Question

What are considered as “good” leaving groups?

Answer 1

Hint: A leaving group is a molecular fragment that leaves the molecule with a pair of electrons in heterolytic bond cleavage. Leaving groups can be cations, anions or neutral molecules, but it is important to note that the leaving groups must be able to stabilize the additional electrons density that results from bond heterolysis.

Complete answer:
Leaving groups are said to be better when it is more stable, and weak bases are considered as “good” leaving groups. This is because when an acid ($H-A$) undergoes dissociation to give $H^{+}$ and $A^{-}$, the $A^{-}$ accepts a lone pair of electrons from the bond $H-A$.
The more stable $A^{-}$ is, the greater is the equilibrium constant which favors the dissociation to give $A^{-}$.
Now we can say that halide ions such as $I^{-},B{r}^{-},C{l}^{-}$, water, sulfonates such as $\rho$-toluenesulfonate and methanesulfonate are excellent leaving groups.

Note:
In general, weaker the base, better the leaving group. But we will never see alkanes or hydrogens as leaving groups because they are strongly basic anions and are very unstable.