Concept of Leaving Group
The SN2 reaction will be easier if it is carried out on a substrate bearing a good leaving group.
But..., what exactly is a good leaving group?
The leaving ability of a group is inversely related to its strength as a base. In short, a good leaving group must have a rather low basicity.
From the following table of possible leaving groups, which would you say are the best ones? Yes indeed, those with the highest pKb, i.e. the least basic ones.
Why is a given species more or less basic?
An atom (or group of atoms) able to accommodate a high electron density will be relatively weakly basic and hence a good leaving group. The factors stabilizing an excess of electron density on a chemical species are SIZE, ELECTRONEGATIVITY and DELOCALIZATION (RESONANCE).
| SIZE | ELECTRONEGATIVITY | DELOCALIZATION |
|---|---|---|
| The larger the atom, the more disperse and stable the excess electron density and the lower its basicity. This is the predominant factor in alkyl halides: larger size I− > Br− > Cl− > F− smaller size weaker base I− < Br− < Cl− < F− stronger base better leaving group I− > Br− > Cl− > F− worse leaving group | Comparing atoms belonging to the same row of the Periodic Table, the higher the electronegativity of the atom bearing the excess electron density, the lower its basicity and the better it is as a leaving group. In the halides, the size effect is predominant because from one halide to the next we must change rows. bad leaving group F− > HO− > NH2− > CH3− very bad leaving group | Resonance allows extensive delocalization of the excess electron density and its strong stabilization. That's why the sulfonates are extremely weak bases and thus excellent leaving groups.  |