Mechanistic classification substitution reactions

The mechanistic classification generally accepted for ligand substitution reactions was proposed by Langford and Gray in 1965 (19). This classification was often discussed in the literature and its principles are only summarized here for convenience. 

Langford and Gray proposed in 1965 (13) a mechanistic classification for ligand substitution reactions, which is now generally accepted and summarized here for convenience. In their classification they divided ligand substitution reactions into three categories of stoichiometric mechanisms associative (A) where an intermediate of increased coordination number can be detected, dissociative (D) where an intermediate of reduced coordination number can be detected, and interchange (I) where there is no kinetically detectable intermediate [Eqs. (2)-(4)]. In Eqs. (2)-(4), MX -i and... 

The substitution reactions of organometallic compounds, in which heterolytic attack at the carbon is usually by electrophiles, are not susceptible to the simple mechanistic classification (SnI, Sn2, and S i) which is so useful in discussing nucleophilic reactions of organo-nonmetallic compounds. The situation is complicated by the propensity of the metal to increase its coordination number, and a spectrum of mechanisms has to be considered in which electrophilic attack at carbon is accompanied by or preceded by nucleophilic attack at the metal. 

The classification within this section is based on the structural (rather than the mechanistic) relationship between the starting materials and products. Mechanistically, all of the reactions considered in this section involve nucleophilic substitution as the first step, except for aromatic substitution via the aryne mechanism, which involves elimination followed by nucleophilic addition. 

The interest in photoreactions that involve chemical change in an aromatic ring continues at a high level. Aromatic photosubstitution reactions have assumed a greater importance than they once had, but the classification of these reactions on a mechanistic basis is not easy because one of several different mechanisms may operate, and published information may not be sufficient to distinguish between the likely possibilities. However, it is clear that straightforward photochemical electrophilic substitution is as yet of very limited importance. 

Lastly, most mechanisms commence by the reaction of an electrophile or nucleophile on a reactant, or by a bond homolysis, and this in part forms a classification chemists give to each mechanistic possibility. For example, electrophilic aromatic substitution, or nucleo-... 

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