Arrange the following compounds in decreasing order of their basic character.

Options

A. `I gt II gt III`
B. `II gt III gt I`
C. `III gt II gt I`
D. `I gt III gt II`

Answer

C

Explanation

The basicity of the compounds is compared based on the availability of lone pair. In compound (I):
, Pyrrole is an aromatic compound but both the lone pair electrons contribute to the `pi`-electron system. The lone pair, thus, takes part in resonance and is delocalized over the ring. Hence, it is less available for a proton and the basic character is the least.
In compound (II):
Pyridine is an aromatic system with six-`pi`-electrons, so the ring is planar and lone pair is held `sp^2` hybridized orbital of nitrogen. The increased s character of this orbital as compared to the `sp^3` orbital in piperidine means that the lone pair electrons are held closer to the nitrogen and hence less available for protonation. Hence the basicity is less than that of piperidine.
In compound (III):
, In piperidine, nitrogen is `sp^3` hybridized, so the lone pair is held in `sp^3` orbital. It is more basic than pyridine where the lone pair resides in `sp^2` orbital. Hence, the order is `III gt II gt I`