Question

The increasing order to P_kb for the following compounds will be :

• (A) < (B) < (C)
• (C) < (A) < (B)
• (B) < (A) < (C)
• (B) < (C) < (A)

Hint:

A stronger base has a lower pK_b. Compare how well each conjugate acid is stabilised by resonance (guanidinium and amidinium ions) versus simple inductive effect.

Answer 1

The Correct Option is C

Compound B is a bicyclic guanidine (three N atoms on one carbon). Its conjugate acid, a guanidinium ion, is stabilised by delocalising the positive charge over three nitrogens, so B is the strongest base (lowest pK_b).
Compound A, NH₂-CH=NH, is an amidine: protonation on =NH gives an amidinium ion with charge delocalised over two nitrogens, so it is a strong base, but weaker than guanidine.
Compound C, CH₃NHCH₃, is an ordinary secondary amine whose conjugate acid is stabilised only by +I effect of two methyl groups (pK_b about 3.3), the weakest base here.
Basicity: B \(\gt\) A \(\gt\) C, so pK_b increases in the order \(B \lt A \lt C\).
Answer: Option 3: (B) \(\lt\) (A) \(\lt\) (C)