Question

Which is the best leaving group?
(A) ${{N}_{2}}$
(B) $O{{H}^{-}}$
(C) $N{{H}_{2}}^{-}$
(D) $C{{H}_{3}}CO{{O}^{-}}$

Answer 1

In general, the leaving group is the one which can leave a molecule without causing any harm to the molecule and also to itself. The best leaving group will be the one which can self-detach itself without any external help.

Complete step-by-step answer: Let us first study about the leaving groups;
Leaving group-
In order for any group to leave from a molecule, it must be able to accept electrons (generally). Hence, weak bases constitute leaving groups.
They can be either anions, cations or neutral molecules. The reason why the leaving group accepts the lone pair of the electrons as the bond between the leaving group and neighbour atom (usually carbon) is broken.
Now, the stability of the lone pair of electrons defines the stability and viability of the leaving group.
Thus, from the above given groups;
${{N}_{2}}$ is the best leaving group as it is self-detachable and stable when left from the molecule.

Note: It is a saying in general- “The leaving group is a nucleophile acting in reverse” and is in fact true in every possible case.
Also, note as the electronegativity increases the ability of the leaving group increases.