Leaving group definition

Intrinsic Nucleophilicity vs. Leaving Group Ability. As pointed out earlier, our definition of intrinsic nucleophilicity in terms of barriers to degenerate exchange reactions necessarily implies that nucleophilicity and leaving group ability are equivalent. This can be seen in either of two ways. First,... 

We consider as dihydro derivatives those rings which contain either one or two 5p3-hybridized carbon atoms. According to this definition, all reactions of the aromatic compounds with electrophiles, nucleophiles or free radicals involve dihydro intermediates. Such reactions with electrophiles afford Wheland intermediates which usually easily lose H+ to re-aromatize. However, nucleophilic substitution (in the absence of a leaving group such as halogen) gives an intermediate which must lose H and such intermediates often possess considerable stability. Radical attack at ring carbon affords another radical which usually reacts further rapidly. In this section we consider the reactions of isolable dihydro compounds it is obvious that much of the discussion on the aromatic heterocycles is concerned with dihydro derivatives as intermediates. 

According to the definition, electrophiles are electron pair acceptors. They therefore contain either a deficiency in the valence electron shell of one of the atoms they consist of or they are indeed valence-saturated but contain an atom from which a bonding electron pair can be removed as part of a leaving group. Concomitantly this atom accepts the electron pair of the nucleophile. Electrophiles are therefore, as a rule, cations or neutral compounds but not anions. In this book electrophile is abbreviated as E+, regardless of charge. 

The proposal of the general-acid catalysed expulsion of the leaving group (28) raises the question of possible catalysis by other electrophilic species. No definitive case of catalysis by metal ions has been reported (Lam and Miller, 1966 Giles and Parker, 1970), which may be a consequence of competition from the uncatalysed step which is believed to occur via the entropically favoured transition states [13] or [14] (Kirby andjencks, 1965 Bemasconi, 1973). 

A cross-coupling reaction can be partially defined by equation (1), where Nu is a carbon (or heteroatom) nucleophile see Nucleophile), R X is an electrophilic substrate, X is a halogen or other appropriate leaving group, and M is a metal or metalloid. At first glance, it would appear that simple nucleophihc substitution reactions should fall under this definition. However, what makes the cross-coupling chemistry special is its ability to perform transformations that cannot be accomplished with simple substitution chemistry. 

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