Chapter 1.4, Problem 1.14P

In the following compound, identify whether each lone pair is available to function as a base, and explain your choice:

Interpretation Introduction

Interpretation:

The availability of lone pair in the compound to function as a base has to be explained.

Concept Introduction:

If a lone pair is not a part of the ring means not taking part in resonance, then it will be available to function as a base.  It also can be explained in other way.  Like if a lone pair is present in a $p$ orbital rather than in a $s{p}^2$ hybrid orbital, there will exist a continuous overlapping of $p$ orbitals in the ring means that lone pair will take part in resonance.  Pyrrole and Pyridine can be taken as example.

Nitrogen in both pyrrole and pyridine has a lone pair and it is $s{p}^2$ hybridized.  But in Pyrrole, the lone pair is present in the third $p$ orbital whereas the lone pair in pyridine is present in one of its $s{p}^2$ orbitals, as the third $p$ orbital is involved in formation of $\pi$ bond with the adjacent carbon atom.  Hence, the lone pair in Pyrrole is not available to function as a base whereas in pyridine it is available to function as a base.

Explanation of Solution

By properly examining the structure, it can be seen that the numbered $1$ nitrogen which is attached to a methyl group is Pyrrole type nitrogen.  The lone pair on this nitrogen is a part of the ring as it is present in the third $p$ orbital of nitrogen atom which can takes part in resonance.  Thus it will not be available to function as a base.

The numbered $2$ nitrogen is Pyridine type nitrogen.  The lone pair on this nitrogen is not a part of the ring as it is present in one of the $s{p}^2$ hybrid orbitals of nitrogen atom.  Thus it is available to function as a base.