of the inductive effect, solvation effect and steric hinderance of the alkyl group which decides the basic strength of alkyl amines in the aqueous medium.

Which base among the following is least basic in aqueous medium?

$NH_3$

The correct answer is Option (4) → $NH_3$
In aqueous medium, the basic strength of amines depends on the inductive effect of alkyl groups and solvation of the ammonium ion.

Alkyl groups show a +I effect, donating electron density to nitrogen and increasing its ability to accept a proton. Therefore, amines with alkyl groups are generally more basic than ammonia.

Order of basicity: Secondary amine > Primary amine > Tertiary amine > NH₃

For the given compounds:

• OPTION 1 - (CH₃)₂NH (Dimethylamine) – Secondary amine; strong +I effect and good solvation → very basic.

• OPTION  2 - CH₃NH₂ (Methylamine) – Primary amine; +I effect increases basicity compared to ammonia.

• OPTION 3 - (CH₃)₃N (Trimethylamine) – Tertiary amine; +I effect present but steric hindrance reduces solvation, yet still more basic than NH₃.

• OPTION 4 - NH₃ (Ammonia) – No alkyl groups, so no +I effect → least basic.

Thus, the order of basicity in aqueous medium is:

(CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃

Therefore, the least basic base is NH₃.