The pKb value of N,N-diethylethanamine is 3.25. Which of the followin…

The correct answer is Option (4) → 3.29

The smaller the pK_b, the stronger the base. For the ethyl-substituted amines in water, the order of basic strength is set by the +I effect, solvation of the ammonium ion and steric hindrance together:

    \( \displaystyle \ce{(C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3} \)

• Lower limit: ethanamine is a weaker base than N,N-diethylethanamine, so its pK_b is more than 3.25.

• Upper limit: ethanamine is a stronger base than ammonia, so its pK_b is less than 4.75.

• Result: the only value between 3.25 and 4.75 is 3.29.

NCERT (2026-27 edition):
9.6 Chemical Reactions – “… Larger the value of K_b or smaller the value of pK_b, stronger is the base. The pK_b values of few amines are given in Table 9.3.
pK_b value of ammonia is 4.75. Aliphatic amines are stronger bases than ammonia due to +I effect of alkyl groups leading to high electron density on the nitrogen atom.”

Table 9.3: pK_b Values of Amines in Aqueous Phase

“… The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows:
    (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃
    (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃”