A leaving group is a nucleophile acting in reverse; it accepts a lone pair as the bond in between it and also its neighbor (normally carbon for our purposes) is broken.
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So what provides a good leaving group?
The “happier” and more stable that lone pair is, the much better a leaving group it will be. The the majority of predictive ascendancy for leaving group capacity is….
Good leaving groups are weak bases.
Think about the dissociation of an acid H–A to provide H+ and A– . The species A– is the conjugate base of HA. It accepts a pair of electrons from the H-A bond. It’s a base acting in reverse.
The even more stable A- is, the better the equilibrium constant will be that favors dissociation to provide A- .
This transforms out to be the exceptionally definition of acidity! The negative log of the equilibrium constant is the familiar pKa.
In other words pKa is a straight measurement of how “happy” and also stable a lone pair of electrons is – the incredibly meaning of what we have to be in search of when trying to quantify leaving group ability.
So it should be no surprise to discover that exceptionally weak bases such as halide ions (I-, Br-, Cl-) water (OH2), and also sulfonates such as p-toluenesulfonate (OTs) and methanesulfonate (OMs) are terrific leaving groups.
These are the conjugate bases of solid acids. (Respeak to that the stronger the acid, the weaker the conjugate base).
On the various other hand, strong bases are poor leaving groups. This is why alcohols don’t get involved in SN2 reactions very much! the hydroxyl group (HO-) is a destructive leaving team.
If you’re not sure where a reactivity is going to happen on a molecule, look for a great leaving group. That’s commonly wbelow the activity is!
The trfinish is pretty clear – in basic, the weaker the base, the better the leaving team. In addition, note how we (almost) never before check out alkanes or hydrogens as leaving groups. That’s because they’re strongly fundamental anions – and incredibly unstable.
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You might note that I have actually carefully avoided mentioning fluorine. Fluorine often tends to be an extremely bad leaving group for SN1/SN2/E1/E2 reactions. In Org 2, you might view some examples wright here F have the right to act as a leaving team as soon as it is attached to a carbonyl carbon or an aromatic ring. These reactions (addition-elimination reactions) are a small bit different in that the rate determining action is not so regarded loss of the leaving team. There are some additional factors at work-related in these instances that we have the right to talk about if you’re curious.