Question

Pyridine is a base with $K_b$ value:

• A. $1.7 \times 10^{-9}$
• B. $2.3 \times 10^{-12}$
• C. $3.2 \times 10^{-6}$
• D. $3.8 \times 10^{-7}$

Hint:

Remember: Pyridine is a weak base with a $K_b$ around $10^{-9}$ — significantly less basic than aliphatic amines due to aromatic delocalization effects.

Answer 1

The Correct Option is A

- Pyridine is a heterocyclic aromatic organic compound with the molecular formula \ce{C5H5N}. It consists of a six-membered ring with five carbon atoms and one nitrogen atom.
- The nitrogen in pyridine is sp$^2$-hybridized and its lone pair of electrons is not involved in the aromatic $\pi$-system. This makes the lone pair available for protonation, hence making pyridine act as a Lewis base.
- The basicity of a compound is often expressed using the base dissociation constant ($K_b$), which measures the extent to which a base can accept a proton.
- Pyridine has a $K_b$ value of $1.7 \times 10^{-9}$, indicating it is a weak base. This low $K_b$ corresponds to a p$K_b$ of approximately 8.77.
- The low basicity compared to aliphatic amines is due to the electron-withdrawing effect of the aromatic ring, which reduces electron density on the nitrogen atom.
- Among the options, the correct $K_b$ value of pyridine is $1.7 \times 10^{-9}$.