A leaving group is a nucleophile acting in reverse; that accepts a lone pair together the bond between it and also its ar (usually carbon because that our purposes) is broken.
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So what makes a good leaving group?
The “happier” and an ext stable the lone pair is, the far better a leaving group it will certainly be. The most predictive dominance for leave group ability is….
Good leaving groups are weak bases.
Think around the dissociation that an acid H–A to give H+ and also A– . The varieties A– is the conjugate base of HA. The accepts a pair of electrons from the H-A bond. It’s a base exhilaration in reverse.
The much more stable A- is, the higher the equilibrium consistent will be the favors dissociation to offer A- .
This transforms out to it is in the very meaning of acidity! The an adverse log the the equilibrium consistent is the acquainted pKa.
In various other words pKa is a direct measurement of just how “happy” and stable a lone pair of electrons is – the very an interpretation of what we should be looking for when trying come quantify leaving group ability.
So it have to be no surprise to discover that an extremely weak bases such as halide ions (I-, Br-, Cl-) water (OH2), and also sulfonates such as p-toluenesulfonate (OTs) and also methanesulfonate (OMs) are excellent leaving groups.
These space the conjugate bases of solid acids. (Recall that the more powerful the acid, the weaker the conjugate base).
On the various other hand, solid bases are bad leaving groups. This is why alcohols don’t participate in SN2 reactions very much! the hydroxyl group (HO-) is a disastrous leaving group.
If you’re not certain where a reaction is going to occur on a molecule, look because that a great leaving group. That’s typically where the action is!
The tendency is pretty clear – in general, the weaker the base, the far better the leave group. Furthermore, note just how we (almost) never ever see alkanes or hydrogens as leaving groups. That’s due to the fact that they’re strongly straightforward anions – and an extremely unstable.
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You can note that i have carefully avoided mentioning fluorine. Fluorine has tendency to be a really poor leaving team for SN1/SN2/E1/E2 reactions. In Org 2, you may see some instances where F have the right to act as a leaving team when the is attached come a carbonyl carbon or an aromatic ring. These reactions (addition-elimination reactions) are a tiny bit various in the the price determining action is no so related to loss that the leave group. There space some extra determinants at work in these situations that we can comment on if you’re curious.