Depto. Química Orgánica

CONCEPT OF LEAVING GROUP

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15/07/2017
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The SN2 reaction would be easier if effected over a substrate bearing a good leaving group.

But..., what the heck is a goog leaving group?
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The leaving abilities of a functional group is in inverse relationship with its strength when it acts as a base.
In short, a good leaving group must have a pretty moderate basicity.
From the following table of possible leaving groups, which would you tell are the best ones?
Yes indeed, those bearing the highest pKb, i.e. the less basic ones. Why a given species is more or less basic?
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An atom - or group of them - capable of accommodating a high electron density will display relatively low basicity and hence, will be a good leaving group.
The factors stabilizing an excess of electron density over a certain chemical species are SIZE, ELECTRONEGATIVITY and DELOCALIZATION (RESONANCE).
SIZEELECTRONEGATIVITYDELOCALIZATION
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
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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 in the Periodical System, the higher the electronegativity of the atom bearing the excess electron density, the lower its basicity and the better its capabilities as leaving group.
In the halides, the size effect is predominant because from one halide to the next we must change rows.
From a really bad leaving group F- > HO- > NH2- > CH3- to the worst leaving group ever
Resonance allows for the heavy delocalization of excess electron density and its strong stabilization.
That's why the sulfonates are extremely weak bases and thus excellent leaving groups.

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