Which of the following have the lowest value of pK_b in their aqueous…

The correct answer is Option (1) → \((CH_3)_2NH\)

The lower the pK_b, the stronger the base. In aqueous solution the basic strength of methyl amines is decided by three factors together: the +I effect of the methyl groups (more groups, more electron density on N), solvation of the protonated amine by hydrogen bonding with water (more N–H, better solvation) and steric hindrance to that solvation. For methyl groups the balance gives

    \( \ce{(CH3)2NH} \) > \( \ce{CH3NH2} \) > \( \ce{(CH3)3N} \) > \( \ce{NH3} \)

pK_b: 3.27, 3.38, 4.22 and 4.75 respectively. So dimethylamine has the lowest pK_b.

Why the other options are incorrect

• Option (2) (CH₃)₃N — Incorrect. It has the largest +I effect, but its ion \( \ce{(CH3)3NH^{+}} \) has only one N–H for hydrogen bonding and is poorly solvated, so in water it is weaker than both 1° and 2° methylamines (pK_b 4.22).

• Option (3) CH₃NH₂ — Incorrect. It is well solvated, but with one methyl group its +I effect is smaller than that of dimethylamine (pK_b 3.38).

• Option (4) NH₃ — Incorrect. It has no electron-releasing alkyl group and is the weakest base of the four (pK_b 4.75).

NCERT (2026-27 edition):
Alkanamines versus Ammonia – …