Question

In the following compounds, the order of basicity :

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

Answer 1

The correct option is D $I>III>II>IV$

Two of the factors which influence the strength of a base are:

-the ease with which the lone pair picks up a hydrogen ion,

-the stability of the ions being formed.

I) Methyl Piperidine (I) is the most basic. The lone pair is in an $s{p}^3$ hybrid orbital and there is no resonance with $+I$ effect of methyl.

III) In (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 $C=O$ from ring which makes them less available for donation .

IV) This is a very weak base. 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.

In (II) 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 sp3 hybrid, so there is a stronger attraction to the nucleus, so less basic than I and III.

Hence, order of basicity is,

$I>III>II>IV$