A leaving team is a nucleophile acting in reverse; it accepts a lone pair together the bond between it and its ar (usually carbon because that our purposes) is broken.

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So what provides a an excellent leaving group?

The “happier” and an ext stable the lone pair is, the far better a leaving group it will certainly be. The many predictive ascendancy for leaving group ability is….

Good leaving teams are weak bases.

Why?

Think about the dissociation that an mountain H–A to provide H+ and A– . The species A– is the conjugate base of HA. It accepts a pair of electron from the H-A bond. It’s a base acting in reverse.

The an ext stable A- is, the better the equilibrium continuous will be that favors dissociation to offer A- .

This transforms out to it is in the very meaning of acidity! The an adverse log that the equilibrium constant is the acquainted pKa.

In various other words pKa is a straight measurement of just how “happy” and also stable a lone pair of electron is – the very meaning of what we must be looking for when trying come quantify leaving group ability.

So it should be no surprised to discover that an extremely weak bases such as halide ions (I-, Br-, Cl-) water (OH2), and sulfonates such together p-toluenesulfonate (OTs) and also methanesulfonate (OMs) are great leaving groups.

These are the conjugate bases of strong acids. (Recall the the more powerful the acid, the weaker the conjugate base).

(think “differences in energy”), whereas leaving group capability is based upon reaction rates. So although the correlation is an extremely good, the isn’t perfect.>

On the other hand, solid bases are negative leaving groups. This is why alcohols don’t take part in SN2 reactions an extremely much! the hydroxyl group (HO-) is a terrible leaving group.

If you’re not certain where a reaction is walking to occur on a molecule, look for a great leaving group. That’s generally where the action is!

The trend is pretty clean – in general, the weaker the base, the much better the leaving group. Furthermore, note exactly how we (almost) never see alkanes or hydrogens as leaving groups. That’s due to the fact that they’re strongly straightforward anions – and very unstable.

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You could note that ns have carefully avoided stating fluorine. Fluorine often tends to be a very poor leaving team for SN1/SN2/E1/E2 reactions. In Org 2, you may see some examples where F deserve to act together a leaving team when the is attached to a carbonyl carbon or an fragrant ring. These reactions (addition-elimination reactions) space a little bit various in that the price determining action is no so regarded loss of the leaving group. There room some extra factors at occupational in these cases that us can discuss if you’re curious.