Question

For the compounds the order of basicity is :

• A. IV > I > III > II
• B. III > I > IV > II
• C. II > I > III > IV
• D. I > III > II > IV

Answer 1

Answer: (D)

I > III > II > IV

Piperidine ($I$) is the most basic (conjugate acid pKa = 11.2). The lone pair is in an $s{p}^3$ hybrid orbital and there is no resonance.

In pyridine ($II$)(conjugate acid pKa = 5.2) the $N$ lone pair is in an $s{p}^2$ hybrid orbital but is not part of the electron aromatic system of ring nor is it involved in any resonance (perpendicular). The $s{p}^2$ hybrid is smaller than the $s{p}^3$ hybrid, so there is a stronger attraction to the nucleus, so less basic.

In morpholine ($III$) , the $N$ lone pair The lone pair is in an $s{p}^3$ hybrid orbital and there is no resonance. But the electron withdrawing effect of $O$ from ring which makes them less available for donation .

In pyrrole is a very weak base (conjugate acid pKa = -4). The $N$ lone pair is involved in the electron aromatic system. Protonation will destroy the aromaticity of the electron aromatic system and this makes it an unfavourable process. Therefore the correct order of basicity is $I>III>II>IV$