Leaving group
In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules, but in either case it is crucial that the leaving group be able to stabilize the additional electron density that results from bond heterolysis. Common anionic leaving groups are halides such as Cl−, Br−, and I−, and sulfonate esters such as tosylate (TsO−). Fluoride (F−) functions as a leaving group in the nerve-agent sarin gas. Common neutral molecule leaving groups are water and ammonia.
The physical manifestation of leaving group ability is the rate at which a reaction takes place. Good leaving groups give fast reactions. By transition state theory, this implies that reactions involving good leaving groups have low activation barriers leading to relatively stable transition states.
It is helpful to consider the concept of leaving group ability in the case of the first step of an SN1/E1 reaction with an anionic leaving group (ionization), while keeping in mind that this concept can be generalized to all reactions that involve leaving groups. Because the leaving group bears a larger negative charge in the transition state (and products) than in the starting material, a good leaving group must be able to stabilize this negative charge, i.e. form stable anions. A good measure of anion stability is the pKa of an anion's conjugate acid, and leaving group ability indeed generally follows this trend, with lower pKaH's being associated with better leaving group ability.
The correlation between pKaH and leaving group ability, however, is not perfect. Leaving group ability represents the difference in energy between starting materials and a transition state (ΔG‡), and differences in leaving group ability are reflected in changes in this quantity (ΔΔG‡). The quantity pKaH, however, represents the difference in energy between starting materials and products (ΔG) with differences in acidity reflected in changes in this quantity (ΔΔG). Also, the starting materials in these cases are different. In the case of pKa, the "leaving group" is bound to a proton in the starting material, while in the case of leaving group ability, the leaving group is bound to (usually) carbon. It is with these important caveats in mind that one must consider pKaH to be reflective of leaving group ability, but nevertheless the trends in each tend to correlate well with each other. Consistent with this picture, strong bases such as OH−, OR− and NR2− tend to make poor leaving groups, due their inability to stabilize a negative charge.
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